SEARCH

SEARCH BY CITATION

Keywords:

  • natural resources;
  • renewable polymers;
  • rosin;
  • terpene;
  • terpenoids

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Terpenes
  5. 3. Terpenoids
  6. 4. Copolymers of Terpenes, Terpenoids, and Other Monomers
  7. 5. Polymers from Rosin
  8. 6. Conclusions
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information
  12. Biographical Information

The development of sustainable renewable polymers from natural resources has increasingly gained attention from scientists, engineers as well as the general public and government agencies. This review covers recent progress in the field of renewable bio-based monomers and polymers from natural resources: terpenes, terpenoids, and rosin, which are a class of hydrocarbon-rich biomass with abundance and low cost, holding much potential for utilization as organic feedstocks for green plastics and composites. This review details polymerization and copolymerization of terpenes such as pinene, limonene, and myrcene and their derivatives, terpenoids including carvone and menthol, and rosin-derived monomers. The future direction on the utilization of these natural resources is discussed.


1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Terpenes
  5. 3. Terpenoids
  6. 4. Copolymers of Terpenes, Terpenoids, and Other Monomers
  7. 5. Polymers from Rosin
  8. 6. Conclusions
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information
  12. Biographical Information

One of the most pressing issues for future generations is the development of a sustainable society. Fossil fuel used for both energy production and plastic manufacturing has finite availability. Within the next century, it will be nearly depleted. Approximately 7% of the global production of fossil fuel goes into the synthesis of plastic materials.1–13 In the United States alone, approximately 13% of fossil fuel consumption goes toward nonfuel chemical production. Global energy demands are expected to increase in the coming decades, further increasing the price of petroleum and other nonrenewable resources as the supply struggles to meet the demand. Although there is an increased investment in finding nonrenewable energy sources for global transportation and heating, the chemical industry should not be neglected. In addition to the economic influence, the undesirable environmental impact by nonrenewable resources has contributed to the rebirth of renewable resource alternatives. Burning fossil fuel has led to increased greenhouse gas emissions, reduced air quality, and global warming.14 Most plastics derived from nonrenewable resources have led to water and land pollution due to their inability to undergo biodegradation.

The environmental concerns, along with depleting oil reserves, have led to an increased interest in the development of green plastics derived from renewable natural resources.3, 6, 7, 10–12, 15–29 The large-scale production of green plastics primarily depends on the integration of biorefineries.8, 9, 30 A biorefinery, as defined by the National Renewable Energy Laboratory, is a facility that integrates biomass conversion processes and equipment to produce fuels, power, and chemicals from biomass and is analogous to today's petrochemical refineries.31 The importance of these refineries to develop green plastics, without impacting the food and feed production in a negative manner, is essential for future growth.

Green plastics can be classified into three primary categories. The first class of natural resources is natural polymers including lignin, cellulose, hemicellulose, polysaccharide, and chitin.22, 32–43 Many of these biopolymers display excellent biocompatibility and biodegradability. These natural polymers have long been exploited without any modifications. Currently, common approaches involve physical blending and limited chemical modifications. However the ill-defined already-built-in macromolecular structures could be further manipulated by implementing advanced polymerization techniques and potentially serve as a building block toward diverse polymeric architectures and therefore rich properties.44 The second class of renewable polymers can be obtained in nature by microorganism fermentation of sugar or lipids. These polymers include polyhydroxyalkanoates (PHAs) such as poly(hydroxybutyric acid).27, 45 The third category of green plastics pertains to the use of small molecular biomass that can be derivatized and further polymerized.7, 15, 18, 19, 46–51 Vegetable oils, fatty acids, and lactic acids are a class of small molecular biomass, which are usually obtained directly from forestry and agriculture products or by microorganism fermentation. These materials could be precisely engineered at a molecular level into renewable polymers in a way similar to some plastics derived from petroleum chemicals. For example, poly(lactic acid) has been commercially used for over 50 years. It should be worthwhile to mention that there are increased efforts to synthesize olefins such as ethylene, propylene, and isoprene using biological routes including fermentation of biomass. Mathers17 has just published an excellent review on this topic.

As one of the major classes of petroleum chemicals, cycloaliphatic, and aromatic compounds such as benzene, cyclohexane, and cyclohexene offer rigidity and hydrophobicity to polymers derived from them. There are great opportunities to develop renewable polymers from natural resources containing rich cycloaliphatic and aromatic structures.17, 51, 52

Terpenes, terpenoids, and rosin are a class of important natural molecular biomass containing cycloaliphatic and/or aromatic structures.28, 29, 53–55 Some natural resources containing them are shown in Figure 1. Particularly, they are also major components of resin, which is an exudate obtained from trees, especially pine trees and conifers. The volatile fraction of resin, turpentine, is composed of a mixture of terpenes. Most terpenes have a basic cycloaliphatic structure with isoprene elementary unit. Terpenoids can be considered as modified terpenes, wherein methyl groups have been moved or removed, or oxygen atoms added. Rosin is the nonvolatile solid form of resin and produced by heating fresh liquid resin to vaporize the volatile liquid terpene components. Rosin consists primarily of abietic- and pimaric-type resin acids (or rosin acids) with characteristic hydrophenanthrene structures and about 10% neutral materials. The hydrophenanthrene rings are considered to have cycloaliphatic and aromatic structures.

thumbnail image

Figure 1. Some examples of natural resources producing terpenes, terpenoids, and rosin.

Download figure to PowerPoint

The first comprehensive review on rosin-containing polymers dated back to more than two decades ago written by Maiti et al.53 During 2008–2011, Gandini and co-workers published numerous monographs, book chapters, and reviewer papers on rosin and terpenes.10, 11, 16, 28, 29 A book on rosin-based chemicals and polymers was released in 2012.51 To the best of our knowledge, there have been no single comprehensive review articles to combine terpenes, terpenoids, and rosin. In the last two decades, polymer science has overseen several breakthroughs in the areas of controlled polymerization techniques,44, 56–71 and high efficiently organic group functionalization such as click chemistry,72–78 which have reinvigorated new life for the use of terpenes, terpenoids, and rosin to prepare renewable polymers with advanced architectures and novel properties. The focus of this review paper is to highlight recent advances in the synthesis of terpenes, terpenoids, and rosin-based monomers and polymeric materials. There is an abundance of work in which terpenes, terpenoids, and rosin are used as additives for formulations of a variety of engineering products, however, beyond the scope of this review.

2. Terpenes

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Terpenes
  5. 3. Terpenoids
  6. 4. Copolymers of Terpenes, Terpenoids, and Other Monomers
  7. 5. Polymers from Rosin
  8. 6. Conclusions
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information
  12. Biographical Information

Terpenes are hydrocarbons that contain one or more carbon–carbon double bond and share the same elementary unit of isoprene (2-methyl-1,4-butadiene).8, 10, 11, 16, 28, 29, 51 Representative unsaturated monoterpenes, with a general formula of C10H16, are shown in Figure 2. Turpentine is by far the main source of terpenes, whose world yearly production amounts to some 350 000 tons.11 Terpenes, especially pinene and limonene, are abundant and inexpensive, making them an ideal starting material for the synthesis of new important chemicals for use as fragrances, flavors, pharmaceuticals, solvents, and also chiral intermediates.8 Even though terpenes can undergo numerous catalytic chemical processes, only a few of these molecules have been the subject of studies related to their use as polymeric starting materials. Based upon their low cost and ease of isolation, only α-pinene, β-pinene, limonene, and myrcene have been studied relatively extensively as starting materials for synthesis of polymers.

thumbnail image

Figure 2. Chemical structures of some common terpenes.

Download figure to PowerPoint

2.1. Polymers Derived from Pinene

Pinenes are the most abundant and easily isolated terpenes. Obtained by the steam-distillation of resinous sap from pine or conifer trees, pinenes can undergo a multitude of chemical reactions to produce valuable monomeric compounds. Out of these chemical reactions, isomerization is the most useful as it leads to a wide variety of products such as camphene and limonene. Figure 3 shows isomerization (some with further functionalization) of pinenes into some of the more utilized compounds that can undergo cationic polymerization.8, 29 All compounds displayed in Figure 3 have a methyl group or other electron-donating groups on the double bond. Thus, cationic polymerization has been the most suitable chain reaction tool for polymerization of these monomers. A better understanding of the cationic polymerization mechanism, along with improved reaction conditions, has led to the synthesis of homo and copolymers of pinenes. It should be mentioned that the vast majority of pinene polymerization involves with the use of β-pinene, not α-pinene, mostly due to intrinsic difference between the two pinene structures. The structure of α-pinene lacks the highly reactive exo-methylene double bond that is however present in β-pinene, thus making α-pinene highly inert toward cationic polymerization. Only oligomers of α-pinene can be obtained by cationic polymerization.79

thumbnail image

Figure 3. Various terpenes derived from pinenes by isomerization.

