Catalysts Immobilized on Organic Polymeric Monolithic Supports: From Molecular Heterogeneous Catalysis to Biocatalysis

Abstract

This review describes the synthetic routes to various types of organic polymeric monoliths. Significant concentration is applied to the role of these continuous, porous structures in both heterogeneous catalysis and biocatalysis. A monolith is composed of a solitary mass filled with interconnected pores, which include both large flow-through pores and smaller meso- or micropores. These porous monolithic materials have several advantages over conventional packed beds of porous polymeric beads, owing to their macroporosity and lack of interstitial spacing. Their large pores contribute to mass transfer, which allows the structure to withstand higher back pressures than conventional packed beds, whereas their small pores still operate by diffusion. The effect of multiple parameters, such as the temperature, the cross-link density, and the type and content of porogenic solvent on the pore formation and pore size distribution is outlined for monoliths prepared through free radical polymerization and ring-opening metathesis polymerization (ROMP). Post-functionalization of these monoliths to control the surface chemistry of the supports and/or affix functional catalysts is elucidated, as well as employment of these supports in continuous catalytic reactions. Significant advances in supported catalysis for metathesis, Heck, Suzuki, Sonogashira–Hagihara, and biocatalytic reactions are described.

1. Introduction

Primary rewards of employing monolithic media in catalysis include their simple preparation and the facile control of their surface chemistry.1 Early research in supported catalysis focused on fine chemical and pharmaceutical syntheses.2, 3 Although the history of solid supports dates back to Merrifield resins, a cross-linked polystyrene-based solid support that revolutionized solid-phase peptide synthesis,4 the advent of combinatorial libraries has rekindled interest in their widespread application.2, 3 The advantages of using supported catalysts include 1) simplicity of recovery and reuse of the catalyst in subsequent reactions, 2) their malleability to automated and continuous flow systems, and 3) reduced metallic impurities left in the final products.2 This methodology has become increasingly attractive in recent years for expensive transition metals and biocatalysts. Although general concerns still include cost and loading capacity, the large flow-through pores of monolithic materials offer the benefit of mass transfer, compared with supports formed from packed, uniform, spherical porous beads, in which catalysis is diffusion limited.2, 5–8

Although macroporous polymeric beads for use as solid supports materialized as early as the 1950s,7, 9 organic polymeric porous monoliths have ascertained great interest since the advent of robust, “continuous rods” by Fréchet et al. in the early 1990s.10 A polymeric monolith is composed of one, solitary portion or continuous block of porous, cross-linked polymer and is characterized by its interpenetrating network of usually micrometer-sized pores (Figure 1).2, 5, 11

Figure 1. SEM of a porous polymeric monolith.

These porous, cross-linked materials are often compared to porous, spherical beads produced by suspension polymerization. However, the advantages of monolithic materials over conventional packed beds include high throughput, greater tolerance to high flow rates, and the simplicity of their preparation.8 Monolithic structures are predominantly attractive for use as chromatographic separation media for HPLC,12–19 size-exclusion chromatography,20 capillary electrochromatography,21, 22 or the facile separation of biomolecules and proteins.23–32 They also find applications as solid-phase extraction media,33 ion-exchange resins,34, 35 matrices for desorption/ionization mass spectroscopy,36 and supports for continuous catalysis.37–45 With photolithographic post-functionalization, monoliths can even serve as both chromatographic media and solid-phase extraction media.33 This review is by no means a full discussion of these applications of monolithic media or their history. In fact, several extensive reviews exist that outline the history and the properties of polymeric monoliths.1, 46–64 Our objective, rather, is to describe the contemporary advances in monolithic science and noteworthy achievements toward continuous supported catalysis.

