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Keywords:

  • green chemistry;
  • polymerization;
  • polymer reaction engineering;
  • renewable resources;
  • sustainability

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Conclusion
  5. Acknowledgments
  6. Biographies

The 12 principles of green chemistry are reviewed and applied specifically to polymer production. Examples of how the principles relate to current practice in polymer reaction engineering and which areas show the greatest potential impact for implementation of these principles are discussed. This paper does not attempt to be exhaustive but rather to target specific areas for further development.mren201300103-gra-0001

1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Conclusion
  5. Acknowledgments
  6. Biographies

Sustainability refers to the ability to meet current needs without putting in jeopardy the ability of future generations to meet their own needs. The development of sustainable technologies has been dealt with in several ways, one of which is the application of the principles of green chemistry to various processes. As a mature and versatile field, the polymer industry plays a significant role in our society as polymers have become ubiquitous. Issues with the extensive use of fossil-based raw materials and large amounts of reagents that are of environmental concern, in addition to the accumulation of polymeric materials in the environment, gives scientists and engineers the responsibility to re-examine the polymerization process in light of the 12 principles of green chemistry.[1] The 12 principles are:

  1. Prevent waste: design chemical syntheses to prevent waste, leaving no waste to treat or clean up.
  2. Design safer chemicals and products: design chemical products to be fully effective, yet have little or no toxicity.
  3. Design less hazardous chemical syntheses: design syntheses to use and generate substances with little or no toxicity to humans and the environment.
  4. Use renewable feedstock: use raw materials and feedstock that are renewable, rather than depleting. Renewable feedstocks are often made from agricultural products or are the wastes of other processes; depleting feedstocks are made from fossil fuels (petroleum, natural gas, or coal) or are mined.
  5. Use catalysts, not stoichiometric reagents: minimize waste by using catalytic reactions. Catalysts are used in small amounts and carry out a single reaction many times. They are preferable to stoichiometric reagents, which are used in excess and work only once.
  6. Avoid chemical derivatives: avoid using blocking or protecting groups or any temporary modifications if possible. Derivatives use additional reagents and generate waste.
  7. Maximize atom economy: design syntheses so that the final product contains the maximum proportion of the starting materials. There should be few, if any, wasted atoms.
  8. Use safer solvents and reaction conditions: avoid using solvents, separation agents, or other auxiliary chemicals. If these chemicals are necessary, use innocuous chemicals. If a solvent is necessary, water is a good medium as well as certain eco-friendly solvents that do not contribute to smog formation or destroy the ozone.
  9. Increase energy efficiency: run chemical reactions at ambient temperature and pressure whenever possible.
  10. Design chemicals and products to degrade after use; design chemical products to break down into innocuous substances after use so that they do not accumulate in the environment.
  11. Analyze in real time to prevent pollution: include in-process real-time monitoring and control during syntheses to minimize or eliminate the formation of by-products.
  12. Minimize the potential for accidents; design chemicals and their forms (solid, liquid, or gas) to minimize the potential for chemical accidents including explosions, fire, and release to the environment.

Given a general acceptance of these principles, we herewith examine each principle as applied to current practices in polymer reaction engineering.

1.1 Prevent waste: design chemical synthesis to prevent waste, leaving no waste to treat or clean up

The generation of waste in polymer production arises primarily from three sources: residual monomers due to incomplete reaction conversion, waste from chemicals used during the process but not consumed in the reaction such as solvents and suspending agents, and off-spec material generated due to excursions from controlled conditions.

Residual monomers are especially problematic because of their typically hazardous nature. Since most monomers demonstrate significant toxicity to human health, reducing the residual monomer content is desired to prevent workplace exposure as well as exposure to the consumer. Residual monomer removal techniques are generally classified into two categories: chemical methods and physical methods. Chemical methods include the reaction of the residual monomer to generate additional polymer or to produce non-toxic or at least, easily removable compounds. Physical methods include the removal of residual monomer from the polymer by evaporation, by solvent extraction or with the aid of an ion-exchange resin.[2] Extraction of the residual monomer using supercritical carbon dioxide was reported to give better performance than steam stripping techniques.[3]

Commonly used chemical methods include ramping the reaction temperature and/or using a finishing catalyst often referred to as a “chaser”. This protocol is usually implemented immediately after the polymerization and may lead to additional costs due to the extra processing stage. Post-polymerization methods and/or chemical monomer removal are often employed to reduce the monomer content before the use of devolatilization processes. Other techniques of this type include post-catalysis procedures followed by spray-drying,[4] and hydrolytic slitting of the monomers followed either by distillation or by the use of an oxidizing agent.[5]

In addition to the above-mentioned methods, there are ways to ensure that the monomer consumption in the polymerization is as high as possible. An example is the formation of poly(styrene) (PS) latex via ultrasonic initiation.[6] The presence of ultrasonic irradiation results in the increase of the polymerization rate. The molecular weight of the polymer latex obtained from this method is higher than the conventionally manufactured one, yielding a higher monomer conversion and lower residual monomer content in the final polymer product.[6] Modifying the reaction formulation by changing some of the components (e.g., initiator type) can also give promising results.[7] Similarly, optimization of process conditions can also lead to significant improvements to residual monomer levels, as shown for the emulsion polymerization of vinyl acetate (VAc).[8]

Choosing the best residual monomer removal technique is highly case specific as each polymerization method and monomer system presents its own peculiarities. However, one major principle that governs the choice of residual monomer techniques is that the method used should not affect the final properties of the polymer product.

Solution polymerization is a widely used polymerization technique, which uses a solvent as the polymerization medium. Solution polymerization is often preferred over bulk polymerization due to viscosity and heat transfer issues.[6] One major disadvantage of solution polymerization is the removal of the solvent from the polymer, which may require energy intensive methods such as distillation.[6] However, not all polymerizations can be practically carried out in a solvent-free environment, and many solution polymerization products exhibit superior properties. Therefore, many research efforts are underway to achieve similar properties in either alternative, safer solvents or by other polymerization techniques.[9-11]

The issue of off-spec material concerns many industries. In the polymer industry, the production of off-spec material is not unusual. The viscous nature of high molecular weight polymers and the typically exothermic nature of the reaction make the polymerization process prone to temperature excursions. These excursions can affect the polymer molecular weight and lead to branching, cross-linking and eventually, the formation of gel. In some cases, the presence of cross-linked materials and gels is desired to impart certain performance properties. However, in many instances, these may have a deleterious effect on product properties. Ensuring proper mixing, heat removal and process control methods can be helpful in those cases.

In addition, off-spec material can result from the presence of impurities found, e.g., in the monomer feed stream. Impurities that scavenge radicals in free-radical polymerizations can have a significant effect on the final product.[12] One should also consider that bad batches are often blended with good ones. Finally, recent advances in polymerization methods such as controlled radical polymerization (CRP) have served to restrain the formation of unwanted compounds and have also enabled remarkable microstructural control.[13-15]

As can be deduced from the above discussion, there is not a single pot solution for the minimization of waste production in polymerization processes. Often, a combination of several steps must be considered. These steps include measures taken to reduce the use of solvents or their replacement, the implementation of catalysis to improve yield, and means to either prevent or reduce residual monomer. The use of alternatives to current solvent-based synthesis may play a significant role in reducing waste, but the maintenance of high quality final product properties is of concern.

1.2 Design safer chemicals and products: design chemical products to be fully effective, yet have little or no toxicity

In a relative way, polymer products tend to be inherently safer than their starting materials (e.g., monomers, solvents). In general, most polymers are in and of themselves non-toxic. However, in the event that they are degraded or burned, their degraded components may present serious environmental effects.[16-18] In addition, incomplete conversion of the starting materials may result in residual monomers in the product, which upon release, can prove to be quite harmful.[16-18] Moreover, in many instances, a variety of additives are often used to modify polymer properties and these also can pose health and environmental risks.[19, 20] These additives can be used as processing aids, antioxidants, flame retardants, solvents, electrostatic agents, stabilizers, pigments, mechanical property modifiers (e.g., plasticizers), etc.[20]

Solvents are a major input in the polymerization industry and comprise a significant fraction of the waste generated as well as a large portion of workplace hazards. A significant source of toxicity in polymer products occurs when solvents are used either as diluents or to enable the application of the polymer (e.g., as a paint or adhesive). Volatile organic compounds (VOCs), highly polar conventional solvents such as N-methylpyrrolidone, N,N′-dimethylacetamide, N,N′-dimethylformamide, pyridine, and chlorinated solvents, which are used in condensation polymerizations, result in increased air pollution, most of them being volatile and most of them toxic and flammable.[21-23] These solvents are high on the list of harmful chemicals because they are used in significant volumes and their volatility makes them difficult to handle. Hence, based on these concerns, certain less hazardous surrogates have been suggested for use. Among the suggested solvent alternatives are monoterpenes (MTs), ionic liquids (ILs), and supercritical fluids (mainly, supercritical carbon dioxide), to name but a few.[21-23] These environmentally benign substituent solvents perform comparable to conventional fossil-fuel based solvents.[21-23] A more in depth discussion on solvent alternatives is presented in Section 8.