Download figure to PowerPoint

Some of the earliest work on the cationic polymerization of β-pinene employed a Lewis acid, aluminum chloride, or ethyl aluminum dichloride, as a catalyst. These polymerizations resulted in only low-number-average-molecular-weight polymers Mn ≤ 4000 g mol−1).79–81 The first example of the living cationic polymerization of β-pinene was reported by Sawamoto et al.82 The mechanism for this polymerization is shown in Figure 4. The addition of cationic initiator onto β-pinene yields a tertiary carbocation, which then undergoes isomerization to produce the key propagating isobutylene-like tertiary carbocation. This is very similar to the cationic polymerization of isobutene. Compared with isobutene, the reactivity of β-pinene-based cationic addition is much higher.82, 83 There are several factors to contribute to this increased reactivity, including the reactive exo-methylene double bond, the release of ring strain by opening the fused cyclobutane ring, and the formation of a stable tertiary carbocation by growing-end ring-opening isomerization. In order to achieve living conditions for cationic polymerization, a proper Lewis acid activator must be needed.84 The Lewis acid and a cationic initiator should be well suited for a particular monomer based upon the monomer's reactivity. Initiating systems composed of 1-chloro-1-(2-chloroethoxy)ethane and isopropoxytitanium trichloride in the presence of tetra-n-butylammonium chloride was used to polymerize β-pinene.82 It resulted in controllable molecular weight and low polydispersity index (PDI). When the temperature was in the range of −40 °C to −70 °C, polymers with molecular weight of 4000 g mol−1 and a PDI of 1.3 were obtained. The living nature of this polymerization also allowed for the synthesis of block copolymers of β-pinene with styrene or p-methylstyrene, which will be further discussed in “Section 4.”85

thumbnail image

Figure 4. Mechanism of the cationic polymerization of β-pinene.82

Download figure to PowerPoint

Even though homopolymers and block copolymers were prepared by living cationic polymerization, only low-molecular-weight polymers were obtained. High-molecular-weight poly(β-pinene) was achieved by Keszler and Kennedy86 and Kamigaito et al.87 using an H2O/EtAlCl2 co-catalyst system. High-molecular-weight polymers up to Mn = 40 000 g mol−1 were obtained, however temperature as low as −80 °C was necessary to achieve these high-molecular-weight polymers. In addition to cryogenic temperature, very dilute solutions and high initiator concentrations were also employed. These polymers exhibit a glass transition temperature (Tg) around 90 °C, belonging to a class of thermoplastic polymers. The low reaction temperature, dilute reaction conditions, and high initiator concentrations would limit large-scale production of poly(β-pinene).

To further optimize the polymerization conditions for preparation of high-molecular-weight β-pinene polymers, different reaction conditions and catalyst systems were explored.88, 89 Lu et al.89 synthesized a Schiff-based nickel complex catalyst (Figure 5) to produce poly(β-pinene) by cationic polymerization. Single-site late transition metal-based catalysts are more tolerant of functional monomers and much less oxophilic. Nickel- and palladium-based catalysts are often chosen for the polymerization of α-olefins. This catalyst alone did not produce any polymers. An activator, methylaluminoxane (MAO), was incorporated into this polymerization system. Using this catalytic system at 40 °C, they were able to achieve molecular weight of 11 000 g mol−1 with a PDI of 1.70. The ratio of MAO to nickel complex determined the rate of polymerization, and the molecular weight of polymers. When the ratio was too high, there was a decrease in monomer conversion, while a too low ratio resulted in a decrease in molecular weight even though a higher polymerization rate was observed. A ratio of [MAO]:[Complex] at 500:1 yielded the best results for both polymerization rate and monomer conversion. The excess amount of MAO was necessary to achieve high polymerization activity. The ability to obtain high molecular weighthigh-molecular-weight polymers at elevated temperatures, while utilizing low dose of metal-containing catalysts, would be beneficial to potential industrial production of poly(β-pinene).89

thumbnail image

Figure 5. A Schiff-based nickel complex used for cationic polymerization of β-pinene.89

Download figure to PowerPoint

The major drawbacks to the above-mentioned polymerizations are that the catalysts are not commercially available and require multiple steps to synthesize, and the fact that MAO is a rather expensive chemical. To overcome these obstacles, Kukhta et al.88 used a H2O/AlCl3•OPh2 (OPh2:diphenyl ether) coinitiator system to produce relatively high-molecular-weight poly(β-pinene) (Mn = 9000−14 000 g mol−1), which has a Tg at ≈82–91 °C. This was achieved by using a lower initiator concentration (5.5 × 10−3 M) and a lower temperature (20 °C) than the work done by Lu et al.89 The proposed mechanism for this polymerization is shown in Figure 6. There is the formation of free Lewis acid due to the dissociation of the AlCl3•OPh2 complex. The in situ generation of a weakly nucleophilic counteranion through the interaction of AlCl3OH with diphenyl ether, which leads to the suppression of chain transfer reactions, is one of the reasons that high-molecular-weight poly(β-pinene) was obtained.88 Another reason for the production of high-molecular-weight polymers is the basicity of the diphenyl ether electron-donating group. The use of a stronger base like dibutyl ether leads to β-H abstraction, yielding only low-molecular-weight polymers, which is different from cationic polymerization of styrene.90, 91 With a weaker base like diphenyl ether, this side reaction is no longer present. These friendly reaction conditions make this method of cationic polymerization very attractive for industrial utilization.

thumbnail image

Figure 6. Proposed mechanism for β-pinene polymerization using H2O/AlCl3•OPh2 initiating system.88

Download figure to PowerPoint

As mentioned above, some cationic polymerization of terpenes leads to low-molecular-weight polymers (or oligomers). These oligomers, along with other terpene monomers, can be further modified to produce epoxy resins and polyols. Wu et al.92–94 used hydrogenated terpinene-maleic anhydride to synthesize hydrogenated terpinene-maleic ester type epoxy resin. After curing, these epoxy resins, however, exhibited poor deformability and were quite brittle. To improve these properties, the epoxy resins were combined with polyurethane to create a class of crosslinked polymers. As shown in Figure 7, different polyols were prepared by ring-opening polymerization of the hydrogenated terpinene-maleic type epoxy resins in the presence of secondary amines such as diethylamine, N-methylethanolamine, and diethanolamine.92–94 These polyols were then reacted with polyisocyanate to produce crosslinked polyurethanes. The crosslinked polymers had Tg in the range of −5 to 37 °C and displayed excellent impact strength, flexibilities, and chemical-resistance and thermal-resistance properties. These new crosslinked products combined the rigidity and weatherability of the saturated terpinene alicyclic epoxy resin with the flexibility and tenacity of the polyurethane.

thumbnail image

Figure 7. Synthesis of polyols derived from hydrogenated terpinene-maleic ester type epoxy resin.92–94

Download figure to PowerPoint

2.2. Polymers Derived from Limonene/Myrcene

As shown in Figure 3, the isomerization of pinene leads to a myriad of monoterpenes. One of these isomers, myrcene, has gained interest in recent years. Hillmyer et al.95 prepared high-molecular-weight poly(3-methylenecyclopentene) using a combination of ring-closing metathesis and cationic polymerization.95 The monomer, 3-methylenecyclopentene, was produced by ring-closing metathesis of myrcene using Grubbs second-generation catalysts (G-2), as shown in Figure 8. The byproduct of this reaction is isobutene, which is the main component for the production of butyl rubber. The use of initiation system i-BuOCH(Cl)Me/ZnCl2/Et2O led to the formation of poly(3-methylenecyclopentene) with molecular weight of 22 000 g mol−1 and a PDI of 1.12. The low PDI and excellent control on the molecular weight indicated that living cationic polymerization could be achieved. End-group analysis suggested that there was a cyclopentadienyl group present at the chain end. This was further confirmed by a Diels–Alder reaction with maleic anhydride, opening the possibility for many end-functionalization reactions. The final polymers were hydrogenated by using p-toluenesulfonylhydrazide and the thermal properties of these two polymers were compared. The hydrogenated polymer exhibited a Tg of −28 °C and a melting point (Tm) of 106 °C, while the non-hydrogenated polymer displayed a Tg of 11 °C and a Tm of 65 °C and 105 °C. The presence of both Tg and Tm provided evidence that both polymers are semicrystalline.

thumbnail image

Figure 8. Ring-closing metathesis of myrcene and cationic polymerization of 3-methylenecyclopente.95

Download figure to PowerPoint

Another interesting terpene is limonene. Although limonene can be obtained from the isomerization of pinene; it is naturally produced by more than 300 plants.96–98 Limonene is a chiral molecule. The (R)-enantiomer consists of 90–96% of citrus peel oil and its worldwide production exceeds 70 000 tons per year.98 The primary use for limonene is in flavor and fragrance industry with little emphasis on its polymeric potential.99 As mentioned above, cationic polymerization has been utilized to produce pinene-based homopolymers. However, the restriction on the limited number of monomers that can undergo cationic polymerization has directed researchers to incorporate other polymerization methods to produce terpene-based homopolymers.98, 100 Condensation polymerization is one of the oldest and most exploited polymerization techniques. Terpenes lack the necessary functional groups to participate in condensation polymerization, however they contain one or more unsaturated C5C bond. These bonds can be converted to desirable functional groups by thiol-ene click chemistry.

Thiol-ene click chemistry is a very useful technique to convert unsaturated C5C bonds to a wide variety of functional groups.49, 73–78, 101–104 Thiol-ene reaction has attracted significant interest in the area of polymer chemistry due to its recognition as “click chemistry.”72 The thiol-ene click reaction, as shown in Figure 9, is a radical process that produces a sulfur-carbon bond, mainly producing the anti-Markovnikov product. The reaction can be initiated by either a thermal or a UV method. It can also proceed by a catalytic process mediated with the aid of nucleophiles, acids, and bases. The thiol-ene reaction is extremely versatile, working for both internal and terminal double bonds, with terminal ones being more reactive. It also works for a multitude of thiols. One of the most attractive aspects of thiol-ene reactions is that they are, in many cases, fast, and can be performed at ambient temperature in the presence of air and water.

thumbnail image

Figure 9. Mechanism of a typical thiol-ene click reaction.