2. Polymeric Monolithic Materials

Organic polymer-based monoliths are derived through various types of reactions, which include free radical polymerization, ring-opening metathesis polymerization (ROMP), polycondensation, and polyaddition reactions.5 Free radical polymerization is the most common route used to create monolithic structures and is initiated through electron beam-triggered reactions, thermal initiation, UV-based initiation, or even controlled, tetramethylpiperidyloxy (TEMPO)-mediated initiation.5 The most popular polymeric backbones include poly((meth-)acrylate-co-ethylene dimethacrylate) and poly(styrene-co-divinylbenzene) systems. The acrylate-based monoliths are by far the most studied type of organic polymer-based monolith, and comprehensive reviews exist simply on these types of monoliths.1, 65 Today, this type of monolith is available commercially as ion-exchange and reverse-phase chromatographic material from Dionex Co. and is labeled as ProSwift. Convective Interaction Media (CIM) Disk is also a commercial product of BIA Separations Company.1 Although the acrylate-derived monoliths have received much esteem, the ROMP-based approach has also led to significant advances in post-functionalization and grafting on monolithic media.66–68 There are two main types of ROMP-based monoliths composed of either norborn-2-ene (NBE)-based monomers or cis-cyclooctene (COE)-based monomers. This review describes the synthesis, the structure formation, the control of porosity, the post-functionalization, and the characterization of pore sizes and pore size distributions for numerous types of monolithic supports.

2.1. Free radical polymerization-derived monoliths

2.1.1 (Meth-)acrylate- and polystyrene-divinylbenzene-based monoliths

Fréchet et al. introduced the first free radical polymerization-derived monoliths in 1992.10 Today, this chemistry still remains one of the most popular methods to synthesize monolithic supports. Synthesis of this type of monolithic support usually requires a monomer, a cross-linking reagent, one or more porogenic solvents of which at least one is a poor solvent for the polymer formed, and an initiator. These are mixed and poured into an unstirred container for polymerization by various types of initiation (Figure 2).

Figure 2. Synthesis of porous polymeric monoliths through electron beam-triggered free radical polymerization.66

The majority of acrylate-based monolithic compositions use glycidyl methacrylate as a functional monomer because the epoxide group allows post-functionalization through hydrolysis to vic-diols or ring opening through various amines.5, 69, 70 This epoxide-based monomer is often used in accordance with the ethylene glycol dimethacrylate (EDMA) cross-linker and cyclohexanol and dodecanol as macro- and microporogens. Many studies of varied acrylate-based monolith compositions have also coincided with the syntheses of varied styrene/divinylbenzene compositions, especially if these polymers were later compared to suspension polymerization products.71 With the styrenic monoliths, the porogenic solvents used are often dodecanol and toluene. Additional syntheses of monoliths through TEMPO-mediated free radical polymerization have been explored but are limited, owing to slow reaction kinetics and the formation of nonideal structures.5, 72 However, this approach permits facile grafting of monomers such as 2-hydroxyethyl methacrylate or vinyl benzyl chloride to the monolithic backbone. Poly(styrene-co-divinylbenzene)-based monoliths have been synthesized through TEMPO mediation in the presence of toluene and aliphatic alcohols or polyethylene glycol as porogenic solvents. A cation-exchange resin was also prepared by grafting 3-sulfopropylmethacrylate to the monolithic surface.5, 73–75

2.1.2. Initiation

There are several ways to initiate free radical polymerization for monolith synthesis. Most monoliths are synthesized by initiating the reactions with heat and common radical initiators, such as 2,2'-azobisisobutyronitrile (AIBN).1, 71, 76–77 AIBN possesses a half-life of 37 h at 55 °C and 6 h at 70 °C.71 Although the initiator is not often the most popular or primary tool chosen to control porosity, the use of 2,2-azobis(2,4-dimethyl)valeronitrile has led to lower overall surface area and larger flow-through pores than those formed by AIBN-derived monoliths.1, 77 Similar effects contributing to larger flow-through pores occurred with dibenzoyl peroxide-initiated cross-linking, rather than with AIBN-initiated monolith formation.1, 76 The slower decomposition of dibenzoyl peroxide compared to that of AIBN plays an important role in determining the final porous structure.