Functional fillers represent a significant component of polymer products.[24] They are primarily used to modify mechanical and thermal properties. Many synthetic fillers such as silica, titanium dioxide, calcium carbonate, glass fiber, and carbon fiber are used extensively. An increasing number of naturally sourced fillers such as clays, natural fibers (e.g., cellulose, hemp, flax, kenaf, and wood) and starch are also employed. Attempts to address environmental concerns regarding these composite materials have been reviewed recently.[25, 26] Some specific examples include the use of pine wood flour,[27] natural jute fiber,[28] sugarcane bagasse fiber,[29] cellulosic fibers,[30] and kenaf biofibers.[31]

An interesting subclass of fillers, nanofillers, are materials having at least one dimension in the nanometer range. Due to their high aspect ratio and high surface area, even low loadings of these nanomaterials impart unusually strong modifications to polymer properties.[32] Whether synthetic or natural, when these materials are modified to the nanoscale, it is not known how these might impact the environment and human health. At least one of these, cellulose nanocrystals, has been approved in some sectors as environmentally benign.[33]

Plasticizers are incorporated into a material to increase its flexibility and processability.[34] These are generally non-volatile, low molecular weight materials, which consequently, can leach out of the polymer product. Phthalates, which are the diesters of 1,2-benzenedicarboxylic acid (phthalic acid), are the most common class of plasticizers used in industry. Others include benzoates, tartrates, chlorinated hydrocarbons, phosphates, etc.

Other examples of toxic additives used in plastic processing for which their impacts on human health have been extensively discussed[19] are bisphenol A and polybrominated diphenyl ethers (PBDEs). Bisphenol A is used in a variety of polymer products mostly as food can lining[35] and also in poly(carbonate) plastics as used for baby bottles and other containers.[36] PBDEs are flame retardant chemicals used in many thermoplastics. Studies have shown that exposure to bisphenol A and PBDEs causes altered endocrine function and reproductive effects[37] and might also result in cancer in the case of higher percentage accumulation.[38]

Bisphenol A has been utilized in four main classes of polymers: epoxies, polycarbonates, polyesters, and polyimide. In order to minimize the level of this endocrine-disrupting additive, several pathways and replacements have been widely investigated.[39] For example, aliphatic-aromatic components derived from 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCBD)[40] and isosorbide have been studied as promising candidates for bisphenol A replacement.[41]

Another class of highly toxic chemicals which have been used for cross-linking and network formation are highly reactive isocyanate containing compounds.[42] Isocyanates are used extensively in polyurethane production resulting in outstanding mechanical and thermal properties.[43] Replacement of isocyanate components with other, less harmful di- and tri-functional molecules such as bis-propargyl-succinate has been reported.[44] Additionally, less toxic routes to the synthesis of isocyanate-free polymers or non-isocyanate polyurethanes (NIPUs) have been explored using different monomers. For example, monomers such as cyclic carbonates, for the synthesis of poly(urethane-carbonate)[45] and carbonated soybean oil for the catalytic synthesis of NIPUs have been studied.[46]

A logical approach has been to replace these compounds with higher molecular weight materials to achieve lower solubility and migration rates. Research on a number of natural-based plasticizers is underway.[47] Notable examples include soybean oil, epoxidized triglycerides from vegetable oils, water, polyols (e.g., glycerol, sorbitol, and xylitol), monosaccharides (e.g., mannose, glucose, fructose, and sucrose), amino acids, saturated fatty acids. The use of triethyl citrate in the development of a cellulose acetate biopolymer is a good example of combining a renewable, non-toxic plasticizer with a renewable polymer.[48]

The presence of VOCs as solvents, residual monomers, and additives in the polymer industry poses many potential negative consequences to human health and the environment.[19, 49, 50] The replacement or removal of these components without substantially affecting the quality and properties of the polymer products is an area of ongoing study. The use of alternative solvents as suggested in Section 8 presents an area of significant impact. The presence of residual monomers, while not as significant in quantity as other volatile components, nonetheless, provides an important area for improvement, especially in light of the increased use of polymers in medical applications. Polymer additives have only begun to be targeted by various concerned groups and their replacement with environmentally benign compounds is also at the forefront of several research efforts.

1.3 Design less hazardous chemical synthesis: design syntheses to use and generate substances with little or no toxicity to humans and the environment

The transformation of one or more monomers into a final polymer product often includes a number of additional components to aid in the synthesis or to modify the final product properties. As noted in the previous section, large volume of solvents, often petroleum-based, as well as plasticizers, fire retardants, pigments, stabilizers, catalysts, and the like, are used throughout the synthesis stage and these often pose serious toxicity concerns. In addition to the workplace hazards posed by these materials, their release to the environment during product use, under unexpected conditions (e.g., in a fire), and after disposal may also pose significant risks (see Section 2). Hence, a less toxic polymer synthesis suggests either an alternative processing technology or the replacement of toxic starting materials with environmentally benign substances (see also Sections 4, 5, 6, and 8). The discussion that follows is focused on the use of less toxic monomers, solvents, catalysts, and additives as possible surrogates for conventional components in polymer product synthesis, processing, and modification.

It is widely accepted that many monomers pose serious health hazards but in a polymerized form, the hazard is no longer present or is significantly reduced. Of course, the hazard of exposure to monomers does exist during polymer synthesis in the workplace but may also exist in the product in the event of incomplete conversion, where residual monomer remains. One also should not forget cases where the curing of the polymer must take place (e.g., in epoxies). Eventually, depending on the usage, degradability, and disposal methods, a polymer may revert to its original monomer components and once again, the toxicity of the monomer would become important. Although there are synthetic polymers that are not bio-sourced and still environmentally benign, a shift to the use of so-called “green” monomers often implies the use of starting materials from biomass.

Bio-based polymers, like polymers from cellulose and ligno-cellulosic materials, were reported to present poorer performance compared to those derived from petroleum-based monomers, especially in terms of thermal processability.[51] The relative difficulty of converting biomass into plastics is due to cellulose's rigid backbone. This is a common refrain when it comes to replacing long-existing products with more benign options. It is clear that work has to be done to improve the performance of bio-based materials but successes are indeed possible.[52, 53] Below, we report on the numerous examples of bio-sourced monomers being successfully transformed to quality polymer products.

Cellulose derivatives are attractive bio-based materials, among which nitrocellulose (NC) and cellulose acetates (CA) are the most common. Many recent studies have focused on CA, mostly because it is produced in large quantities.[54]

Lignocellulosic material, primarily wood, is another class of biomass that can be processed into plastics. However, lignocellulosic materials cannot flow easily and methods for processing them are limited. Several reactions such as esterification and etherification were reported during the late 1970s and early 1980s to convert wood into a plastic material by chemical modification.[55] Such a conversion was mainly related to the internal plasticization of wood, and the degree of plasticization was found to be insufficient, especially when small substituent groups or polar groups were used. Therefore, external plasticization was used to achieve better properties. Benzylated wood (BzW) is one of the most common externally plasticized lignocellulosic materials with reported tensile strengths of 42.7 MPa, which are significantly higher than that of PS at 29.4 MPa under the same conditions.[51] In addition, differences are observed in their flow properties, with PS starting to flow at 153 °C whereas BzW begins to flow at 175 °C.[56]

Poly(lactic acid) (PLA) is another class of bio-based polymers which is highly biocompatible; not surprising given that its monomer, lactic acid, is a natural product found in the body.[57-60] Early application of PLA was in the biomedical field[58-60] and greater improvements in its synthesis have been achieved over the past decade, particularly from fermentation pathways to convert corn starch into lactic acid. PLA is used in a wide range of applications such as packaging materials, plastic bags, and thermoforms.[61, 62]

Biodegradable poly(carbonate) synthesized by ring opening polymerization of six-membered cyclic carbonates is another interesting class of polymers that can be used for the production of thermoplastics. These polymers are widely used in the medical field, e.g., in the production of sutures, drug delivery systems, implants, and tissue engineering.[63-66]

Several other benign monomers exist either as naturally occurring compounds or through certain chemical modifications based on natural substances. Conversion of starch into environmentally benign monomers via catalytic reduction has been studied and compounds such as polyols, organic acids (lactic and glycolic acid), 5-hydroxymethylfurfural, and levulinic acid have been formed as a result of the catalytic breakdown of starch.[67]

Glycerol, which can be obtained via various sources, such as biodiesel production, is another interesting renewable source for the production of poly(glycerol).[52] Stimuli-responsive hydrogels were produced from bio-based poly(glycerol).[53] A wide variety of polyurethane materials have been produced from different types of vegetable oil-based monomers. These polyurethanes have been evaluated and proven to be of comparable, or even better performance than some fossil-based polymers.[68]