Download figure to PowerPoint

Janes et al.105 prepared terpene-based thiols by reacting hydrogen sulfide with various monoterpenes (Figure 10). Reactions of limonene, α-pinene, α- and γ-terpinene, terpinolene, car-3-ene, and pulegone with hydrogen sulphide in the presence of aluminum trichloride or tribromide at ≈0 °C yielded a mixture of thiols, and sometimes additionally bridged epi-sulphides of the monoterpenoids in yields up to 20%. The main product from reaction of limonene or α-pinene with hydrogen sulphide–aluminium trihalide was p-menth-1-en-8-thiol. The main product from pulegone was 8-mercapto-trans-p-menth-3-one, an important component of Buchu leaf oil. However, thiol-ene click reactions involved with these novel thiols have not been carried out. This may be an interesting direction worthy for further exploration.

thumbnail image

Figure 10. Preparation of terpene-based thiols by reacting hydrogen sulfide with various monoterpenes.105

Download figure to PowerPoint

There are a few examples in the literature on the use of thiol-ene click reaction involved with terpenes. Recently, Meier and co-workers98 made use of thiol-ene click chemistry to produce limonene-based homopolymers. The monomers synthesis is shown in Figure 11. Limonene has two unsaturated double bonds, one being terminal and the other being internal. Depending on the choice of thiols, monoaddition product or a mixture of monoaddition and diaddition products can be generated due to the difference in reactivity of the terminal and internal double bonds. The reaction between limonene with thioglycerol at room temperature under vacuum produced an exocyclic addition product, even with the use of excess thiols. In order to increase the overall yield, the terpene to thiol ratio was manipulated to optimize the reaction efficiency. An initial ratio of 1:1 led to less than 50% double bond conversion. Using a slight excess of thiol (1.2 mol equiv.) produced the highest yield of monosubstituted product (≈80%). The addition of 2-mercaptoethanol or methyl thioglycolate to limonene showed both exocylic and endocyclic addition to the double bonds, when excess thiols were used. It should be mentioned that the ester gave a higher yield than the alcohol. The increased reactivity of the ester was attributed to the weakening of the thiol bond by the formation of a hydrogen bonding with the carbonyl group. The addition of the second thiol followed the same reaction conditions as the first one with a similar yield.

thumbnail image

Figure 11. Thiol-ene click chemistry between limonene and thiols.98

Download figure to PowerPoint

Condensation polymerization was carried out on four different di-substituted monomers.98 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD) was chosen as a catalyst due to its high transesterification activity. Polymerization of the diester with the diol only produced oligomers, most likely due to the steric effect from the bulky cyclic structure, which may hinder the catalyst to access reacting centers. During polycondensations, little deviations from an ideal 1:1 ratio between monomers can substantially decrease molecular weights through a chain-stopper effect. The polymerization between two heterodifunctional monomers, shown in Figure 11, produced polymers with molecular weight of 8000 g mol−1 and 10 000 g mol−1 and PDI as low as 1.65. All resultant polymers exhibited Tgs around −10 °C. Even though all monomers experienced the same steric hindrance, the monomers with both an ester group and an alcohol group have the ideal 1:1 ratio of functional groups, leading to high-molecular-weight polymers.

In order to achieve high-molecular-weight polymers with either the diol or diester monomers, comonomers that provide spacing between limonene units were used.98 The comonomers, shown in Figure 12, were synthesized by either the self-metathesis or thiol-ene homocoupling of methyl-10-undecenoate or 10-undecen-1-ol, both of which are derived from castor oil. The condensation polymerization was carried out at 120 °C under vacuum using the same TBD catalyst. Higher molecular weight polymers (Mn between 9000 and 25 000 g mol−1 with PDI between 1.75 and 2.47) were obtained, probably due to the reduction of steric hindrance. The Tg of all polymers were ≈ −45 °C and the Tm varied from −15°C to 50 °C. The long alkyl chains in the backbone allowed polymers to crystallize without any interference from the bulky limonene structure. Polymers with the longer alkyl chains were observed to have higher melting temperature. Polymers that contained the sulfur atoms also had a higher melting temperature than those without sulfur. Comparing saturated to unsaturated polyesters, the unsaturated displayed higher melting temperature.

thumbnail image

Figure 12. Preparation of limonene-castor oil polymers by condensation polymerization. Reproduced with permission.98 Copyright 2011, American Chemical Society.

Download figure to PowerPoint

Another method employed in the synthesis of limonene monomers is the epoxidation of the unsaturated double bonds.96, 97 As shown in Figure 13, Coates et al.97 utilized carbon dioxide and limonene oxide to produce linear polycarbonates based upon prior success of petroleum-based cyclohexane oxide/carbon dioxide copolymerization.97 β-Diiminate zinc acetate complexes were chosen as catalysts. When the polymerization was carried out at 25 °C, only regioregular alternating polycarbonates were produced with molecular weight of 25 000 g mol−1 and a PDI of 1.16. The increase of temperature resulted in the formation of a more random copolymer of polycarbonate with a noticeable decrease of molecular weight and broadened molecular weight distribution (PDI = 1.34). Low temperature polymerization facilitated the stereo- and regioregular ring-opening of limonene. Although longer reaction time was needed to produce high-molecular-weight polymers, the prospect of creating biodegradable polycarbonates from inexpensive and renewable starting materials has a lot of promise in commercial applications.

thumbnail image

Figure 13. Copolymerization of limonene oxide and carbon dioxide. Reproduced with permission.97 Copyright 2004, American Chemical Society.

Download figure to PowerPoint

Limonene dioxide is also commercially available and used industrially as a diluent for epoxy resin and as a solvent. Mülhaupt group exploited limonene dioxide/CO2 to produce nonisocyanate polyurethanes and thermosetting polymers.96 Nonisocyanate polyurethanes are an attractive green alternative to traditional isocyanate polyurethanes due to the absences of toxic monomers and phosgene. The synthetic route to prepare nonisocyanate polyurethanes is shown in Figure 14. The catalyst used for the synthesis of limonene dicarbonate was (3-iodopropyl)-trimethoxysilan (1-butyl)-imidazol. No catalyst or solvent was added for the polymerization of limonene dicarbonate with a diamine. Using various diamines, they prepared polymers with molecular weight up to ≈ 1800 g mol−1. The polymer prepared from isophorone diamine exhibited the highest Tg at 70 °C and the highest Tm ≈90–100 °C, due to the rigid nature of the isophorone unit. The thermosetting polymers were prepared by reacting limonene dicarbonate with various tri- and polyfunctional amines. Lupasol PR, a highly branched polyamine from BASF, provided the thermosets with the highest Young's Modulus (4100 MPa). Lupasol PR contains primary, secondary, and tertiary amines, with the primary amines accounting for 17 mol% of the entire polyamine. Only the primary amines reacted with limonene dicarbonate, most likely due to the low curing temperature.

thumbnail image

Figure 14. Linear nonisocyanate polyurethanes prepared from limonene dioxide. Reproduced with permission96 from the Royal Society of Chemistry.

Download figure to PowerPoint

3. Terpenoids

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Terpenes
  5. 3. Terpenoids
  6. 4. Copolymers of Terpenes, Terpenoids, and Other Monomers
  7. 5. Polymers from Rosin
  8. 6. Conclusions
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information
  12. Biographical Information

The above-mentioned terpenes usually provide the unsaturated double bonds to produce low-molecular-weight polymers that are nondegradable. Even though the thiol-ene click reactions allow for the polymerization of terpene-based degradable polyesters, challenges still exist, such as preparation of high-molecular-weight polymers. Aliphatic polyesters are produced by either step-growth polycondensation or ring-opening polymerization (ROP) of lactones or lactides. Only relatively low-molecular-weight terpene-based polymers are produced by step-growth polycondensation. Steric hindrance and the lack of stoichiometric control are the primary reasons for the observed low molecular weight. High reaction temperature and longer reaction time are needed to achieve high-molecular-weight polymers. Unfortunately these harsh conditions lead to the formation of undesirable side products. To overcome these obstacles, ROP of lactides and lactones is often employed to produce high-molecular-weight aliphatic polyesters.8, 21, 106, 107 ROP of lactides and lactones has led to the synthesis of versatile biocompatible and biodegradable polymers, which can find a myriad of biomedical applications.6, 21, 107–109 Polymers derived from lactide, glycolide, and ϵ-caprolactone are produced commercially. A number of catalytic systems have been developed for ROP of cyclic esters and diesters.21, 68, 106, 110 Many organometallic compounds, such as oxides, carboxylates and alkoxides, are effective initiators for the controlled synthesis of polyesters. ROP can proceed by either cationic, anionic, coordination, or radical mechanism, depending on the catalytic system used. Coordination polymerization usually produces polymers with the highest molecular weight. Tin and aluminum carboxylates and alkoxides initiation systems yield polymers with low PDIs, well-defined end groups, and controlled molecular weight. In addition, metal-free organocatalysts are receiving more and more attention.68, 110

Direct ROP of the terpenes discussed thus far is impossible due to the lack of a carbonyl group. A second class of terpenes, referred to as terpenoids, possess the necessary functional groups that can be converted to a cyclic ester.21, 100, 111, 112 Carvone and menthol (Figure 15) are two common terpenoids that have been successfully derivatized and further polymerized by ROP. Carvone is a natural terpenoid found in both Mentha spicata (spearmint) and Carum carvi (caraway) oils, with an global production of nearly 10 000 metric tons per year.113 Carvone has been widely used as a flavor in food and beverages, as well as in toothpaste and mouth wash. It has also found applications as an insect repellent and antimicrobial agent.113 Menthol is found in Mentha arvenis and Mentha piperita (peppermint) oils.112, 114 Menthol is commonly associated with peppermint and the cooling sensation experienced when ingested or applied to the skin. The primary uses for menthol are in the flavoring industry, as well as in various medicinal applications.114

thumbnail image

Figure 15. Two common naturally occurring terpenoids: carvone and menthol.