UV radiation alongside UV-based initiators has also been used to initiate monolith formation.1, 5 This route is more popular for small-scale columns, such as silica capillaries and microfluidic discs. On a small scale, UV penetration and light intensity are less of a factor for fast and uniform curing of the monolith. For example, curing of glycidyl methacrylate and trimetholpropane trimethacrylate has been achieved with benzoin methyl ether UV initiator, and isooctane and toluene porogenic solvents.5, 78 Application of 365 nm UV light led to porous structures, 33–4500 nm in diameter. However, some other approaches have been developed to circumvent issues of penetration or light intensity found in UV initiation. These methods include using gamma radiation79 and redox chemistry.80 Gamma radiation at 10–40 kGy h^−1[79] has led to the curing of porous EDMA-based monoliths without any added initiator. Alcohols served as macroporogens, whereas the microporogens included various organic solvents, such as acetone and THF.8, 79

Electron beam-triggered synthesis of monolithic media is a new approach developed by our research group (Figure 2).25 Curing occurs without any added initiator. This type of initiation has been used for the synthesis of monolithic media for HPLC in both capillary size (50–200 μm inner diameter) and preparative size (up to 7 cm inner diameter) reactions. A typical electron beam dose of 22 kGy was applied,18 and various monomer structures were viable including ethyl methacrylate (EMA), trimethylol propane triacrylate (TMPTA), lauryl methacrylate, and EDMA. Pore formation occurred for acrylate-based monomers in the presence of porogenic solvents such as 2-propanol, 1-dodecanol, and toluene. Solutions were mixed, injected into silanized capillaries that were functionalized with 3-(trimethoxysilyl)propyl methacrylate, and then sealed inside these capillary tubes before irradiation.18 The “grafting-from” approach that uses electron beam-triggered free radical polymerization on preformed free radical-polymerized monoliths has also been employed to functionalize acrylate-based monoliths with grafted methacryloyl-substituted N-heterocyclic carbene (NHC) polymer precursors.42 The NHC-functionalized monomer was simply dissolved in degassed methanol, injected onto the monolith, and sealed in the column. An electron beam dose of 22 kGy was applied. Subsequent copper functionalization of the NHC groups led to the use of these materials in continuous hydrosilylation and hydrocyanation reactions.42 This same grafting-from approach has also allowed the functionalization of polyacrylate-based capillary monoliths for biocatalysis using immobilized trypsin.40 Grafting of N-hydroxysuccinimidyl ester-substituted monomers led to the facile attachment of trypsin through aminolysis.40

2.1.3. Control of porosity

Monoliths are formed in an unstirred state and possess several types of porosity that are maintained even in dry conditions, which include micro- (<2 nm), meso- (2–50 nm), and macroporosity (>50 nm) (Figure 3).1 One of the most advantageous aspects of monoliths is their large flow-through pores that allow for fast mass transfer or convection. Compared with conventional packed beds, the structures also withstand high back pressures. However, having a large population of small pores in addition to the large flow-through pores is necessary for the retention and separation of analytes.1, 43, 47 The large pores operate by convection, but the smaller pores still operate by diffusion as those in porous beads. Monoliths are often compared to conventional suspension-synthesized porous beads that are packed into columns; however, these two types of porous media have distinct differences.71 Packing of porous suspension-polymerized beads leaves interstitial void volume between their regular, spherical surfaces. Although their uniform structure supplies advantageous, uniform flow properties compared to those of crushed polymer gels, flow to “active sites” or functional sites on the polymer is diffusion limited.1 Macroporous monolithic media avoid these problems with their large flow-through pores.

Figure 3. Representation of the structure of porous monolithic media. Reproduced with permission from Ref. [81], Copyright 2000 American Chemical Society.

Many publications by Svec, Fréchet et al.7, 8, 71 and Buchmeiser et al.5, 37, 58–62, 81 have defined the ways to control composition, porosity, and flow properties of monolithic supports synthesized by using free radical polymerization and ROMP. Targeting specific porosities and pore size distributions with novel compositions unlike the ones previously published is much attributed to art. Hydrophobicity of the monomers and the degree of solvation of them in the porogenic solvents contribute greatly to the morphology of the monolith, and the resulting core monolith structure can be altered quite significantly with changes in monomer functionality. Therefore, there is much advantage in using previously derived, well-defined monolithic scaffolds to target specific porosities and pore size distributions. Owing to this consideration, much effort has been focused on chemical modification and postgrafting techniques on monoliths derived from current formulations.66–68