In order to modify the final properties of polymer products, several additives may be added such as emulsifier, plasticizers (see Section 2), stabilizers, flame retardants, and other fillers (see Section 2). For example, nonylphenol ethoxylates (NPEOs) are extensively used as surfactants for emulsion polymerization and for stabilization in latex applications. But NPEOs are undesirable because alkyl-phenol residues are formed when these surfactants are released to the environment.[69] Therefore, the substitution of ionic (NPEOs) with environmentally friendly non-ionic alkyl phenol free emulsifiers is highly encouraged.[70] Fatty alcohol ethoxylates (FAEOs) as less toxic surfactants are suitable substitutes for these surfactants. FAEOs, having a degree of ethoxylation between 7 and 40, show excellent water miscibility and stability performance in emulsion polymerization.[70] The hydrophobe in the surfactants is synthesized from renewable resources and the use of these bio-based surfactants has proven to be successful for many systems such as acrylates.[70] Other examples of environmentally friendly surfactants are mannuronate moieties derived from alginates, fatty hydrocarbon chains derived from vegetable resources[71] and ILs[72] and surfactants based on lipo-peptides.[73]

Flame retardants are also common additives used especially in the plastics industry. Two significant classes of flame retardants are organohalogen and organophosphorous compounds.[74-76] The organohalogen compounds, particularly brominated aromatics, have been the most widely used because of their efficiency and low cost. Alternatively, organophosphorous compounds have equal efficiency to the organohalogen compounds, but are much more expensive. The bioaccumulation of organohalogen compounds in the environment and the related health risks are significant concerns.[77, 78] Recent research has been conducted on a non-toxic flame retardant derived from tartaric acid, a by-product of the wine industry, and can be used in the synthesis of bio-based, environmentally friendly flame retardants for polymers.[79] One can also look to bio-based nanofillers for improved flame retardant properties.[32]

Commercial polymers are often processed in the presence of solvents. Application of environmentally friendly solvents in polymer production is discussed extensively in Section 8. Less hazardous syntheses and the use of substances with little or no toxicity to humans and the environment is one of the most important areas of research in polymer science and many current studies are focused on this field. This is not surprising as there is great potential for significant impact on harm reduction from these approaches.

1.4 Use renewable feedstock: use raw materials and feedstock that are renewable rather than depleting. Renewable feedstocks are often made from agricultural products or are the wastes of other processes; depleting feedstocks are made from fossil fuels (petroleum, natural gas, or coal) or are mined

The use of macromolecular materials based on renewable resources had long been a low priority with the surge of coal-based chemistry starting in late 19th century, followed by the petrochemical revolution of the 20th century.[80] However, in recent years, increasing environmental concerns and the need for more versatile polymer-based materials has led to greater interest in polymers from renewable feedstocks. This includes the design and synthesis of renewable polymers with various macromolecular architectures and enhanced properties.[81] The consumption of fossil fuels in the production of plastics was reported to be about 7% of worldwide oil production.[82] Therefore, given the increasing oil prices and its limited sources, it is necessary to develop new pathways to polymeric materials using renewable resources. Some interesting renewable sources are discussed below.

The use of renewable monomers can be attractive due to their abundance, the typical presence of high energy heteroatom functionality, and in many cases, low cost. Challenges may arise due to the frequent need to eliminate superfluous heteroatom functionality in these monomers and in some cases, high cost. It is also frequently difficult to achieve high monomer purity from bio-based sources and therefore, high molecular weight polymer production can be hampered.

Gandini[83] classified renewable monomers from the following starting materials (limited examples are shown):

  1. vegetable oils: soybean oil, castor oil, palm oil, rapeseed oil, linseed oil, tung oil, etc.
  2. lignin fragments: cellulose, chitin, chitosan, starch, lactic acid, etc.
  3. sugars
  4. rosin: resin acids
  5. glycerol
  6. furans: furfural, furfuryl alcohol
  7. tannins
  8. suberin
  9. terpenes: α-pinene, β-pinene, and limonene
  10. and others: citric acid, tartaric acid, carbon dioxide

Lactic acid is a product of the fermentation of D-glucose, usually derived from corn or sugar feedstocks. The lactic acid is thermally and catalytically converted into its cyclic dimer and lactide. The lactide can then undergo ring-opening polymerization to yield PLA (a.k.a. poly(lactide) or PLA).[61] PLA makes an interesting replacement for polyolefins, yet degrades to metabolites. It has found extensive application in the packaging and fiber industries.[61]

Plant oils, such as soybean oil, palm oil, and rapeseed oil are some of the most important renewable raw materials for the chemical industry. Plant oils consist of a broad composition of fatty acids (i.e., triglycerides) and are typically liquid at room temperature. Their physical and chemical properties are greatly affected by the stereochemistry of the double bonds of the fatty acid chains, their degree of unsaturation, and the length of the carbon chain of the fatty acids. Plant oils find broad application in foods, pharmaceuticals, cosmetics, fuels (e.g., biofuels), lubricants, paints, and construction materials.[84] Over 120 million tons are produced per year worldwide, with 12 million tons available to the oleochemical industry; the rest is used for food. Due to their natural abundance and reactive functionality, they make an obvious choice as polymer building blocks. They can be classified into two product groups: base chemicals (e.g., fatty acids, methyl esters, fatty alcohols) and derivative compounds. Plant oil-based polymers have found extensive use in paints, coatings, resins, and as flooring materials. The most famous example of these is linoleum flooring, which has been industrially produced for well over 100 years.[85] Linoleum provides a durable and environmentally friendly alternative to PVC flooring. In recent years, extensive studies have been carried out on the production of various polymers from plant oils.[86-90] However, their use as monomers presents two major challenges: (i) their chemical structure is often heterogeneous and variable; and (ii) extraction of plant oils can be costly due to their typically low concentrations and seasonally variable compositions.[82]

Carbon dioxide (CO2) is an attractive monomer replacement option since it is abundant, inexpensive, non-flammable, and a waste product of many chemical processes. It is estimated that over 200 billion tons of glucose are produced from CO2 via photosynthesis each year. However, the development of effective processes that use CO2 is lagging.[82] For example, the viable production of only urea, salicylic acid, and some cyclic carbonates are possible at this time. Nonetheless, interesting copolymerization products are possible by combining CO2 with heterocycles (e.g., epoxides, aziridines, and episulfides). On the larger scale, CO2 can be used as a feedstock for the synthesis of poly(carbonate).[91] Poly(carbonate) is a thermoplastic material and can be efficiently produced from CO2 and epoxides.

Due to a significant glut in the glycerol market because of increasing biodiesel production, there is great interest in finding economic ways to utilize the emerging surplus of glycerol. Glycerol as a bio-based monomer with high functionality is an interesting candidate for sustainable step-growth polymerization. Recently, new opportunities for the conversion of glycerol to high molecular weight polymers have emerged.[52] Poly(glycerol) produced from this cost-effective renewable monomer was used to produce hydrogels suitable for a wide range of pharmaceutical and biomedical applications.[53, 92]

Renewable bio-based aromatic monomers have been synthesized and used as alternatives to monomers such as terephthalic acid and styrene which are widely used fossil-based aromatics.[93] Efficient conversion of solid biomass feedstock to high quality aromatics has been investigated using different catalytic methods such as catalytic fast pyrolysis.[94] From these, one can derive furan heterocycles and other similar molecules.[95] Conversion of the furan aglycon into γ-keto-carboxylic acid moieties results in a renewable building blocks for different applications.[96] In another approach, furan moieties were reacted with epoxidized triglycerides, another monomer from two renewable sources, in a Diels–Alder reversible polycondensation.[97] Additional discussions on bio-based aromatics (e.g., terpenes) can be found in other sections.

In addition to renewable monomers, natural polymers such as starch can be used as renewable feedstocks.[98] Starch can be found in nature in different plant forms and this high molecular weight polymer can be modified to be used as a replacement for fossil-based polymers. For example, starch was used as the starting material for production of EcoSphere biolatex binder which has been proven to be a successful replacement for petrochemical-based latexes such as styrene butadiene rubber (SBR) used in the manufacture of paper and paperboard products.[99, 100]

Research in the use of renewable monomers is expanding rapidly and one cannot hope to provide an exhaustive review of each of the possibilities. From terpenes[101, 102] to furans[103] to carbohydrates,[104] and many others,[105, 106] interest in this area continues to grow. While current limits to the feedstocks for renewable polymers imply that they are not expected to overtake the market for commodity polymers in the near future, the use of renewable polymers in a wide range of specialty products is highly promising.