Download figure to PowerPoint

Hillmyer and co-workers100, 111, 112 have carried out some pioneering work on the ROP of terpenoids. Menthol is commercially available in its ketone form, a compound known as menthone. Menthone is readily converted to a lactone monomer, menthide, by the Baeyer-Villiger oxidation using meta-chloroperoxybenzoic acid as an oxidizer (Figure 16).112 The ROP of menthide took advantage of a highly active zinc alkoxide catalyst, allowing the polymerization to run at room temperature in toluene for only 8.5 h. By adjusting the monomer to catalyst ratio, Hillmyer group synthesized polymers with molecular weight ranging from 3300 to 91 000 g mol−1 with the PDIs below 1.6. The polymerization can be performed in a controlled manner.

thumbnail image

Figure 16. Ring-opening polymerization of menthide derived from menthone.112

Download figure to PowerPoint

Based on the above menthide polymerization, Hillmyer group further proceeded to polymerize carvone in a similar fashion. Carvone is commercially available as a partially hydrogenated ketone. Using Wilkinson's catalyst, dihydrocarvone was hydrogenated and then converted to carvomenthone (Figure 17).100 To avoid epoxidation of the terminal alkene, Oxone® was chosen for the Baeyer-Villiger oxidation of dihydrocarvone and only 10% of the final product was converted to undesirable epoxide. The polymerization was carried out in bulk at 100 °C using diethyl zinc as a catalyst and benzyl alcohol as an initiator. The change of monomer to initiator ratio led to the formation of carvomenthone-based polymers with a molecular weight of 62 000 g mol−1, a PDI of 1.16 and a Tg of −27 °C and dihydrocarvone-based polymers with a molecular weight of 10 500 g mol−1, a PDI of 1.24 and a Tg of −20 °C. Copolymers of these two monomers displayed a Tg between the two homopolymers. A monomodal gel permeation chromatography (GPC) trace with a PDI between 1.1 and 1.2 confirmed the presence of a copolymer, as opposed to two independent homopolymers.

thumbnail image

Figure 17. Carvone-based lactone synthesis and their ROP.100

Download figure to PowerPoint

The polymerization of carvomenthone-based monomers displayed good control on the molecular weight up to 50 000 g mol–1. Above it, there was a significant difference between the calculated and the actual molecular weight, most likely due to trace amounts of impurities in either the monomer feed or the catalyst.100 For the dihydrocarvone-based polymers, the difference between measured and calculated molecular weight was observed for low molar mass polymers. This could also be attributed to impurities in the catalyst and the small amount of epoxidized monomer present in the initial monomer feed. In the absence of any initiator, ROP occurred at elevated temperatures possibly due to the opening of the epoxide ring, which could serve as an initiator for the ROP of dihydrocarvone. Crosslinking of the final dihydrocarvone-based polymers was made possible by the thiol-ene reaction between a dithiol and the pendant double bonds. This interesting work by Hillmyer group may open a new avenue for the production of high-molecular-weight terpenoid-based homopolymers and copolymers.

4. Copolymers of Terpenes, Terpenoids, and Other Monomers

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Terpenes
  5. 3. Terpenoids
  6. 4. Copolymers of Terpenes, Terpenoids, and Other Monomers
  7. 5. Polymers from Rosin
  8. 6. Conclusions
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information
  12. Biographical Information

One of the first block copolymers that incorporate a terpene was the block copolymers of β-pinene with styrene or p-methylstyrene, carried out by Sawamoto and co-workers.85 Since then, a number of copolymers have been produced with various terpene fractions employing a variety of polymerization methods.98, 100, 115–120 Lu et al.115–120 prepared random and block copolymers by a combination of living cationic polymerization, ROP, and controlled radical polymerization. β-Pinene was used exclusively for each copolymerization. Due to the electron-donating effect of the two alkyl groups, β-pinene has good activity toward cationic polymerization.118, 119 A random copolymer of β-pinene and isobutylene was synthesized.117 The initiating system consisted of 1-phenylethyl chloride, titanium tetrachloride, titanium isopropoxide, and tetra-n-butylammonium chloride. β-Pinene and isobutylene have similar structures in terms of propagating cationic species (Figure 18), thus their reactivity should be similar to produce a random copolymer.117 The polymerization was carried out in dichloromethane at −40 °C. By adjusting the molar ratio of β-pinene to isobutylene, they were able to synthesize a number of random copolymers. The polymerization progressed in a controlled manner, yielding polymers with molecular weight as high as 25 000 g mol−1 with a PDI below 1.2. The change of two monomer ratios did not affect the rate of copolymerization. The reactivity ratios determined by the Kelen-Tüdos method were rβ-pinene = 1.1 and risobutylene = 0.89. These ratios are nearly identical and the composition of final polymers was nearly same as the monomer feed ratio. The increase of β-pinene fraction, which offers rigidity to resultant polymers, from 10% to 30%, increased the Tg of final polymers by 60 °C.

thumbnail image

Figure 18. Propagating cations of β-pinene and isobutylene polymerization.

Download figure to PowerPoint

Living cationic polymerization allows the synthesis of well-defined polymers with a variety of terminal or side reactive groups. These functionalized polymers can serve as macroinitiators for other living polymerization techniques. Lu et al. utilized living cationic polymerization and ROP to produce diblock copolymer poly(β-pinene)-b-polytetrahydrofuran.115 Using the similar reaction conditions as mentioned above, poly(β-pinene) was prepared and then end-capped with styrene to produce a benzyl chloride end group (Figure 19). The macroinitiator, in the presence of AgSbF6 and propylene oxide, was able to polymerize tetrahydrofuran at 20 °C. Propylene oxide was added due to the extremely slow polymerization of tetrahydrofuran in the presence of the macroinitiator and the silver salt alone. The propylene oxide served as a catalyst that reduced the reaction time from more than 40 h to less than 20 min. A block copolymer with low molecular weight of 4500 g mol−1 and a PDI of 1.61 was obtained. The clear shift in the GPC traces, along with the emergence of oxymethylene protons in NMR spectra from the polytetrahydrofuran segment, confirmed the successful synthesis of the block copolymer.

thumbnail image

Figure 19. Synthesis of poly(β-pinene)-b-poly(tetrahydrofuran) block copolymers by sequential cationic polymerization and ROP.115

Download figure to PowerPoint

Lu et al.116 also used the same macroinitiator, shown in Figure 19, to synthesize block and graft copolymers of β-pinene and styrene. The poly(β-pinene) block was again end capped with benzyl chloride, which was a useful initiator for atom transfer radical polymerization (ATRP). Using a copper chloride catalyst and 2,2′-bipyridine (bpy) as a ligand, they prepared block copolymer poly(β-pinene)-b-polystyrene with clean GPC trace shift to higher molecular weight (Figure 20). In comparison, tert-alkylchloride-capped poly(β-pinene) was also used as an ATRP macroinitiator. The tert-alkylchloride-capped poly(β-pinene) could initiate ATRP of styrene, however the polymerization was slow. There was also a bimodal molecular weight distribution with high-molecular-weight block copolymers and lower molecular weight macroinitiator due to the slow initiation of the tert-alkyl chloride-capped poly(β-pinene). In order to synthesize the graft copolymer, a brominated poly(β-pinene) macroinitiator was prepared by reacting poly(β-pinene) with N- bromosuccinamide (NBS) and azoisobutyronitrile (AIBN). The partially brominated macroinitiator underwent ATRP of styrene, yielding a poly(β-pinene)-graft-polystyrene copolymer. The level of bromination was controlled by varying the feed ratio of NBS and poly(β-pinene). With the use of the same polymerization conditions, the graft copolymer had a molecular weight of 33 000 g mol−1 and a PDI of 1.51. The GPC results, along with the kinetic data, suggested that the ATRP grafting of styrene initiated with brominated poly(β-pinene) was living and gave the desired poly(β-pinene)-graft-polystyrene.

thumbnail image

Figure 20. Synthesis of poly(β-pinene)-b-polystyrene block copolymer and poly(β-pinene)-graft-polystyrene graft copolymer by ATRP.116

Download figure to PowerPoint

Radical copolymerization of β-pinene with vinyl monomers would greatly increase the diversity of β-pinene-containing copolymers.118, 119 Reversible addition– fragmentation chain transfer (RAFT) has gained interest in recent years for the polymerization of β-pinene-based polymers. There have been a few examples, in which β-pinene was copolymerized with either methyl acrylate or n-butyl acrylate.118, 119 Compared with other controlled radical polymerization techniques, RAFT is more tolerant to a wide range of monomers and reaction conditions.62, 121–125 Figure 21 shows the general RAFT polymerization mechanism.122 The Z and R groups of the RAFT transfer agent can be manipulated to provide control of the polymerization depending on the monomers used. A free-radical initiator, typically AIBN, is used to initiate the polymerization.

thumbnail image

Figure 21. A general mechanism of RAFT polymerization and synthesis of copolymers of β-pinene and methyl acrylate by free-radical polymerization. Reproduced with permission118 from Elsevier.