The three principal tactics to alter the porosity and pore size distribution of monolithic materials in a relatively predictable way are as follows: 1) to change the type or amount of porogen, 2) to change the amount of cross-linking reagent, and 3) to change the temperature of the polymerization.1, 5, 7, 8, 37, 58–62, 71, 81 The first consideration involves the porogenic solvents. A porogenic solvent is often chosen for its poor solvation of the monomers, but this solvation can range from moderately poor to very poor based on monomer functionality and hydrophilic/hydrophobic considerations. Solvents are often chosen as microporogens and macroporogens to sustain multiple levels of porosity. Monoliths are synthesized by mixing monomers and initiators in the presence of one or even two other porogenic solvents. After mixing, the reagents are placed in a mold without additional stirring for the curing process. For acrylate-based monoliths, cyclohexanol is a typical microporogen, whereas 1-dodecanol is a typical macroporogen;5 however, many other binary and ternary solvent combinations exist. For ROMP-derived, NBE- or COE-based monoliths, 2-propanol is a macroporogen, whereas toluene can serve as a microporogen.11

Monolith formation depends greatly on phase separation, which is affected by both the precipitation and the cross-linking of growing polymer nuclei in solution. According to the Flory–Huggins theory, the interaction parameter X can be used to tailor this process.82 Adding a good solvent to the monomers shifts the pore size distribution to smaller pore sizes, and phase separation occurs late in the polymerization and is very dependent on cross-linking rather than just solubility.8 A good solvent solvates free monomer. This effect lowers the local monomer concentration in the subsequently smaller nuclei, and smaller pore sizes result. If more porogenic solvent is added that is a poor solvent for the monomer, larger pores often form. Phase separation also occurs very early in the polymerization as a result of solvation effects.8

Other ways to control the porosity include changing the amount of cross-linking reagent and the polymerization temperature. As the content of the cross-linker is increased, the pore sizes decrease.5, 8 Many cross-linked microglobules are formed and precipitate at an early stage in the reaction, and their high cross-link density leads to the low probability of coalescence between globules as they continue to polymerize. However, changing the cross-link density also requires changing the amount of the cross-linker added to the formulation. Therefore, composition of the final monolith in terms of monomer structure is also altered. Temperature is also a parameter used to control porosity and is directly related to the reaction kinetics.5, 8, 76 As described previously, the half-lives of common initiators, such as AIBN, are lower at higher temperatures. Therefore, a greater number of growing polymer nuclei exist at higher temperatures, and the formation of smaller pore sizes results.5

2.1.4. Characterization

Of great importance to supported catalysis is the measurement of both catalyst loading and catalyst leaching. The latter can be analyzed by using NMR and other standard methods, whereas catalyst loading is best analyzed with inductively coupled plasma optical emission spectroscopy (ICP–OES).42 A typical experiment would involve thoroughly drying a sample of monolith, dissolving it in aqua regia, and then measuring the metal content by using ICP–OES.42 For reactions and separations that involve large molecules such as polymers or proteins, flow properties of these macromolecules through the monolithic columns are also important. Beckert, Buchmeiser et al. studied the self-diffusion of large polymeric molecules through acrylate-based monoliths by using pulsed field gradient NMR. Self-diffusion could be correlated to the reptation of polymer chains by using the Zimm model and the Doi–Edwards model.83 The other characterization methods for monolithic materials have been described boundlessly, but the basic methods are listed only briefly. The most important tools used for the evaluation of porosity are inverse size-exclusion chromatography (ISEC), mercury intrusion, and Brunauer–Emmett–Teller gas adsorption studies.48 Although porosities, specific surface areas, pore volumes, and mean pore diameters are determined through these methods, only ISEC is performed in the solvated state. Mercury intrusion is valuable for measuring macroporosity but analysis is completed in a dry state, in which smaller pores may collapse.5 Reflectance electron microscopy can be used for the determination of diameters. Scanning electron microscopy (SEM) is a common tool used to analyze macroporosity, but resolution is not sufficient to identify smaller structures (Figure 1).1, 65, 84 Atomic force microscopy (AFM) can also be applied under solvated conditions to image both macroporosity and mesoporosity (Figure 4).1, 65, 85

Figure 4. AFM images of porous monolithic media a) under topographical tapping mode in air and b) in contact mode in water through a contour plot, in which each line represents a height increase of 100 nm. Reproduced with permission from Ref. [85], Copyright 2006 Elsevier.