1.5 Use catalysts, not stoichiometric reagents: minimize waste by using catalytic reactions. Catalysts are used in small amounts and carry out a single reaction many times. They are preferable to stoichiometric reagents, which are used in excess and work only once

Catalytic agents are used to not only increase reaction rate but to manipulate the final product properties. In free radical polymerization, e.g., a small amount of catalyst say, 10−9–10−3M, results in the reaction of 0.1–10 M reactive groups. Usually, by using catalysts, waste formation is significantly decreased. In some cases, it is possible to recover or recycle the catalyst after completion of reaction but often separating the catalysts from the polymer product is very challenging.[107] However, the catalysts are often used at very low concentrations and can be deactivated.[108, 109]

The types of catalysts used in polymerization are either homogeneous or heterogeneous. A majority of homogeneously catalyzed processes exist in the liquid phase. Homogeneous catalysis is advantageous in terms of increased activity but problems may arise due to corrosion of reaction vessels and difficult separation processes.[109] On the other hand, the use of heterogeneous catalysts has proved to be an increasingly interesting alternative as solid catalysts are rarely corrosive, are capable of withstanding a wide range of temperatures and pressures without a significant impact on their activity, and separation of the catalyst from the product may be straightforward.[109]

Catalysts have been used in polymer synthesis since 1953, where some transition metal compounds (e.g., TiCL4) were used in combination with aluminum alkyls for polymerization of ethylene into linear poly(ethylene) (PE).[110] The use of Ziegler–Natta polymerization has increased tremendously in the polymer industry over the years for the production of polyolefins. By using Ziegler–Natta catalysts, it has been feasible to achieve outstanding balance between properties such as fluidity, impact strength, and transparency. The production of polyolefins via Ziegler–Natta polymerization now exceeds 70 million tons globally.[110] After some studies were performed on titanocenes,[111] a group of catalysts containing homogeneous and single-site systems were developed, based on metallocenes[112-118] or late transition-metal complexes.[119, 120] These catalysts seemed to be much better than conventional Ziegler–Natta systems. However, heterogeneous Ziegler–Natta catalysts are still the most popular catalyst in most industrial applications. Polymer production technologies have improved to a great extent with the evolution of Ziegler–Natta catalysts. For example, exceptional control of morphology over both the catalyst and polymer particles was achieved and some important polymer parameters such as molecular mass, molecular weight distribution, and co-monomer incorporation were obtained.[119, 120]

Catalysts are also used in CRP methods such as atom transfer radical polymerization (ATRP). This method, which was originally established in the 1990s enables great molecular weight and functionality control in polymerization with minimal catalyst concentrations. Copper complexes are the most popular catalysts used via this method.[121-123] Other metal catalysts are also used for polymer syntheses[124] such as ruthenium catalyst for living radical polymerization of star polymers[125], iron for ATRP of methyl methacrylate (MMA),[126] and cobalt for polycarbonates and cyclic carbonates syntheses.[127]

The concentration of catalysts used in polymerizations can be an issue and, in many cases, residual catalyst in the final polymer product could result in changes in final properties as well as inducing toxicity. In previous decades, advances in catalyst technology and properties such as activity, selectivity, and specificity have led to the use of low concentrations of catalysts with very high yield, which minimizes the chance of occurrence of undesirable side reactions.[110] For example, for PE synthesis, efficiencies of catalysts based on either MgCl2 or its precursors, range from approximately 10 to 40 kg polymer · g−1 catalyst.[110]

During the past two decades, polymer synthesis using environmentally benign enzymatic catalysis has received a great deal of attention.[128] These catalysts are interesting because they are reported to induce faster reaction rates, result in efficient reactions at mild conditions and they have high stereo-, regio-, and chemoselectivities.[129] However, the use of enzymatic, bio-based catalysts, and organo-catalysis is usually less straightforward than that of chemical catalysis.[130, 131] Their use often eliminates the need for severe polymerization conditions such as elevated temperatures for condensation polymerization, as enzymes function under mild conditions (≤100 °C).[132] In addition, precision syntheses are often achieved using enzymatic catalysis. The synthesis of polyesters is a great example of enzyme-catalyzed condensation polymerization at mild conditions[132] and that of poly(ethylene glycol) using Candida antarctica lipase B is an example of precision polymerization.[133] Table 1 shows the different types of polymers produced with their respective enzymes.[129]

Table 1. Examples of in vitro production of polymers catalyzed by enzymes[132]
EnzymesTypical polymers
OxidoreductasesPolyphenols, polyanilines, vinyl polymers
TransferasesPolysaccharides, cyclic oligosaccharides, polyesters
HydrolasesPolysaccharides, polyamino acids, polyamides, polyesters, polycarbonates

In step growth polymerizations, the catalyst plays a critical role. In addition, stoichiometric imbalances can greatly and negatively affect the degree of polymerization. The use of superacid catalysts for polydyroxyalkylation reactions provides an example of overcoming this difficulty and permitting the synthesis of polymers with a broad range of molecular architecture.[134]

Catalysis is a highly evolved and advanced sector in polymer reaction engineering and serves a key industrial role in increasing polymerization rates and manipulating final polymer product properties. For instance, most step-growth polymerizations require catalysts in order to proceed at a practical rate of reaction. The introduction of catalysts to such systems will effectively result in the decrease in waste and minimization of the energy consumption for running the reaction.[135]

1.6 Avoid chemical derivatives: avoid using blocking or protecting groups or any temporary modifications if possible. Derivatives use additional reagents and generate waste

Throughout the polymerization process, there may be instances in which the monomer must be modified either to make it compatible for future steps (e.g., modification of polymer surfaces) or to impart certain characteristics vital to the polymer product. Such temporary or permanent modifications are sometimes affected by the use of blocking or protecting groups, which introduce additional reagents into the process. These additional reagents may lead to the generation of waste. For example, poly(vinyl alcohol) (PVA), is usually modified temporarily using a t-butoxycarbonyl group (t-BOC) into poly(vinyl-t-butyl carbonate) and then thermolyzed to achieve the final PVA product. Such a modification is done in order to make PVA soluble in a wide variety of organic solvents, but during the process, carbon dioxide and 2-methyl-propene are released, which are of concern for workplace safety and the environment.[136] Therefore, alternative means by which the desired product performance properties can be achieved without the use of additional reagents is important.

Another example is 2-hydroxyethyl methacrylate (HEMA), a functional monomer which has been used in many applications such as in the manufacture of soft contact lenses.[137, 138] HEMA homopolymer is a hydrophilic gel synthesized through a fairly complicated pathway. There have been several reports on the synthesis of controlled-structure HEMA-based block copolymers via anionic polymerization and in most of these approaches, the protection of the alcohol functionality in the presence of additional reagents is required.[139, 140] These methods are harmful to the environment due to the release of wastes. In addition, the three-step synthesis method includes synthesis of the protected monomer followed by the controlled polymerization and finally, the removal of the protecting groups.

The application of blocking and protecting groups for temporary modifications on polymeric substances is often applied via CRP, but the outstanding property control afforded by these methods presents an intriguing dilemma. The ability to create unique polymer architectures under highly controlled conditions would not be possible without these methods. One form of CRP, ATRP, was discovered in 1995[121] and has been used effectively for the polymerization of hydrophilic monomers in aqueous solution under mild conditions.[141-143] For the production of poly(HEMA) in aqueous media, the process was made more efficient by switching the medium to a 50:50 methanol/water mixture which resulted in a useful product without the use of a protecting group to aid in solvation.[144] Similar results were obtained when ATRP was employed to form 3-sulfopropyl methacrylate (SPMA) homopolymers and amphiphilic block copolymers with MMA in a water/dimethylformamide (DMF) mixed solvent at 20 °C using Cu/bipyridine catalyst.[145] It is interesting that in polymerizations using a mixed solvent system, it is possible that hydrophobic monomers can also be block-polymerized in the same solvent. This allows for the direct synthesis of hydrophobic–hydrophilic block copolymers without the application of protecting group chemistry or post-polymerization derivatization. However, despite the lack of protecting groups, there were some drawbacks. In particular, the small amount of residual SPMA monomer detected during the initial stage of the MMA polymerization had a significant effect on the micellization properties.[146]

Frequently, one finds the application of blocking and protecting groups in the preparation of polymeric materials for biomedical applications. In fact, blocking and protecting groups are used widely in biomedical polymers because of the need for bulk and surface modification. Such modifications are put in place to improve the acceptance and healing of the synthetic device or medical implant. Many innovative processing technologies are used to produce functional devices resembling natural tissues.[146] Often, the synthetic materials fabricated for medical purposes need to be made compatible with reactions taking place in the body, via the incorporation of reagents as blocking or protecting groups.

The generation of waste that arises as a result of the introduction of further reagents is undoubtedly an issue to be dealt with in designing sustainable chemical processes. In the case of polymer reaction engineering, the related impacts are not as serious as some of the other issues presented herein.[147, 148] Many of the polymer products requiring protecting/deprotecting are niche products. Thus, given that worldwide production levels of these products are significantly lower than that of commodity polymers such as, e.g., PS or PE, the total impact of additional chemicals used in their production is comparatively small.

1.7 Maximize atom economy: design syntheses so that the final product contains the maximum proportion of the starting materials. There should be few, if any, wasted atoms

Efficiency is a key factor in sustainable polymer synthesis; it should be addressed not only in terms of selectivity [chemo- (functional group differentiation), regio- (orientational control of two reacting partners), diastereo- (control of relative stereochemistry), and enantio-selectivity (control of absolute stereochemistry)], but also through the number of atoms in the reactants (monomers and other reagents) that are retained in the final polymeric product.[149] This concept is referred to as atom economy, and is calculated by dividing the molecular weight of the desired product by the sum of the molecular weights of all the substances produced in the stoichiometric equation. In ideal terms, a 100% atom efficient process signifies one in which the product would include all of the atoms of the reactants and no waste is produced. The degree of atom economy is commonly defined by the E-factor, the ratio of the amount of waste produced to the amount of product that results (kg of waste/kg of product).[150] Therefore, the lower the value of the E-factor, the less waste is produced, and vice-versa. Accordingly, the major benefits derived from an atom economical process are most notably the effective use of valuable raw materials, decreased emissions, and low waste production.