Download figure to PowerPoint

For the copolymerization of β-pinene and either methyl acrylate or n-butyl acrylate, free-radical polymerization was first attempted to determine the reactivity ratios of the monomers using AIBN as an initiator (Figure 21).118, 119 As expected, β-pinene exhibited very low reactivity and only a small fraction of final copolymers contained β-pinene. In both cases, β-pinene had a reactivity ratio of approximately zero. This indicates that no β-pinene homopolymers were produced and that the random copolymers consisted of large acrylate portions separated by a single β-pinene repeat unit. In an attempt to overcome this problem, a Lewis acid was added to the reaction mixture. Lewis acids are known to increase the tendency toward alteration for some comonomer pairs.84 The early termination reactions are suppressed by a binary complex between the Lewis acid and the electron-acceptor monomer. The best Lewis acid was diethylaluminum chloride. Other Lewis acids led to the formation of β-pinene oligomers, due to cationic polymerization of β-pinene initiated by the Lewis acids.118, 119 In the presence of diethylaluminum chloride, the reactivity ratios of the acrylates approached 1.0, however, reactivity ratio of β-pinene did not change. RAFT polymerization was attempted to produce copolymers. 1-Methoxycarbonyl ethyl phenyl dithiobenzoate and 2-cyanopropyl-2-yl dithiobenzoate were respectively chosen as the RAFT agents for preparation of methyl acrylate copolymers and butyl acrylate copolymers with β-pinene.118 Both copolymerizations displayed a controlled behavior. When the molar fraction of β-pinene in the feed increased, there was a noticeable decrease in the overall monomer conversion. This may be due to either slow fragmentation of the intermediate adduct or termination by chain transfer to β-pinene. In either case, β-pinene had a very small fraction in the final compositions, indicating limited success of RAFT polymerization of β-pinene. The problems associated with radical polymerization of β-pinene are not fully solved, but CRP appears to provide a possible tool to prepare β-pinene-based copolymers.

Compared with synthesis of pinene-based copolymers, copolymerization of limonene or terpenoids with other monomers is still in their infancy stage. Limonene and limonene oxide have been used as a solvent for ring-opening metathesis polymerization (ROMP) of several monomers, as carried out by Mathers et al.126, 127 Molecular weight of final polymers was lower than those polymerized in toluene, due to chain transfer to the solvent. ROMP of 1,5-cyclooctadiene in toluene produced a polymer with a molecular weight of 58 000 g mol−1, while the same monomer in limonene led to a polymer with a molecular weight of 11 000 g mol−1. This difference in molecular weight could be explained by the ability of the vinylidene alkene of both limonene and limonene oxide to participate in metathesis reactions as a chain transfer agent.127 The use of dicyclopentadiene as a monomer led to a hyperbranched polymer with terminal monoterpene end groups confirmed by 1H NMR.126

The first attempt to use limonene as a comonomer was done in 1981 by Doiuchi et al.128 using maleic anhydride as the comonomer. Sharma and Srivastava129–131 carried out the free-radical copolymerization of limonene with a number of vinyl monomers. Using acrylonitrile as a comonomer, they were able to produce a copolymer containing limonene and acrylonitrile as characterized. The reactivity ratio of limonene was zero, same as β-pinene, indicating that it cannot undergo self- polymerization.129 The increase of molar fraction of limonene in the feed ratio lowered the intrinsic viscosity of final polymers, indicating the reduction of molecular weight due to chain transfer effect from limonene. Similar trends were observed for the copolymerization with methyl methacrylate or styrene.130, 131 A possible explanation is the hydrogen abstraction from the limonene radical. The polymerization is terminated once the end of a polymer chain has a limonene radical. The radical removes an allylic hydrogen from the limonene monomer, thus making the polymer chain inactive and producing a stable radical that is incapable of initiating polymerization again (Figure 22).131

thumbnail image

Figure 22. Proposed termination of styrene/limonene copolymerization.131

Download figure to PowerPoint

Kamigaito and co-workers132 were able to synthesize a copolymer of limonene and N-phenylmaleimide by free-radical polymerization and RAFT polymerization. With an initial feed ratio of N-phenylmaleimide to limonene at 2:1 and fluorinated cumyl alcohol as a solvent, the copolymerization produced high-molecular-weight polymers with a unique AAB monomer sequence. Other solvents such as dimethylformamide and nonfluorinated cumyl alcohol led to the formation of low-molecular-weight polymers. The reactivity ratios suggest that limonene would not react with itself and that there is no chance that more than two N-phenylmaleimide monomers would react with one another, eliminating the possibility of forming a homopolymer. This observed trend is probably due to the interaction between the fluorinated solvent and N-phenylmaleimide (Figure 23) and the bulky limonene structure. A bridging interaction occurs between the alcohol group of the solvent and the carbonyl group of N-phenylmaleimide. The association constant between the N-phenylmaleimide monomer and the fluorinated solvent is significantly lower than that of the dimer and solvent. This strong coordination at the chain end enhances the addition of the electron-rich limonene monomer. The final polymers prepared by free-radical polymerization had a molecular weight of 9200 g mol−1 and a Tg of 243 °C. The RAFT polymerization displayed good control on molecular weight and produced polymers with a molecular weight of 8400 or 7600 g mol−1, depending on the RAFT agents that were employed. This solvent-specific polymerization of limonene opens a new approach for the incorporation of terpenes into polymer structures in a controlled fashion.

thumbnail image

Figure 23. Mechanism of AAB-sequence radical copolymerization of limonene (M1) and N-phenylmaleimide (M2). Reproduced with permission.132 Copyright 2010, American Chemical Society.

Download figure to PowerPoint

As mentioned earlier, Hillmyer and co-workers100 have carried ROP of carvone-based monomers. They prepared random copolymers of two monomers shown in Figure 17 using the same reaction conditions as used for the homopolymerization. The polymerization displayed good control, a monomodal GPC trace with a PDI between 1.1 and 1.2, and the final polymer showed a single Tg. Using an epoxidized monomer, they prepared random and crosslinked copolymers with ϵ-caprolactone (Figure 24).111 The crosslinked copolymers displayed a decrease in Tm and an increase in Tg with an increasing content of oxidized dihydrocarvone. This was expected as oxidized dihydrocarvone disrupts the crystallization of the ϵ-caprolactone segments. The final polymers were also tested to determine their shape memory properties. The amount of crystallinity plays a vital role in the shape memory properties such as percent shape recovery. The polymers with the highest amount of crystallinity, less than 10% oxidized dihydrocarvone, displayed excellent shape memory properties. The shape memory properties and their prospect of biodegradability make these crosslinked polymers viable candidates for biomedical applications.111

thumbnail image

Figure 24. Copolymerization of oxidized dihydrocarvone and ϵ-caprolactone to prepare linear and crosslinked polymers using tin(II)-2-ethylhexanoate and diethyl zinc as respective catalysts. Reproduced with permission.111 Copyright 2009, American Chemical Society.

Download figure to PowerPoint

5. Polymers from Rosin

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Terpenes
  5. 3. Terpenoids
  6. 4. Copolymers of Terpenes, Terpenoids, and Other Monomers
  7. 5. Polymers from Rosin
  8. 6. Conclusions
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information
  12. Biographical Information

According to its source, rosin is classified into three main types: gum rosin, wood rosin, and tall oil rosin. Wood rosin is obtained from aged pine stumps, which are chipped and soaked in a solvent solution. Solid wood rosin is obtained through distillation. Tall oil rosin is produced during the distillation of crude tall oil, a by-product of the Kraft process of wood pulp manufacturing. Gum rosin is the most common rosin from pine resin obtained by tapping living pine trees. Gum rosin is the nonvolatile fraction of pine resin, while turpentine is the volatile fraction. The production of rosin is more than 1 million metric tons per year.28, 51, 53, 55, 133 It consists primarily of abietic- and pimaric-type rosin acids with characteristic hydrophenanthrene structures and about 10% neutral materials, as shown in Figure 25. The hydrophenanthrene structures are considered to have cycloaliphatic and aromatic structures, thus providing rosin with renowned hydrophobicity, which has been utilized in marine antifouling materials for decades by US Navy.55 The predominant rosin acid is abietic acid (AA) with the empirical formula C20H30O2. Other acidic constituents of rosin differ mainly from abietic acid in that they are isomers of abietic acid having double bonds at different positions in the hydrophenanthrene moieties, which are often further hydrogenated or dehydrogenated with the aid of transition metal catalysts. The intrinsic acidity and rigidity, coupled with other chemical properties, enable rosin acids to be converted to a large number of downstream derivatives such as salts, esters, and maleic anhydride adducts, and hydrogenated, disproportionated rosins, which are used in a wide range of applications such as in the manufacture of adhesives, paper sizing agents, printing inks, solders, and fluxes, surface coatings, insulating materials, and chewing gums.52, 134–142 It should be noted that rosin acids are a class of stereoisomers (3 or 4 chirality centers depending on rosin acids). This review will not address the work on isolation of these stereoisomers.

thumbnail image

Figure 25. A few representative components of rosin.