2.2. Monoliths prepared by using ROMP

2.2.1. NBE- and cis-COE-based systems

The controlled, “living” character of the ROMP mechanism allows for the synthesis of well-defined monoliths that are easily post-functionalized (Figure 5).2, 68

Figure 5. Post-functional grafting of ROMP-derived monolithic supports.68

There are two main types of published ROMP-based architectures: NBE-based and COE-based monoliths. COE-based supports have a backbone structure with sec-allylic carbons, which have a higher oxidative stability than NBE-based monoliths with tert-allylic carbons.17, 26 Although the “living” nature of the polymer chains allows for greater reproducibility, the same basic principles that govern the pore sizes of free-radical-polymerized monoliths also govern pore sizes in ROMP-based monoliths. Increasing the temperature leads to smaller pore sizes, and the ROMP monoliths are typically synthesized around 0 °C.5, 11 Increasing the content of the cross-linking reagent leads to smaller pores, and changing the porogen content and type will also affect porosity. However, one additional concern is present in the case of ROMP-based monoliths: the deactivation of the “living” ruthenium- or molybdenum-based end groups.5 For example, Grubbs initiation is facilitated through phosphine dissociation,86, 87 hence the presence of excess phosphine during polymerizations delays monolith formation and produces lower amounts of microstructure, larger pores, and large-diameter microglobules.5, 29, 81

Our research group has used ROMP extensively for the synthesis of functional supports for both separation and catalysis,12–45 and we have also established procedures for high loadings of organometallic catalysts and biocatalysts through a grafting-from approach.66–68 In the last five years, there were several advances in ROMP-based monolithic media and most of them are also related to catalysis, biocatalysis, and bioseparations. These included the development of trypsin-loaded continuous bioreactors on both COE- and NBE-based monoliths through attachment to aldehyde end functionalities.44 ROMP-based monoliths for HPLC and methods for the efficient separation of insulin from its analogs were developed,16 and voltage-assisted capillary liquid chromatography of peptides by using monolithic capillary columns was successfully implemented.21 We also reported on the successful synthesis of monolithic materials through Schrock-based catalysts and their protein and amino acid separation properties,89 and we have also explored reinforcing monolithic materials with CaCO₃ nanoparticles for applications in biocatalysis and tissue engineering.63, 88 Furthermore, weak cation-exchange monoliths were synthesized from NBE-based monomers,34 and Pd-functionalized ROMP-based monoliths were prepared for Heck- and Suzuki-type reactions.11, 41 Continued efforts also exist toward the development of continuous organometallic catalysis on monolithic supports.11, 41

2.3. Other syntheses

Polycondensation and polyaddition are two other methods that have been implemented to synthesize monoliths. Hosoya et al. reported on monolithic capillary columns from epoxy-containing monomers reacted with diamine-functional monomers.90 The columns were capable of separating nucleic acids, and a chiral column based on chiral epoxy monomers was also synthesized for separations of racemic mixtures. The monoliths were synthesized from 4-[(4-aminocyclohexyl)methyl]cyclohexylamine, trans-1,2-cyclohexanediamine, and tris(2,3-epoxypropyl)isocyanurate with low molecular mass polyethylene glycols as porogens.90 Hosoya et al. also reported the synthesis of monoliths by using bisphenol A diglycidyl ether and 4,4'-methylenebis(cyclohexylamine) monomers with polyethylene glycol porogen and without any added initiator.91 In addition, Irgum et al. synthesized epoxy-based monoliths in the presence of epoxy monomers, added surfactant, porogenic solvent, H₂O, CaCl₂, and 1-diaminohexane.92 These monoliths were hydrophilic, but they did not possess enough mechanical integrity to withstand high pressures for liquid chromatography.