In the case of straightforward reactions, atom efficiency can simply be enhanced by using catalysts. Using catalysts facilitates the pathway by which a desired product can be achieved by better selectivity and activity. Therefore, the formation of unwanted by-products is minimized and if possible, leads to a 100% atom economical process.[151] However, in the case of polymerization, there is not any one-pot solution to transform a less atom economical process towards a more atom efficient one as there are multiple factors responsible for the production of unwanted by-products. Atom wastage in polymerization, in some instances, is unavoidable due to the nature of the process and the absence of alternative ways to synthesize the polymer. The most common example of polymerization in which by-products are produced is found in many step-growth polymerizations. There is usually a specific type of chemical reaction which enables monomers to combine to form macromolecules. Example reactions in step-growth polymerization include esterification, amidation, formation of urethanes, and aromatic substitution.[135] Due to a limited number of chemical reaction options, the polymer industry has been forced to follow these pathways even though unwanted by-products were continually being produced. For instance, Nylon 66 is produced according to:[135]

mren201300103-gra-0002

As seen in the reaction scheme above, there is always one mole of water produced as a by-product for every linkage produced. Hence, this reaction is not fully atom efficient, with an atom efficiency of 91.26%. On the other hand, there are also 100% atom efficient step-growth polymerizations such as in the production of polyurethanes, as shown in the following reaction scheme:

mren201300103-gra-0003

Unlike most step-growth polymerizations, which proceed via a single pathway, there are cases such as in the formation of poly(ethylene terephthalate) (PET), which can proceed via several pathways. PET can be formed in two distinct ways:mren201300103-gra-0004

The first pathway is moderately more atom efficient than the second, where atoms are lost as 2n-H2O. Based on this, one may decide to proceed with the first pathway in view of the atom economy, but availability of the corresponding monomers, price, reaction conditions, energy consumption, and safety also must be considered.[135] A similar scenario can be witnessed in the production of poly(glycerol), which can be synthesized via two pathways, i.e., (i) using glycidol as a monomer via free radical polymerization, which has a higher atom economy and (ii) from glycerol via step-growth polymerization, which has a relatively lower atom economy.[52, 53] In the first case, a highly toxic, non-renewable monomer is used as a starting material while in the second case, a renewable, non-toxic monomer is used. In other words, this example highlights how one should use caution and not consider each green chemistry principle in isolation.

Among the different step-growth polymerization techniques, it is the interfacial type that is referred to as the most atom economical. Another advantage of this method is that higher molecular weight polymers are usually produced.[152, 153] Interfacial polymerization is characterized by the formation of polymers at or near a phase boundary of two immiscible monomer solutions. These reactions are carried out at lower temperatures, which results in lower relative rates of side reactions. Monomer purity, in this case, is not as critical as in typical step-growth polymerizations. Nevertheless, interfacial polymerization has not been applied widely industrially due to the high cost of the reactive monomers and the significant solvent removal and recovery step.[135] However, in the case of many other step-growth polymerizations, monomer purity is of prime significance as it determines the possibility of formation of unwanted by-products through side reactions; hence higher atom economy is to be expected only when high purity monomers are employed.

Contrary to step growth polymerizations, chain growth polymerizations are highly atom efficient and all atoms in the monomers are consumed during the course of polymerization. Nonetheless, it is worth mentioning that a chain growth polymerization might indeed lose atoms via side reactions and waste generation. As discussed in Section 1, the generation of waste will be influenced by monomer purity and control of the reaction conditions (e.g., temperature, ingredient feed rates).

In conclusion, atom economy (or E-factor) is an important criterion to be accounted for, when considering sustainable processes. Several methods to reduce waste formation in, e.g., the fine chemical industry have been implemented and resulted in higher atom economy. Polymer processes, for the most part, tend to have an inherently high atom economy due to the reaction mechanism. Achieving an even higher atom economy may involve a trade-off among several other equally or more important factors that contribute toward the attainment of a more sustainable process. Often, one would need to employ an alternative polymerization mechanism to obtain the same polymer and this may not always be feasible.

1.8 Use safer solvents and reaction conditions: avoid using solvents, separation agents, or other auxiliary chemicals. If these chemicals are necessary, use innocuous chemicals. If a solvent is necessary, water is a good medium as well as certain eco-friendly solvents that do not contribute to smog formation or destroy the ozone

Solution polymerization continues to be an important method to produce polymers with valuable properties. The use of a solvent as a polymerization media prevents increases in viscosity due to the generation of high molecular weight polymer chains by diluting the reaction mixture. This translates into improved heat transfer and prevention of thermal runaway by absorbing the heat of polymerization. Many polymer products have been developed over time, which present desirable molecular weight qualities and other final properties that cannot be easily met by other polymerization technologies (e.g., emulsion polymerization). However, with strict regulation and concerns over VOCs in various polymer production processes, alternatives to conventional solvents are being sought. Of course, alternatives may not be necessary if one is working with water-soluble polymerization systems. However, it is worth considering whether hazardous wastewaters are produced in such processes. Some important environmentally-friendly alternative solvents are discussed below.

Monoterpenes (MTs) are a class of terpenes and have the molecular formula C10H16. Terpenes are a broad class of molecules found in nature and their structure may be cyclic or acyclic with functional groups such as alcohols, aldehydes, ketones, esters, and carboxylic acids. MTs have several similarities with petroleum solvents based on available data for dielectric constants and densities, and they resemble aromatic solvents, such as toluene and xylene.[154] Therefore, they have the potential to act as solvents for a broad range of monomers and catalysts and have the ability to replace non-renewable solvents in some polymerization processes. The substitution of petroleum-based solvents such as toluene and methylene chloride with MTs has been reported for ring-opening metathesis polymerization (ROMP),[155] and for the synthesis of a wide variety of polyesters, polyethers, polyamides, and unsaturated polyolefins.[156-158] MTs have also been utilized for the production of hyperbranched polymers in a one-pot process that provides an economic alternative to multi-step dendrimer synthesis.[159] The prepolymers produced using MTs show good physical properties compared to the network produced in the presence of toluene. However, in the presence of MTs, chain transfer reactions were observed which limit the formation of growing insoluble networks; which can be important in some processes.[154]

Ionic liquids (ILs) are organic salts composed of ions in liquid form, close to room temperature.[160] In the last decade, ILs have been considered the “green” media of the future[161-163] and have shown potential as non-volatile organic solvents for polymerizations because of their near-zero vapor pressure, non-flammability, and ease of production.[164] The term “IL” also includes neoteric solvents, ionic fluids, liquid organic salts, and molten salts.[165-168] ILs are relatively non-volatile, therefore they do not produce VOCs.[169-172] They show good thermal stability over a wide temperature range, and can thus be utilized at higher temperatures (e.g., 800 °C).[169, 173-175] Being both polar and non-coordinating solvents, ILs exhibit good solvent properties for a wide range of monomers in different chemical processes.[172, 174] In addition, they have the ability to interact through hydrogen bonding, and dipolar and electrostatic interactions.[173, 174] Their highly ionic character improves the reaction rates in various reactions such as microwave-assisted organic synthesis and polymerizations.[176]

The use of ILs as a polymerization solvent has been reported for polycondensations, free-radical polymerizations, and ionic (anionic and cationic) polymerizations, where they were mostly used in the synthesis of polyamides, polyimides, and polyesters.[177, 178] In some cases, it was observed that while ILs were used as solvents they may act as catalysts as well; e.g., in the polycondensation of phenol and formaldehyde.[179] Another major advantage of using ILs is the absence of enzyme deactivation, which usually takes place in most polar solvents (e.g., methanol).[180] Free-radical polymerizations were also performed in ILs and interestingly, the molecular weights and rates of reactions were reported to be higher compared to that in organic solvents.[174-176, 181, 182] The rate constants of propagation tend to be higher and the rates of termination lower than in bulk or conventional solution polymerization. This phenomenon has special significance for CRP, especially atom transfer radical polymerization.[183]

Despite numerous advantages in using ILs, they exhibit some technical challenges such as high viscosity. In addition, they are sometimes sensitive to moisture, which is not typically desirable in many chemical syntheses. Furthermore, their toxicity has not been fully studied.[21, 184]

Supercritical fluids like supercritical carbon dioxide are a small class of solvents which have been employed in polymerization reactions. Supercritical CO2 has been extensively studied for polymer production,[185] has interesting physical properties, and poses few health hazards.[186] Supercritical fluids can have gas-like diffusivities which are very beneficial for reactions and have liquid-like densities which allow solvation of many chemicals.[185] In addition, they show changes in solvent density with small changes in temperature or pressure. Of course, CO2 is widely available and is usually released in very large quantities as a by-product of other processes. In general, using supercritical CO2 can be beneficial because of the fact that it is inexpensive and non-toxic.[185] However, there are certain constraints for its use as a solvent in polymer synthesis. Contrary to its outstanding solvent properties for small molecules, it is a poor solvent for most high molecular weight polymers under mild conditions (i.e., <100 °C and/or <350 bar).[187] For instance, poly(methyl acrylate) requires 2 000 bar pressure at 100 °C for a 105 g · mol−1 polymer to dissolve in CO2. These operating conditions are not cost effective. The only polymers that exhibit considerable solubility in CO2 under mild operating conditions are non-crystalline fluoropolymers and silicones.[188-193]