Download figure to PowerPoint

5.1. Rosin-Derived Thermosetting Polymers

Thermosetting polymers are among the most important industrial polymers derived from petroleum chemicals.143, 144 The common adopted method is the use of a curing agent.52, 53 Rosin has been derivatized to contain anhydrides, multiple-carboxyl groups, or epoxy groups as curing agents to replace some of petroleum-derived ones that are widely used in industry today. Exhausted coverage on thermosetting polymers involving the use of rosin is not intended in this review. We show two representative rosin-derived curing agents to potentially replace petroleum-derived 1,2-cyclohexanedicarboxylic anhydride (CHDA) and 1,2,4-benzenetricarboxylic anhydride (BTCA), as shown in Figure 26. These two petroleum-based curing agents are rigid molecules due to their aromatic and cycloaliphatic structures. Rosin also has aromatic and cycloaliphatic structure. Zhang group developed new rosin-derived rigid curing agents.141, 142, 145, 146

thumbnail image

Figure 26. Petroleum-based curing agents (CHDA and BTCA) and rosin-derived diacid (APA) and acid anhydride (MPA).141

Download figure to PowerPoint

The use of rosin as a curing agent was made possible by the manipulation of the acid functional group and the conjugated diene. Abietic acid, the major component of rosin, can undergo isomerization at elevated temperature to produce levopimaric acid. The s-cis conformation of levopimaric acid can then undergo a Diels–Alder reaction to produce either acrylopimaric acid (APA) or maleopimaric acid (MPA) (Figure 26). Comparing MPA and BTCA as curing agents for same polymers, it was found that MPA-based polymers had higher Tg.141 This was attributed to the bulky fused-ring structure of rosin.

The use of rigid curing agents increased the overall Tg of resultant polymers, however there was a significant decrease in the flexibility of these polymers. In order to increase flexibility, Zhang and co-workers146 placed short polymer segment between two rosin molecules as shown in Figure 27. As expected, the Tg of the corresponding polymers decreased while exhibiting an increase in flexibility. Polycaprolactone (PCL) spacer had the greatest effect on the thermal and mechanical properties of resultant polymers. These results open a new avenue for the use of rosin-derived chemicals in formulation of engineering thermosetting polymers.

thumbnail image

Figure 27. Synthetic route to rosin-based curing agents used for preparing more flexible thermosetting polymers.146

Download figure to PowerPoint

5.2. Rosin-Derived Thermoplastic Polymers

Rosin-based thermoplastic polymers can be divided into two categories: main-chain polymers and side-chain polymers, according to the position of the hydrophenanthrene structure of rosin in the polymer structures.

5.2.1. Main-Chain Rosin-Derived Polymers

Main-chain rosin-based polymers are usually produced by step-growth polymerization. A variety of rosin-derived monomers have been developed (Figure 28). Most of monomer synthesis involves a Diels–Alder reaction between levopimaric acid and a dienophile such as acrylic acid, maleic anhydride, and maleimide.52, 53, 134–142, 147–149 Step-growth polymerization between these monomers and other comonomers such as diol, diacid, diamine, etc. allows the preparation of polyesters, polyamides, polyamide-imides, and polyester polyols.

thumbnail image

Figure 28. A variety of rosin-derived monomers for step-growth polymerization.52

Download figure to PowerPoint

Atta et al.147 prepared rosin-based unsaturated polyesters using APA, ethylene glycol, maleic anhydride, and adipic acid as the starting materials. The final polymer had a molecular weight of 5300 g mol−1. The unsaturated polymers were able to undergo curing in the presence of styrene to yield a cross-linked polymer with a Tg of 117 °C. The same group also produced polyamides and polyamide-imides using APA and MPA as starting materials.150 The polymers shown in Figure 29 had a molecular weight of 1600 g mol−1 (APA) and 2100 g mol−1 (MPA), respectively.

thumbnail image

Figure 29. Rosin-derived polyamide and polyamide-imide using monomers APA and MPA.150

Download figure to PowerPoint

Kim et al.148 carried out condensation polymerization between MPA and an azo-dye diamine to produce photosensitive polyamide-imide (Figure 30). The final polymer had a molecular weight of 8300 g mol−1 and a PDI of 1.95. Bicu and Mustata136, 138, 140 were able to produce water-soluble polyamides using APA as the starting material. The final polymer consisted of APA, adipic acid, and triethyleneteramine, shown in Figure 30.136 The reaction between the polymer and epichlorohydrin led to the formation of a substituted polymer that could crosslink in the solid state upon heating to form a thermosetting polymer. Bicu and Mustata[140] further prepared polyesters using APA and ethylene carbonate with a molecular weight of 12 000 g mol−1 and a Tg and Tm of 118 °C and 168 °C, respectively.

thumbnail image

Figure 30. (Left) Photoactive polyamide-imides;148 (Right) Water soluble polyamide.136, 138, 140

Download figure to PowerPoint

Bicu and Mustata134, 135, 137–139 also developed a unique pathway to prepare polymers of APA via a dehydrodecarboxylation reaction shown in Figure 31. Polycondensation reaction led to the formation of a polymer with a molecular weight of 4100 g mol−1 and a Tm of 222 °C.139 The reaction between the diacid and 1,6-hexandiamine produced a polyamide with a molecular weight of 4000 g mol−1 and a Tm of over 360 °C.

thumbnail image

Figure 31. Synthesis of APA ketone diacid and polyamide.139

Download figure to PowerPoint

The conversion of the diacid to diacid chloride and subsequent condensation reaction with 1,4-butanediol produced a polyester with a molecular weight of 11 000 g mol−1 and a Tm of 235 °C.138 The use of dianhydride (DMPK, Figure 28) in conjunction with triethylenetetramine produced a polymer with a molecular weight of 10 000 g mol−1.137 This was achieved by using excess amine, which served as a linker between two polymer chains. The final polymer structure is displayed in Figure 32.

thumbnail image

Figure 32. Polyimide produced by the use of excess amine.137

Download figure to PowerPoint

5.2.2. Side-Chain Rosin-Derived Polymers

Although main-chain rosin-based polymers can be prepared by various condensation methods, only low-molecular-weight polymers could be obtained. Steric hindrance, monomer impurities, and stoichiometric control are the primary reasons why only low molecular weight is obtained.28, 52, 53, 139 The synthesis of side-chain rosin-based polymers by radical polymerization tends to avoid these problems associated with their main-chain counterparts. Radical polymerization of rosin-derived vinyl monomers is utilized to produce side-chain rosin-based polymers.

5.2.2.1. Free-Radical Polymerization

Rosin acids have been converted to vinyl, acrylic, or allyl ester groups that can undergo radical polymerization.52 Vinyl ester monomers, shown in Figure 33, are among the first monomers that were polymerized by free-radical polymerization.147, 151, 152 However, due to steric hindrance, only low-molecular-weight polymers were obtained. To reduce the steric hindrance, allyl and acrylic ester monomers were synthesized (Figure 33). Allyl ester monomers are used primarily as curing agents. Again, low-molecular-weight polymers were produced when free-radical polymerization was explored.52

thumbnail image

Figure 33. A few rosin-derived vinyl ester monomers and ally ester monomers.

Download figure to PowerPoint

Compared with both vinyl and allyl ester monomers, acrylic monomers have shown great potential in the preparation of side-chain rosin-based polymers, in part due to the increased reactivity of the unsaturated C2C (Figure 34).151, 153–158 Lee and Hong153 prepared different copolymers of 1 and methyl methacrylate. The final copolymers had molecular weight of ≈10 000 g mol−1 with a Tg in the range of 97 °C to 122 °C, depending on the length of the alkyl linkers. They further made negative photoresists by photocuring 2 with poly(methyl methacrylate).153 Chu and co-workers155 polymerized 4 to produce a homopolymer with a molecular weight of 4700 g mol−1 and a Tg of −25 °C. However, later work by controlled radical polymerization of monomer 4 showed that these acrylic monomers reported by most early work were not pure.159, 160 Kim and co-workers154, 158 utilized 3 as a tackifier and blended with UV-crosslinkable polyacrylates to make pressure sensitive adhesives.

thumbnail image

Figure 34. A few rosin-derived (meth)acrylic monomers.

Download figure to PowerPoint

5.2.2.2. Controlled Radical Polymerization

Controlled radical polymerization (CRP) allows for the synthesis of well-defined polymers with controlled molar mass, narrow molecular weight distribution, and well-defined architectures and functionalities.44, 56–66, 125, 161 RAFT polymerization62, 122–125, 162, 163 and ATRP61, 65, 164–170 are two of the most widely used CRP methods, and both involve a fast dynamic equilibrium between dormant species and active radical species to provide control. The conditions of the polymerization, including ATRP using transition-metal complexes, and RAFT using dithioesters, are selected so that the equilibrium between dormant and active species is strongly shifted toward dormant species to establish a low concentration of propagating radicals and to reduce proportion of unavoidable termination reactions. The overall mechanisms for RAFT polymerization and ATRP are shown in Figure 21 and Figure 35, respectively. Tang and co-workers52, 157, 159, 160 has carried out work on the use of both ATRP and RAFT to prepare side-chain rosin-based polymers.

thumbnail image

Figure 35. Overall mechanism of atom transfer radical polymerization.