3. Catalysts Immobilized on Polymeric Monolithic Supports

3.1. General aspects of molecular heterogeneous catalysis

Imperative to all supported organometallic catalysts is the preservation of high activities, sufficient reaction rates, simplicity of preparation, extraction and recycling of the catalyst, and contamination or metal leaching in the products.93 The goal is to fuse the benefits of high activities and well-defined reactions by using homogeneous systems with the advantages of improved stability, separation, and recycling in heterogeneous systems.2 Catalyst leaching is still a concern for supported catalysis. Porous catalytic supports besides monolithic media have also suffered from lowered catalytic activity compared with that of unbound catalysts in solution.37, 43 However, the use of monoliths poses an advantage over other porous structures. Their large flow-through pores, which contribute to significant mass transfer, also prevent the reactions from being only diffusion controlled.37, 43

3.2. Organometallic catalysts

3.2.1. Metathesis catalysts

Among one of the most important CC bond-forming reactions for functional molecules is metathesis.93 Developments in metathesis owe largely to the advances in organometallic catalysis, and the range of metathesis products accessible has expanded to the production of rings of various sizes and structures97–99 as well as enantioselective metathesis.93, 96–98 The introduction of ROMP and acyclic diene polymerization has led to novel polymer topologies; in addition, metathesis reactions such as ring-closing metathesis (RCM) and cross-metathesis are becoming increasingly important in pharmaceutical and combinatorial chemistry.99 Typical initiators for the metathesis are Schrock [Mo(N-2,6-R₂-C₆H₃)(CHCMe₂Ph)(OR′)₂] [R=Me, iPr; R′=tBu, CMe(CF₃)₂, etc.] or first and second generation Grubbs initiators [RuCl₂(PR′′₃)₂(CHPh)] and [RuCl₂(PR′′₃)(NHC)(CHPh)] (R′′=Ph, cyclohexyl) (Figure 6).2 Thus, successful removal of the metal catalyst is necessary, and therefore advances in supported catalysis are certainly key to the continued use of metathesis for pharmaceutical applications.

Figure 6. Representative monolith-supported a) Grubbs- and b) Schrock-type catalysts.105

Grubbs- and Grubbs–Hoveyda catalysts: Within the last few years, there has been a major push for the use of metathesis catalysts as heterogeneous catalysts. Early work focused on temporarily immobilized Grubbs catalysts.37 Grubbs-type catalysts are immobilized through several means: through their phosphines or neutral ligands, the alkylidene moieties, pyridine or NHC ligands, or halogen exchange with silver carboxylates.93 Grubbs et al. first immobilized catalysts on organic supports through the phosphine ligand, but this led to low RCM activities, owing partially to incomplete substitution of phosphine, dissociation effects, and diffusion limitations. Attempts to immobilize the catalysts through the alkylidene ligands also often led to lower activities, deactivation problems, and low recycling potential.93 Blechert et al. introduced the earliest recyclable heterogeneous Grubbs catalyst93, 100 as well as anchored ruthenium to the support through the NHC ligands. These first supported versions based on 4-(poly(styrene-co-divinylbenzene)methyloxymethyl)-1,3-dimesityl-4,5-tetrahydrimidazolin-2-ylidene and [RuCl₂(PCy₃)₂(CHPh)] suffered from a diffusion-controlled environment and required long reaction times, but Buchmeiser et al.6, 37, 81, 102, 103 and Hoveyda et al.,104 respectively, synthesized organic and inorganic macroporous monolithic supports for Grubbs metathesis catalysts to circumvent these problems. These macroporous materials possessed limited microporosity, which switches their operation from a diffusion-controlled regime to a convection-controlled regime. However, residual silanol groups in silica-based porous materials irreversibly trapped phosphines, which makes the organic monolithic materials even more desirable.93