Recently, fatty acid methyl esters (FAME or biodiesel) produced from canola oil has been used as a polymerization solvent, where solution polymerizations of four commercially important monomers [i.e., MMA, styrene, butyl acrylate (BA), and VAc] were studied at 60 and 120 °C.[9, 194] In the 1960 s, the use of methyl oleate, a major component of biodiesel, as a polymerization solvent was explored.[195] However, the high cost of the pure compound prevented its widespread application. The solvating ability of biodiesel has not been widely studied.[196] Biodiesel is more commonly used as an alternative to petroleum diesel for use in combustion engines. It is a fatty acid alkyl ester (FAAE) produced via catalytic transesterification of vegetable oils, animal fats, or grease with an alcohol.[197] Biodiesel fulfills the requirements of a good polymerization solvent; it is environmentally benign and has low volatility, low viscosity, and good solubility. In addition, its elevated boiling point (e.g., 326 °C for canola-based biodiesel) points to reduced workplace hazards; reactions can be carried out at elevated temperatures without excessive pressure buildup, aside from the contributions of the monomers.[9] There has been mounting interest in carrying out polymerizations at elevated temperatures.[198] Advantages of using high temperatures include decreasing the required concentration of chain transfer agents and initiators, and increasing the reaction rate. However, there are disadvantages inherent to running reactions at elevated temperatures such as increased energy consumption, safety considerations, and possible undesired side reactions that may occur, i.e., intramolecular chain transfer and depropagation.[199] One drawback of using biodiesel as a high-boiling solvent in solution polymerizations is the difficulty in solvent removal from the final products. In contrast, for polymerizations in traditional low-boiling solvents, the solvent can be easily removed from the polymer product by an evaporation process.

Nowadays, most chemical process industries try to avoid using harmful solvents, separation agents, or other auxiliary chemicals to achieve more environmentally friendly processes. This is also of great concern to the polymer industry and presents an area of high interest.

1.9 Increase energy efficiency: run chemical reactions at ambient temperature and pressure whenever possible

Polymerization reactions often proceed under conditions of mass self-heating because of the exothermic nature of most polymeric reactions.[200] The non-isothermal nature of polymerization reactions has long been considered undesirable as it can lead to inconsistent polymer properties and involves challenges in controlling the reaction temperature. However, in recent years, heat generation has been considered a favorable aspect to be exploited in polymerization, especially in light of efforts to minimize the amount of energy needed to run the reaction. These polymerization reactions can be made to proceed in an almost self-propagating mode and furthermore, the heat generated by the polymerization could be used to preheat various input streams. In other words, there is a case to be made for the use of adiabatic polymerization conditions.[201-203]

To explain the effects of temperature on chain growth polymerization, e.g., it is useful to consider the general polymerization rate equation:[135]:

  • display math(1)

where [I] is the concentration of initiator or catalyst, [M] the total monomer concentration, f the initiator efficiency, and kp, kt, and kd are the propagation, termination, and decomposition rate parameters

According to the polymerization rate equation (see Equation (1)), the effect of an increase in temperature can be estimated by the change in the quantity kp(kd/kt)0.5. Based on the fact that the kinetic rate parameters follow the Arrhenius equation, the activation energy of the polymerization, ER, which is a function of activation energy for propagation, EP, activation energy for initiator decomposition, Ed, and activation energy for bimolecular termination, Et, tends to be high at elevated temperatures largely due to Ed.[135] This indicates that the rate of polymerization increases strongly with temperature. In bulk or solution chain growth polymerization, the average polymer chain length decreases significantly with increasing temperature. This occurs when the initiation and bimolecular termination steps dominate over other molecular mass development steps. When uni-molecular termination (chain transfer to small molecules) dominates the molecular mass development, EP is found to be less than Ef (activation energy for chain transfer reaction), implying that the molecular mass can decrease with an increase in temperature. Adjusting the chain transfer agents or using a mixture of initiators with different kd values can assist with maintaining a constant concentration of free radicals, which results in a constant polymerization rate. This ultimately would lead to a relatively uniform molecular weight distribution.[204] Thus, in the case of an adiabatic polymerization, one could potentially manipulate initiator and chain transfer agent concentrations to compensate for the effect of increasing temperature and therefore control the polymer properties.

Photopolymerization reactions have major capabilities compared to more traditional polymerizations and have found application in many conventional products for over three decades.[205] One of their most outstanding characteristics is the ability of photopolymers to cure rapidly at ambient conditions.[206, 207] Because of the low reaction temperature, fewer side reactions (e.g., chain transfer) are likely. In addition, the polymerization of monomers with low ceiling temperatures (e.g., α-methylstyrene) or polymerization in the presence of temperature-sensitive situations (e.g., in biomedical and dental applications) are made possible. Photopolymerization has been used in a wide range of applications including dentistry,[208-210] contact and other lenses,[211] coatings,[212, 213] photolithography,[214, 215] tissue engineering matrices,[216, 217] and 3D prototyping.[218, 219] However, there are problems related to volume shrinkage and stress, oxygen inhibition, and the presence of unreacted monomer which can limit the utilization of this method.[205, 220-222] The issue arises from the fact that, in most photopolymerization reactions, polymer formation is associated with a dramatic material property change;[223] i.e., an initial liquid mixture with low viscosity rapidly gets converted into a glassy polymer. Several recent photopolymerization examples indicate that this is an area of growth.[224-228]

Another area of potential energy efficiency lies in ambient temperature reactions such as ambient reversible addition fragmentation chain transfer radical (RAFT) polymerization, which is a highly versatile form of CRP.[229-232] This technique has been investigated for the polymerization of many monomers that are able to polymerize under conventional free radical polymerization conditions, providing outstanding control over their molecular weights; this results in the synthesis of polymers with complex architectures.[233-237] One should note, however, that the presence of dithioester groups in the RAFT agents often lead to polymers with undesirable color and odor.[238]

Microwave assisted polymerization (MAP) offers many interesting advantages that begin with the possibility of safe and easy polymer synthesis at elevated temperatures (e.g., 200–300 °C). It follows that one can expect shorter reaction times, higher monomer conversions, more efficient (i.e., even) heating, and straightforward scale-up using MAP.[239] Interesting examples in emulsion polymerization have been reported for styrene,[240] MMA,[241, 242] butyl methacrylate,[243] and many others.[244] Examples over a broader spectrum including synthesis, crosslinking, and processing have also been reviewed.[245, 246]

Microwave heating is perceived to be highly energy efficient though there has been conflicting evidence regarding this efficiency. A review of several published works drew the conclusion that when considering the energy profile of an entire process, greater overall energy efficiency can be achieved through microwave heating in comparison to conventionally heated reactions.[247] This is primarily due to the ability to shorten reaction times by conducting reactions at elevated temperatures and also by considering the higher reaction yields and reduced energy needs for separation equipment and mixing. It should be noted that these efficiencies were realized primarily at the larger reactor scale.

The search for energy efficient and cost-effective polymerization techniques has become more critical of late due to escalating energy costs. Most of the techniques used for increasing the energy efficiency of polymerization reactions are either in terms of running polymerizations adiabatically or implementing photopolymerization, in which UV rays are used to initiate the reaction. There is ample room to exploit adiabatic polymerizations, particularly from the point of view of polymer property control, though it is a mature technology. Photopolymerization also appears to offer fertile ground for greater development.

1.10 Design chemicals and products to degrade after use: design chemical products to break down into innocuous substances after use so that they do not accumulate in the environment

Polymers are employed in a multitude of applications, in some cases, due to their lower weight, flexibility, durability, and lifespan, ease of processing and lower cost. In addition, the physical and mechanical properties of polymeric materials often can be easily manipulated. As a result, many polymeric materials have replaced traditional paper-, metal-, and wood-based materials. Despite their many advantages, most polymeric materials have extremely long persistence times which often go well beyond the intended practical lifespan of the material (e.g., packaging materials, disposable cutlery, and cups). This can result either in limiting the lifetime of landfill sites or, in the case of incineration, the release of greenhouse gases (or worse) into the environment. To further illustrate, polyolefins such as PE and poly(propylene) are produced at a rate of 200 000 000 metric tonnes annually with PE alone being produced in amounts exceeding 80 000 000 metric tonnes per year.[248] Since these polymers remain in the environment for years without any significant decomposition of the polymer, there is an illustrated need for the production of degradable polymers.

It is worth noting some common confusion in terminology: biodegradable polymers are not necessarily biopolymers, which are made from renewable raw materials. Biodegradability does not imply anything about the raw materials used to produce the polymers. Instead, biodegradability refers to the polymer's ability to breakdown into energy and more elementary components such as carbon dioxide, methane, water through the actions of microorganisms such as yeasts, bacteria, and algae within a reasonable period of time.[249, 250] Biodegradation usually takes place via two pathways, aerobically, in which organic matter is converted into CO2, energy, water, etc., by microorganisms in the presence of oxygen or anaerobically, in which organic matter is metabolized by microorganisms in the absence of oxygen.[251] Thus, biodegradability depends on the chemical composition of the polymer and biodegradable polymers can be made from either renewable raw materials or fossil-fuel based feedstocks.