Download figure to PowerPoint

The synthesis and ATRP of rosin-based monomers are shown in Figure 36.160 Dehydroabietic acid (DA) was chosen as the starting material due to its better stability, compared with abietic acid. This difference in stability is due to the aromatic ring in the hydrophenanthrene structure. Tang group were able to prepare highly pure monomers as confirmed by NMR. Different spacers were placed between the vinyl group and dehydroabietic group to vary the steric effect imparted onto the vinyl group, which could have significant influence on the control of polymerization. For ATRP of the monomer with the shortest spacer, the reaction was not controlled most likely due to the steric effect of dehydroabietic moiety. For other acrylates, relatively polar solvents prompted the control of polymerization, while nonpolar solvents resulted in a better control in the polymerization of the methacrylates. The molecular weight of polymers ranged from 10 000 to 100 000 g mol−1 with a PDI below 1.3. Higher molecular weight was obtained for polymers with longer spacers between the rosin moiety and the vinyl group, further indicating the steric effect. All polymers exhibited typical amorphous thermoplastic behaviors, no melting observed. The thermal properties of final polymers were directly related to the length of the spacers between the vinyl group and the dehydroabietic group. The polymer with the longest spacer had a Tg of 22 °C (PABDA in Figure 36), while the one with the smallest spacer had a Tg of 90 °C (PMAEDA in Figure 36). Thermogravimetric analysis (TGA) showed that these polymers have two stages of weight loss behavior: a slight weight loss with onsets at approximately 220 °C and a complete decomposition with onsets at approximately 325 °C, similar to many polymers derived from petroleum chemicals.159, 160

thumbnail image

Figure 36. Synthesis and atom transfer radical polymerization of rosin-based (meth)acrylate monomers: NMR spectra of ABDA monomer and PABDA polymer and differential scanning calorimetric curves of three different polymers. Reproduced with permission.160 Copyright 2010, American Chemical Society.

Download figure to PowerPoint

RAFT polymerization using cumyl dithiobenzoate as the transfer agent and AIBN as the radical initiator was carried out on both methacrylate and acrylate rosin-based monomers (Figure 37).52 Polymerization of methacrylate monomer in toluene at 70 °C produced the best results. There was a linear correlation in kinetic plots, indicating a living polymerization. There was a clean shift in molecular weight with increased reaction time and the final polymer had a PDI below 1.3. For the acrylate monomer, similar conditions were applied for its polymerization. However, resultant polymers had high PDI (>1.5). When the solvent was changed from toluene to THF, polymerization at 80 °C led to a polymer with molecular weight 29 100 g mol−1 and a PDI of 1.3. The polymerization of the acrylate monomer was relatively slow, but controlled if a polar solvent like THF was utilized. Comparing the two CRP methods, RAFT polymerization led to polymers with a molecular weight lower than that of polymers obtained by ATRP, but the PDI was similar.

thumbnail image

Figure 37. Reversible addition fragmentation transfer (RAFT) polymerization of rosin-based (meth)acrylate monomers.52

Download figure to PowerPoint

Recently, Tang and co-workers171 used RAFT polymerization to prepare a series of cationic rosin-containing methacrylate polymers with controlled molecular weight and low PDIs. They polymers showed promising applications as antimicrobial agents.

Tang group further extended controlled polymerization to prepare the first rosin-containing block copolymers. DA and ϵ-caprolactone (CL) block copolymers were prepared by a combination of ATRP and ROP using a difunctional initiator 2-hydroxyethyl-2-bromoisobutyrate (HEBiB).159 Figure 38 shows two-step routes of sequential polymerization (two-pot). The hydroxyl group from the initiator along with tin(II)-2-ethylhexanoate as a catalyst was used to prepare the macroinitiator by ROP of CL. The bromine group from the initiator along with Cu(I) bromide as a catalyst and tris(2-(dimethylamino)ethyl)amine (Me6Tren) as a ligand enabled ATRP of the acrylic rosin monomer. Once the macroinitiators were purified, the second monomer was added to produce the desired block copolymers. Both routes produced high-molecular-weight polymers in a controlled manner, indicating the successful chain extension.

thumbnail image

Figure 38. Preparation of rosin acid-caprolactone block copolymers by ATRP and ROP.159

Download figure to PowerPoint

To simplify the reaction process, block copolymers were also prepared by one-pot polymerization (Figure 38). The first method involved mixing both monomers along with the catalysts and ligand together in one reaction vessel and simultaneously performed ATRP and ROP. Two other one-pot polymerization methods were also explored. The first method was a sequential addition from rosin monomer to CL. The second sequential addition method was just the opposite of the first. Both methods did not need intermediate workup steps. The overall polymerization favored the use of tin(II) catalysts before copper catalysts. If the copper catalysts were used first, the ROP of CL would be significantly suppressed due to the oxidation of Sn(II) to Sn(IV) by the accumulating copper(II) formed from the ATRP reaction system.

The thermal properties of final block copolymers relied on the compositions of the two blocks. Block copolymers with a high PCL content showed a Tm around 60 °C, which originated from the semicrystalline PCL block. However, block copolymers with high rosin content displayed only a Tg of ≈50 °C, indicating the suppression of PCL crystallization. Degradability tests were also performed on the block copolymers in acidic solutions. The GPC traces showed a clear shift toward lower molecular weights, due to the degradation of the PCL block, while the rosin block remained undegradable under this condition.

5.2.2.3. Ring-Opening Polymerization and Click Chemistry

Although ATRP and RAFT were successfully used to prepare well-defined rosin-containing acrylic polymers, the use of single resin acid limited their advantages. Tang and co-workes157, 172, 173 combined ROP and click chemistry to prepare graft copolymers using raw rosin materials to prepare a class of novel degradable graft copolymers, which combine properties of both rosin acids and degradable polymers. The synthetic route for the PCL graft copolymer is shown in Figure 39.172 Copper-catalyzed azide–alkyne click chemistry was utilized to attach the rosin moiety to the PCL backbone. Rosin propargyl esters were prepared by an esterification reaction between rosin acids and propargyl alcohol. The ROP of PCL was carried out using HEBiB as an initiator and tin(II)-2-ethylhexanoate as a catalyst. The substituted PCL was converted from chloride to azide in the presence of sodium azide. The click reactions between PCL and the rosin propargyl esters were performed in the presence of a 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) solution and a copper(I) iodide catalyst at 35 °C. Both NMR and FTIR confirmed the successful attachment of rosin moieties onto PCL. This strategy has a significant advantage as low- cost raw rosin was used as starting materials.

thumbnail image

Figure 39. Synthesis of raw rosin ester-grafted PCL by a combination of ROP and click chemistry, and degradability, thermal properties and contact angle measurements of resultant copolymers. Reproduce with permission.172 Copyright 2011, American Chemical Society.

Download figure to PowerPoint

The addition of rosin moiety had a profound impact on the thermal and surface properties of final copolymers. The Tg of the PCL prior to grafting was −40 ~ −60 °C, while the grafted copolymers had a Tg of 55 ~ 85 °C. The significant increase in Tg is due to the rigidity provided by the hydrophenanthrene structure of rosin. The hydrocarbon nature of rosin structures provided PCL excellent hydrophobicity with contact angle very similar to polystyrene (80° to 90°) and very low water uptake (Figure 39). The degradability of the graft polymers was also tested. Two different methods were employed: acid-catalyzed degradation and hydrolytic degradation. The acidic degradation was carried out in a dilute HCl/THF solution. Within 1 h, the PCL chain completely degraded and only the rosin moieties remained. The bulky and rigid nature of the rosin side groups did not interfere with the degradation of the PCL main chain. The hydrolytic degradation was performed in phosphate-buffered saline solution at 37 °C. There was a gradual degradation and after 60 d nearly 10% of the original PCL mass was lost with molecular weight reduced by more than 20%. The rosin side group improved the thermal properties of the PCL, while maintaining full degradability of the polymer. This work opens an avenue to prepare novel biocompatible and biodegradable hydrophobic polyesters.

5.2.3. Biocomposites of Rosin and Natural Polymers

Rosin has been combined with other natural polymers to provide all organic biocomposites. Tang and co-workers157 explored the grafting of rosin from lignin. Lignin is the second largest renewable resource with an annual production of 50 million tons from paper industry, cellulosic materials, cellulosic ethanol, and many other biomass production processes.33, 36, 41, 42, 174–179 Lignin is a complex material with characteristic aromatic structures. The major functional groups of lignin include hydroxyl, methoxyl, and phenylpropane units. There are two general approaches to utilize lignin for polymer applications. The first approach is to simply blend lignin with traditional polymers. The second method is to chemically modify the lignin structure. Although both methods have their drawbacks, the chemical modification route leads to lignin that is more compatible with polymer blends. Tang and co-workers157 used organosolv lignin to produce rosin-based graft copolymers (organosolv is a pulping technique that uses an organic solvent to solubilize lignin and hemicellulose).