One of the first uses of monolithic media as supports for heterogeneous catalysis employed Grubbs-type initiators with NHC anchors.2, 37 NHC coordination is relatively stable and used in accordance with phosphine dissociation, which permits the production of highly active ruthenium carbenes.37 These continuous NHC-based monolithic supports were synthesized through ROMP of NBE and 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo-endo-dimethanonaphthalene (DMN-H6) with both dichloromethane and 2-propanol as porogens. Monoliths with microglobule diameters of 1.5 μm and interparticle porosities of 40 % were targeted, and subsequent in situ functionalization was accomplished by using a solution of NBE and 1,3-di(1-adamantyl)-4-{[(bicycle[2.2.1]hept-5-en-2-yl-carbonyl)oxy]methyl}-4,5-dihydro-1H-imidazol-3-ium tetrafluoroborate in dichloromethane. The carbenes were formed on the grafted polymer by using bases such as 4-dimethylaminopyridine, and after removal of the residual base, the catalyst was generated by flowing a solution of [RuCl₂(PCy₃)₂(CHPh)] over the monolithic structure. This grafting-from approach led to a catalyst loading of 1.4 wt % on the grafted polymer, which is quite significant relative to the monolith’s low surface area derived from macroporosity. The low relative microporosity of the monolith also led to the reduction of the diffusion-controlled regime and enhanced catalytic activities. Furthermore, the polymers obtained were 90 % trans, just as in homogeneous polymerizations. These NHC-supported catalysts showed high activities in ROMP and RCM reactions, and even surpassed typical turnover numbers for homogeneous metathesis of reagents such as diethyldiallylmalonate (DEDAM). The addition of chain transfer agents also allowed the efficient conversion of ruthenium methylidenes to stable alkylidenes and prolonged the lifetime of the catalysts. Ruthenium-free RCM products that contained a ruthenium content of less than 70 ppm were obtained.37

Monolithic media in the form of a disc are also exceedingly convenient for combinatorial chemistry and high-throughput screening (HTS). The disc format can be designed to fit already commercially available instruments.105 Buchmeiser et al. also synthesized ROMP-based monolithic discs for metathesis with a loading of 0.55–0.7 wt % through NHC anchoring of a Grubbs catalyst, [RuCl₂(PCy₃)(NHC)(CHPh)] [NHC=1-(2,4,5-trimethylphenyl)-3-(6-hydroxyhexyl)-imidazol-2-ylidene]. Immobilization was achieved through chemical modification of acid chloride-containing monolithic grafts. Synthesis of the monolithic core was facilitated through ROMP with NBE and tris(norborn-5-ene-2-ylmethyleneoxy)methylsilane [(NBE-CH₂O)₃–SiCH₃] with 2-propanol and toluene as porogens and [RuCl₂(PCy₃)₂(CHPh)] as the initiator. The use of the Grubbs catalyst-containing monoliths produced high catalytic activities and negligible metal leaching of less than 3 % in ring-opening cross-metathesis and enyne metathesis reactions. They were also used in the cyclopolymerization of diethyl dipropargylmalonate, which resulted in five-membered ring polyene formation with number-average molecular masses around 11 700 g mol⁻¹, monomodal molecular mass distributions, and polydispersity index values around 1.4.105

Alternatively to exploiting the NHC ligand as the binding point, carboxylate-bound second generation Grubbs catalysts were synthesized. Early carboxylate-bound Grubbs catalysts experienced problems with metal leaching, owing to chloro(trifluoroacetate)ruthenium complexes, which undergo ligand scrambling to the dichloro- and the bis(trifluoroacetate)ruthenium complexes.93, 106, 107 However, Buchmeiser et al. proved that deleterious scrambling could be avoided by using a bidentate carboxylate ligand.39, 106 Grafting of anhydride functional monomers, namely, 7-oxanorborn-2-enedicarboxylic anhydride, was achieved on a classic ROMP-based monolithic support.6, 107, 108 Conversion of the grafted anhydrides to silver carboxylates allowed the attachment of second generation Grubbs catalysts through halogen exchange (Figure 7).6, 39, 107, 108

Figure 7. Immobilization of Grubbs–Herrmann ruthenium catalysts with use of silver carboxylates.39

RCM reactions with these supports showed high DEDAM turnover numbers around 1000 and low ruthenium contamination (<3 ppm), with catalyst loadings of approximately 3.5 mg Ru g⁻¹. Because the use of 7-oxanorborn-2-ene-5-carboxylic acid instead of the dicarboxylic anhydride monomer led to significantly lower turnover numbers, the silver carboxylates also proved to function as reversible phosphine scavengers.93 Continued research by Buchmeiser et al. using Grubbs–Hoveyda-type catalysts [RuCl₂(IMesH₂)(CH-2-(2-PrO)-C₆H₄)] (Mes=mesityl group) showed that electron-withdrawing groups further improved catalytic activities.93, 106, 110, 111