A second important issue revolves around the mechanism of polymer degradation. One may mechanically degrade a polymer whereas biodegradation is a certified performance characteristic as can be found in ISO 17088, EN 13432 standards in Europe and the ASTM D 6400 standard in North America. For example, in order to be compliant with EN 13432, a polymer must be converted to carbon dioxide by over 90% within 180 d under certain conditions of humidity, temperature, and oxygen level.[252]

Biodegradable polymers can be obtained from both renewable as well as non-renewable raw materials. Major non-renewable and fossil-based monomers leading to biodegradable polymers are: butanediol, dicarboxylic acids, adipic acid, terephthalic acid, and succinic acid.[252] Among the bio-sourced biodegradable polymers, PLA is a commercial polymer derived from lactic acid which is also highly biocompatible.[57-60, 250-254] PLA which can be produced from sugar and starch over a wide range of molecular weights has been recently used in various applications such as packaging and rigid thermoforms.[61, 62] This renewable polymer possesses chemical and physical properties which make it a suitable replacement for widely used polymers such as PET and PS in the packaging industry.[255]

Although PLA has many interesting properties, it has some drawbacks which limit its application. For example, compared to PS or PET it does not show good mechanical strength and barrier properties at higher temperatures.[61, 82, 255] Many researchers have been focusing on resolving these problems by different methods including, most notably, copolymerizations, reactive extrusion, and blending.[256, 257]

Another class of biodegradable polymers are polyhydroxyalkanoates (PHAs). These polymers, which have structural similarities to PLA, are also produced from renewable feedstocks like sugar and starch and polyesters which differ in chain lengths and in the position of their hydroxyl groups.[258] This class of polymers with different chemical structures has a wide range of chemical and physical properties and has applications in biomedical and non-biomedical industries such as in biodegradable films.[259-261] However, polyesters are prone to chemical hydrolysis, and in many cases, suitable co-monomers should be involved in their syntheses in order to achieve desirable mechanical properties and shelf lives.[262]

Another class of widely available, renewable, biodegradable, and inexpensive starting materials is starch. Starch is found as discrete particles (starch granules) in plants. Pure starch is a very water and temperature sensitive polysaccharide compound and has very limited applications in the plastic and fiber industries. In order to modify the hydrophobicity of starch and improve its stability against solvents, cross-linking of polymer chains and partial substitution of hydroxyl groups has been studied.[263] Nonetheless, these methods can increase the cost of production and lead to a non-biodegradable product.[252]

Other polymers that are designed to be biodegradable include poly(aspartate) (synthesized by the polycondensation reaction of aspartic acid via poly(succinimide), followed by hydrolysis), and poly(ethylene glycol) (a water soluble and completely biodegradable polymer at lower molecular weights).[264-268]

Currently, biodegradable polymers are largely regarded as specialty plastics for selected applications where biodegradability adds value. Extensive research continues to be performed on the development of many more biodegradable polymers such as poly(aspartate), poly(ethylene glycol), and PVA.[269, 270]

1.11 Analyze in real time to prevent pollution: include in-process real-time monitoring and control during syntheses to minimize or eliminate the formation of by-products

Real-time polymerization monitoring techniques are important for the synthesis of polymers with pre-specified properties and for the prevention of waste. These methods improve and/or enable process control procedures and prevent accidents such as runaway reactions, which ultimately serves to protect operators and equipment (see Section 12). Most real-time methods are cost-effective, non-destructive and have little if any negative effect on the environment. During the last two decades, there has been significant growth in sensor technologies and methods for monitoring polymerizations.[271-273] The perfect reaction monitoring tool should be cost effective, easy-to-install, explosion-proof, and preferably calibration and maintenance free. However, until now, only pressure and temperature sensors have come close to meeting these ideal conditions.[273] Nevertheless, pressure and temperature sensors only measure the state of the reactor and do not directly monitor important, ongoing reaction changes like composition of the reaction mixture. Some common monitoring techniques are described below:

1.11.1 Reaction Calorimetry

Most chemical reactions are either exothermic or endothermic, in polymerization usually conversion of monomer to polymer generates heat which can be measured to monitor polymerization.[201, 202, 274-276] One well-known method for controlling and monitoring polymerizations is heat balance calorimetry. In this method, which has been used industrially for some time, the energy balance of the jacket cooling medium is measured.[277] However, several factors can affect the heat balance calculation, which can compromise the results, such as heat loss by radiation, sampling from the reactor and heat dissipation as a result of mixing in reaction mediums with high viscosity. In addition to the above possible sources of error in the calorimetric method, this technique has shortcomings when it comes to systems with several monomers and variables; this is the case in copolymerization systems. Therefore, supplementary analyses and data collection about specific monomers should be obtained via other methods for proper reaction monitoring.

1.11.2 Gas Chromatography (GC)

This method is the most widely used on-line monitoring technique for chemical reactions in the petrochemical industry[278] and for organic syntheses[279] due to its wealth of qualitative and quantitative information.[280] The sample from the reaction mixture should be fed into the GC instrument for analysis via a circulation loop or a transfer line. However, there is usually a time-lag between the sampling time and the analytical results. Therefore, in cases with long delays, this method can be considered an off-line monitoring technique rather than a real-time one. Another drawback of this method for monitoring polymerizations is that the highly viscous or solid polymer product can clog transfer lines, valves, and columns and may require additional dilution. Nonetheless, GC is a common method for monitoring residual monomer and VOCs in polymerization mixtures.[273]

1.11.3 Optical Spectroscopy

The composition of the reaction mixture and the progress of polymerizations can be monitored using different optical methods via in-line or fast off-line analysis. Spectroscopic techniques have many advantages over some other monitoring techniques such as wet chemistry or GC as they usually do not require any additional chemicals or sample pre-processing (e.g., dilution). They are also non-destructive and prevent waste formation. Therefore, these robust techniques enable the fast real-time analysis of samples without the need to extract the samples from the reaction vessel. One concern about monitoring polymerizations with optical sensors is the formation of a polymer film on the optic probe[281] which might be resolved by cleaning the sensor with by-passes, process feeds (e.g., solvents and monomers), or extraction fixtures.[273] Monitoring high pressure polymerization systems is also a significant challenge. Reactors are often cleaned by means of high-pressure jets of cleaning solutions and a direct hit of a sensor probe by this jet might lead to permanent damage to the sensor.[273] Brief descriptions of some common spectroscopic techniques are presented below. More details about on-line sensors can be found in a recent review paper.[271]

1.11.4 Infrared (IR) Spectroscopy

IR spectroscopy is a popular analytical method for preparative and analytical chemistry which provides structural and kinetic information in a non-destructive and waste-free way.[282] The IR band is divided into three regions: near-, mid-, and far-IR. The near-infrared (NIR) spectrum extends from 13 000 to 4 000 cm−1 and it provides information on overtones or a combination of the fundamental stretching bands occurring from 3 000 to 1 700 cm−1. On the other hand, the mid-infrared (MIR) spectrum extending from 4 000 to 400 cm−1 gives information on fundamental molecular vibrations and usually is the preferred choice owing to the unmatched wealth of molecular information contained in this portion of the electromagnetic spectrum.[274] In situ Fourier transform infrared (FTIR) spectroscopy which simultaneously collects spectral data in a wide spectral range, is a modern polymerization monitoring technique that is well suited for obtaining real-time structural and kinetic information. The rapid-scanning capability of FTIR spectroscopy enables monitoring of the time-dependent intensity changes of absorbance of the molecules throughout the polymerization.[283] In order to monitor water-rich reactions, internal reflection spectroscopy (often called attenuated total reflectance or ATR) can be used. Real-time monitoring of conversion of reaction and composition are possible using ATR-FTIR spectroscopy.[271, 281, 284, 285]

1.11.5 Raman spectroscopy

Similar to IR spectroscopy, the molecular structure, and properties of reaction components can be analyzed using Raman spectroscopy based on their vibrational transitions. However, most Raman monitoring techniques use fiber-optic probes[286] which employ a single frequency of radiation to irradiate the sample, in contrast to IR spectroscopy which uses a range of frequencies.[287] This method is suitable for monitoring high water content reactions such as emulsion and suspension polymerizations and is very suitable for monitoring the signal from carbon–carbon double and triple bonds.[273] It is worth mentioning that Raman spectroscopy can be strongly affected by properties of the medium such as turbidity.[288]

1.11.6 UV/Vis Spectroscopy

UV spectroscopy is a well-known analytical technique which is quite fast and requires only a small amount of sample. Therefore, it is a suitable method for on-line monitoring. The UV–Vis part of the spectrum, extending from 200 to 800 nm, corresponds to the excitation of the outer electrons of a molecule.[271] However, only vinyl monomers show adsorption in this spectral range.[289] Successful monitoring of MMA and styrene polymerization was performed via this technique.[273]

1.11.7 Ultrasound

Ultrasonic methods are cost-effective, non-invasive real-time monitoring techniques. Certain properties of a medium such as velocity, viscosity, and elasticity can be measured utilizing ultrasonic waves.[290] Using semi-empirical models, properties like sound velocity can be related to the conversion of polymerization.[291] However, this technique has certain disadvantages, whereas acoustic properties of the material and accurate models are required in order to perform reliable data analysis.[292]

Real time monitoring of reactions is expected to improve significantly by implementation of new technologies in the future and there are several advanced techniques such as acoustic emission, online NMR spectroscopy, ion mobility spectroscopy, and process tomography that are under study[273] and can improve online monitoring of polymerization and prevent the formation of by-products and pollution.