A lignin macroinitiator (Lignin-Br) was prepared by reacting lignin with 2-bromoisobutyryl bromide (BiBB) in the presence of triethylamine, as shown in Figure 40. The hydroxyl groups from the lignin reacted with BiBB to form the brominated ester macroinitiator. Both phenolic and aliphatic hydroxyl groups were confirmed to convert to the 2-bromoisobutyryl ester. The amount of bromine content per gram of lignin can be controlled by changing the ratio of starting materials: lignin and BiBB. ATRP was performed using the same ligand and catalyst as the polymerization of rosin (meth)acrylic monomers. Kinetic studies showed that all polymerization was living and controlled (Figure 41). The GPC traces showed that all polymers exhibited broad molecular weight distribution, which was not surprising given the use of polydisperse macroinitiators-derived from lignin. The lignin was also modified with simple rosin acids by a simple esterification reaction.

thumbnail image

Figure 40. Synthesis of lignin ATRP macroinitiators and rosin polymer-grafted lignin composites. Reproduced with permission157 from John Wiley and Sons.

Download figure to PowerPoint

thumbnail image

Figure 41. Control (kinetic plots and GPC traces) of surface-initiating ATRP from lignin, contact angle measurement, water uptake plot and appearance of lignin and modified lignin. Reproduced with permission157 from John Wiley and Sons.

Download figure to PowerPoint

The thermal properties of the unmodified lignin were compared to those of grafted copolymers. Unmodified lignin had a Tg of ≈97 °C, while the Tg of graft copolymers displayed a dependency on the length of the alkyl side group. Graft copolymers with the longest side group displayed a Tg of ≈20 °C (LGBA), while the shortest had a Tg of 95 °C (LGEMA). The thermal stability of graft copolymers was also compared with that of unmodified lignin. Unmodified lignin displayed good thermal stability and maintained at least 40% weight at 500 °C, mostly likely due to the formation of residual pyrolyzed carbon materials, which are very common for lignin as it is a carbon precursor.180 After grafting, however, there was a two-stage degradation observed at 220–260 °C and 450–480 °C. Although lignin is considered as a hydrophobic material, the modified lignin displayed even greater hydrophobicity and water resistance (Figure 41). The grafting of both DA and rosin polymers significantly enhanced hydrophobicity of lignin. Static contact angle measurement of water droplets showed ≈90° for all these rosin modified lignin composites. X-ray photoelectron spectroscopy demonstrated that the surface of rosin–lignin composites was dominated with chemical compositions originating from the hydrocarbon rich rosin moiety. The impartation of hydrophobicity of rosin into lignin provided excellent water resistance of this class of renewable polymers, as all rosin-modified lignin composites showed water uptake below 1.0 wt%.

Duan et al.156, 181 prepared rosin-grafted chitosan copolymers. Chitosan is a partially deacetylated derivative of chitin. It is nontoxic, biodegradable, and has excellent biocompatibility.32, 182, 183 The primary problem associated with chitosan is that it is only soluble in few dilute acid solutions due to its crystalline nature.43, 184 By grafting rosin and its derivatives onto chitosan it is believed that there should be a significant improvement in solubility. Figure 42 shows the synthetic route used to prepare chitosan-based graft copolymers. Microwave radiation was used to graft the rosin-based acrylate monomer onto the chitosan backbone. The structure of chitosan before and after grafting showed a substantial decrease in crystallinity. The presence of rosin moiety decreased both the amount of crystallinity and the thermal decomposition temperature of chitosan graft copolymers by disrupting the intermolecular hydrogen bonding. The graft copolymers also showed promise in controlled release of fenoprofen calcium, as compared to un-grafted chitosan.156

thumbnail image

Figure 42. Synthesis of rosin-grafted chitosan copolymers.156, 181

Download figure to PowerPoint

6. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Terpenes
  5. 3. Terpenoids
  6. 4. Copolymers of Terpenes, Terpenoids, and Other Monomers
  7. 5. Polymers from Rosin
  8. 6. Conclusions
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information
  12. Biographical Information

There are a variety of demanding challenges for scientists to develop novel biomass-based chemicals and polymers and innovative technologies that can reduce our dependence on fossil fuel and decrease carbon emission, due to the shortage of fossil fuel and global warming. Forestry and agricultural natural resources are defined as source materials derived from a wide range of biological plant systems and processing streams in the food, feed, and fiber industries. Including forestry, rangeland, and a highly productive agricultural system, there are more than 2000 million acres of plant/crop-based resources in U.S., which create 22 million jobs in output processing, handing, and selling fee, food, and fibers.1, 2, 185 In the past 50 years, most of these resources have been largely focused toward food, feed, and fiber production. Expansion of the use of natural resources for producing commodity materials including renewable polymers is urgent. Global economy will be enhanced by environmental needs and improvements in technology. Environmental needs include (1) reducing carbon emissions from fossil fuel and sequestering carbon; (2) reducing our dependence on fossil fuel through development of renewable materials; (3) developing biocompatible and biodegradable commodity materials.

Although widely used as renewable chemicals, terpenes, terpenoids, and rosin have been historically much ignored as biomass for manufacturing of “green plastics” and composites. This is largely due to the difficulty to precisely control the molecular structures and therefore macromolecular engineering into commodity plastics in a low-cost way. For example, this challenge has been well overseen in the development of main-chain rosin-derived polymers, which have unfortunately played a limited role in the field of renewable polymers due to their poor molecular control. On the other hand, terpenes, terpenoids, and rosin have all elements to be renewable natural feedstocks for polymeric materials such as abundance, low cost, and functionality. Our understanding on the chemistry and precise molecular and macromolecular control on these biomass and their derivatives is essential, as this has been well achieved in synthetic plastics derived from petroleum chemicals. It is encouraged from recent efforts that controlled polymerization (mainly CRP, ROP, and ROMP) might pave the way to achieve large-scale utilization of biomass for the development of renewable polymers. The control of molecular weight, functionality, and architectures that controlled polymerization can provide makes it possible to prepare a variety of functional polymers really resembling diverse plastics and composites derived from petroleum chemicals. Another important direction is the use of highly efficient post-polymerization modification techniques such as “click chemistry”, which may take advantage of just a small part of natural biomass (i.e., particular functional groups) although the majority of biomass structures are different. The use of these efficient tools could allow the use of raw natural resources without cost-prohibitive sophisticated process.

Green plastics will play an ever increasing role for future generations. The purpose of this review is to cover one aspect of a pyramid of natural resource utilization. Other plant or crop-based resources, along with biological resources, are equally important for the advancement of green plastics. Production cost and availability of starting materials currently favors petroleum-based plastics, but with rapidly improving technologies and a greater social understanding for the need of nonpetroleum-based materials, renewable biobased plastics will become more prevalent in years to come.

Acknowledgements

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Terpenes
  5. 3. Terpenoids
  6. 4. Copolymers of Terpenes, Terpenoids, and Other Monomers
  7. 5. Polymers from Rosin
  8. 6. Conclusions
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information
  12. Biographical Information

This work was supported by the University of South Carolina Start-up Funds, USDA NIFA under the Award 2011-51160-31205, China International Science and Technology Cooperation (2011DFA32440) and Forestry Public Sector Research Fund of State Forestry Administration of China (CAFYBB2010003-3).

Biographical Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Terpenes
  5. 3. Terpenoids
  6. 4. Copolymers of Terpenes, Terpenoids, and Other Monomers
  7. 5. Polymers from Rosin
  8. 6. Conclusions
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information
  12. Biographical Information
Thumbnail image of

Perry Wilbon received his B.S. from Middle Tennessee State University. Since 2010, he has been a graduate student under the supervision of Dr. Chuanbing Tang in Department of Chemistry and Biochemistry at the University of South Carolina. Currently, he is developing renewable biobased monomers, polymers, and composites.

Biographical Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Terpenes
  5. 3. Terpenoids
  6. 4. Copolymers of Terpenes, Terpenoids, and Other Monomers
  7. 5. Polymers from Rosin
  8. 6. Conclusions
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information
  12. Biographical Information
Thumbnail image of

Fuxiang Chu received his B.S. from Nanjing University and Ph.D. from Chinese Academy of Forestry. Since 1994, he has been a professor in Chinese Academy of Forestry and the major areas of his research work are wood chemistry, wood adhesives, chemical processing, and utilization of forest products. More recently, his research interests focus on the chemical modification of lignin cellulose and the development of biodegradable polymer materials based on forest biomass, with attempts to find new alternatives to petroleum resources.

Biographical Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Terpenes
  5. 3. Terpenoids
  6. 4. Copolymers of Terpenes, Terpenoids, and Other Monomers
  7. 5. Polymers from Rosin
  8. 6. Conclusions
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information
  12. Biographical Information
Thumbnail image of

Chuanbing Tang received his B.S. from Nanjing University and Ph.D. from Carnegie Mellon University with Profs. Krzysztof Matyjaszewski and Tomasz Kowalewski. He was a postdoctoral scholar at University of California Santa Barbara with Profs. Edward J. Kramer and Craig J. Hawker. Since August 2009, he has been an assistant professor in Department of Chemistry and Biochemistry at the University of South Carolina. His research interests focus on organic polymer synthesis, renewable biobased polymers from natural resources, metal-containing polymers, block copolymer self-assembly, and polymers for biomedical application.