1.12 Minimize the potential for accidents: design chemicals, and their forms (solid, liquid, or gas) to minimize the potential for chemical accidents including explosions, fire, and release to the environment

The potential for accidents in the polymer industry is large due to the variety and volume of materials used as well as the presence of these materials in different states (i.e., gas, liquid, or solid). As discussed earlier, many polymerizations are conducted under fairly aggressive conditions (i.e., elevated temperatures and pressures) and many are quite exothermic, posing the threat of thermal runaway. The potential for accidents can be considered from two perspectives: (i) during the synthesis and processing stages of polymer production and (ii) accidents occurring when using the final polymer product. The latter relates primarily to the release of hazardous materials in the event of a fire. It is often mitigated by the use of fire retardants and oxygen scavengers which serve to prevent degradation and release of harmful materials to the environment.[293-295] The use of environmentally benign additives was discussed earlier in Section 2. The focus of the present discussion is on minimizing incidents during polymer synthesis and processing.

A principal reason for accidents in polymer synthesis relates to the transport and handling of chemicals. These chemicals, particularly monomers and initiators, are fairly reactive and usually have the potential to get triggered by external factors, and cause damage to the workplace and harm to workers. One significant incident occurred in the Scottish seaport of Grangemouth, in 2006 where 24 000 L of divinyl benzene (DVB) contained in a tanker exposed to sunlight on the docks self-polymerized.[296] DVB is commonly used as a cross-linker for the production of ion exchange resins. After the accident, firefighters sealed off the area within a 500 m range and residents were forced to remain in their homes for 24 h because of the irritating characteristics of DVB towards skin and eyes. The resulting vapor cloud was eventually completely dispersed by the wind.[296]

Conditions in handling and storage of monomers, most notably temperature, are critical factors that influence the self-polymerization of monomers during transport. In most cases, inhibitors are added to the monomer. For example, 4-tert-butylcatechol (TBC) is added at a level of 900–1 200 ppm by weight to inhibit the self-initiated polymerization of DVB. However, it was noted that the addition of TBC alone was not sufficient to stabilize DVB in the Grangemouth incident and temperature and the level of oxygen in the tank also were very important factors. Obviously, each particular scenario poses its own hazards but temperature and the presence of radical scavengers do play important roles.

In addition to handling and storage, it should be noted that most polymerizations are exothermic reactions. Whenever cooling fails, uncontrolled runaway reaction may take place, leading to increased temperatures and a potential chain of events leading to disaster. As a result of reaction runaway, increase in vapor pressures and side reactions may occur. In these cases, appropriate process control techniques should be implemented.[293] This could include manipulating not only the heat transfer equipment but the inputs of the various reaction ingredients, and in particular the initiator. Several process control methods and equipment and safety devices exist to control the polymerizations such as the simple use of pressure relief valves or emergency cooling systems.

Other considerations include: the implementation of polymerization techniques where the medium acts as a heat sink (e.g., solution, emulsion); fouling, which can in the short and long term, change the heat transfer characteristics of the reactor; mixing which plays a role both in heat transfer and fouling; and regular equipment maintenance and inspection.

In the end, it is important to consider the associated risks in the synthesis, processing, and use of the polymer products from a “cradle to grave” perspective. A study combining classical risk assessment methods with statistics on technological disasters, accidents, and work-related illnesses was carried out to assess the associated risks of certain polymers, such as poly(trimethylene terephthalate) (PTT), PHA, PET, and PE.[297] The total risk to human health throughout a chemical's life cycle was estimated by summing up the following risks:[297]

  1. External risks due to the release of emissions from regular operation;
  2. External risks due to technological disasters;
  3. Risks of work-related accidents;
  4. Risks of work-related illnesses.

Using this risk assessment strategy, estimates for the total number of years of life lost (YOLL) per unit of product throughout the process chain were calculated as can be seen in Table 1 for PTT, PHA, PET, and PE.[297] For PTT synthesis, given the possibility of both petrochemical and bio-based production routes, it is possible to compare these routes for their risk to human health. On the other hand, PET and PE are fossil-based polymers and can be compared to the alternative PHA which is produced from bio-based feedstocks. Based on risk assessment results presented in Table 2, the risks to human health of bio-based polymers are significantly lower than those of the petrochemical polymers.

Table 2. Risk Assessment for PTT, PHA, PET, and PE[297]
ProductRisk (YOLL/Ton product)
Poly(trimethylene terephthalate) (PTT)From ethylene oxide0.003821
 From acrolein0.0043031
 Via anaerobic fermentation of dextrose0.003001
 Via anaerobic fermentation of glycerol0.003227
 Via aerobic fermentation0.003039
Polyhydroxyalkanoates (PHA)Via fermentation0.002501
Poly(ethylene terephthalate) (PET) 0.003211
Poly(ethylene) (PE) 0.004949

Accidents related to the polymerization process are largely due to the handling of several raw materials used in the process such as monomers, solvents, and initiators as well as the highly exothermic nature of the polymerizations. These issues can be mostly resolved by implementing proper environmental and advanced process controls. In addition, substitution of petrochemical feedstocks with bio-based feedstocks may also contribute to decreasing the potential for accidents.

2 Conclusion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Conclusion
  5. Acknowledgments
  6. Biographies

As an economically important and technologically vast and interesting field, polymer reaction engineering is a prime target for transformation towards greater sustainability. There is not a single, unique approach for the transformation of polymerization processes into more sustainable ones, but rather a coordination of several steps can result in an environmentally friendly polymerization process. This transformation is being made possible via the 12 principles of green chemistry:[1]

  1. Prevent waste;
  2. Design safer chemicals and products;
  3. Design less hazardous chemical syntheses;
  4. Use renewable feedstock;
  5. Use catalysts not stoichiometric reagents;
  6. Avoid chemical derivatives;
  7. Maximize atom economy;
  8. Use safer solvents and reaction conditions;
  9. Increase energy efficiency;
  10. Design for degradation after use;
  11. Analyze in real time to prevent pollution; and
  12. Minimize the potential for accidents.

As noted, while there is always room for improvement, several of the principles are essentially being achieved, for the most part, in modern-day polymer production; these include principles 5, 6, 7, 11, and 12. Principles 1, 9, and 10 can be considered as being well underway but still showing promise for important developments. For example, in terms of principle 9, exploiting photopolymerization may require further advances in catalysis while the use of adiabatic polymerization techniques implies a need for improved polymer property modeling, monitoring, and control. Another example, in terms of principle 10, lies in the ongoing development of biodegradable polymers.

One can argue that those principles seeming to have the greatest potential for significant environmental benefit are principles 2, 3, 4, and 8. Principle 2, concerns the design of safer chemicals and products. The replacement of toxic additives with non-toxic alternatives and the reduction in VOCs (e.g., from solvents) in polymer products appear to offer many challenges and significant room for improvement. Designing less hazardous chemical syntheses (Principle 3) offers enormous opportunities from the replacement of solvents to the use of less toxic monomers in polymer production. The use of renewable feedstocks (Principle 4) is a highly active area of research, despite limitations to the supply of these feedstocks. Achieving Principle 8, the use of safer solvents and reaction conditions, also appears to have elicited significant research effort.

Ultimately, keeping an eye towards all 12 green chemistry principles should become standard practice for all polymer scientists and engineers. In this way, the inevitable and necessary transformation of polymer production towards a more sustainable future will be greatly facilitated.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Conclusion
  5. Acknowledgments
  6. Biographies

The authors gratefully acknowledge the support of the Natural Sciences and Engineering Research Council (NSERC) of Canada. Abebe Essayas and Zahra Dastjerdi are also acknowledged for their assistance in the collection of literature data for this paper.

Biographies

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Conclusion
  5. Acknowledgments
  6. Biographies
  • Image of creator

    Marc A. Dubé has over 25 years experience in polymer reaction engineering as well as several years in biodiesel production technology. His expertise is focused on the study of reaction kinetics, polymer characterization and the use of on-line sensors for coatings, resins and adhesive applications. Most recently, his work has focused on sustainable practices in polymer production. He has served as Chair of the Department of Chemical and Biological Engineering and is a founding member of the Centre for Catalysis Research and Innovation (CCRI) at the University of Ottawa in Canada. He is both a Fellow of the Chemical Institute of Canada (FCIC) and a Fellow of the Engineering Institute of Canada (FEIC).

  • Image of creator

    Somaieh Salehpour obtained her Ph.D. degree and M.A.Sc degree in Chemical Engineering at the University of Ottawa in Canada under the supervision of Dr. Marc A. Dubé. Her research work focused on polymer syntheses using renewable and environmentally friendly monomers and solvents. Currently, she is a Senior Research Engineer at EcoSynthetixInc, where her work focuses on development and manufacturing of wide range of new environmentally friendly products with enhanced performance, cost benefits and an improved environmental footprint when compared to the petroleum-based products they replace.