Schrock catalysts: Immobilization of Schrock catalysts on monolithic supports can occur through alkoxide ligands, biphenyl or binaphthyl ligands, arylimido ligands, or alkylidene ligands.93 Research on the immobilization of these catalysts is limited, owing to their sensitivity to moisture and oxygen, and the difficulty of their preparation compared to that of Grubbs initiators. A few organic polymer-based monolithic Schrock catalyst supports are reported.105 However, Schrock catalysts are preferred over Grubbs catalysts in terms of their enantioselectivity and reactivity.105

Although Schrock and Hoveyda et al. described a well-defined supported Schrock catalyst for enantioselective reactions using p-styrylethyl biphenoxide, the supported catalyst showed comparably low recycling potential.112 Promisingly, enantioselectivity was similar to the unbound catalyst, and low metal leaching was observed. Capitalizing on this principle, Buchmeiser et al. supported a chiral Schrock catalyst on ROMP-based materials from bis(NBE)-substituted chiral phenoxide.113 The same chemistry was then applied to synthesize the first monolithic Schrock-type supports: monolithic discs for HTS.105 Support of the catalyst was achieved through modification of the phenoxide groups with potassium hydride and subsequent addition of [Mo(N-2,6-iPr₂-C₆H₃)(CHCMe₂Ph)(OTf)₂⋅DME] (Figure 8).

Figure 8. Immobilization of Schrock-type catalysts on monolithic discs.105

These monoliths possessed porosities around 60 % and functioned in three ways: as a support, as a filtration device, and as a compartment for metathesis reactions.105 Metal leaching into the products was also less than 3 % of the total amount of metal used, and enantioselectivity remained similar to unsupported Schrock catalysts. Buchmeiser et al. also reported the only successful immobilization of a Schrock catalyst through the arylimido ligand by using Ag-perfluoroalkanesulfonate-modified polystyrene-divinylbenzene materials and ω-halogenoalkyl groups on the aryl imido ligands.114 However, this chemistry has not yet been applied to monolithic structures, but it did further support that enantioselectivity is preserved for polymer-bound Schrock catalysts. Continued efforts in the Buchmeiser group exist toward facilitating monolithic-supported Schrock catalysts under continuous flow conditions.

3.2.2. Cu-based catalysts for carbonyl hydrosilylations and hydrocyanations

Hydrosilylation and hydrocyanation are two other industrially important reactions that are catalyzed by copper-based reagents. Buchmeiser et al. synthesized Cu-containing monolithic supports for both hydrosilylation and hydrocyanation reactions (Figure 9).42 The core monolith structure consisted of EMA and trimethylolpropane triacrylate polymerized through electron beam-triggered free radical polymerization in the presence of 1-dodecanol, 2-propanol, and toluene as porogenic solvents. Either 1,3-dimesityl-5-(methacryloyl)-3,4,5,6-tetrahydropyrimidin-1-ium bromide or 1,3-bis(2-propyl)-5-(methacryloyl)-3,4,5,6-tetrahydropyrimidin-1-ium tetrafluoroborate was grafted through electron beam-triggered free radical polymerization onto the preformed monolithic supports as methacryloyl-substituted NHC polymer precursors. Preparation of the ligands was implemented with sodium tert-butoxide, followed by functionalization with CuBr⋅SMe₂. Copper loadings were in the range of 1.3–3.2 mg g⁻¹. These Cu-modified supports were used in continuous flow reactions for cyanosilylation. Although reactions of p-fluorobenzaldehyde and p-chlorobenzaldehyde exhibited slow exchange of bromide and cyanide anions, reactions of p-chlorobenzaldeyde and benzil with trimethylsilyl cyanide maintained reactivity for several days with high turnover numbers, 9500 and 1060, and indicated less than 10 ppm of the catalyst in the final products. Hydrosilylation of p-chlorobenzaldehyde or benzophenone with trimethylsilane produced high turnover numbers, 900 and 830, and metal leaching was less than 13 ppm.42

Figure 9. Synthesis of Cu^I–NHC immobilized monoliths.42

3.2.3. Pd-based catalysts for CC coupling reactions

Undoubtedly, the award of the Nobel Prize in Chemistry in 2010 to Richard F. Heck, Ei-ichi Negishi, and Akira Suzuki exemplifies the applicability of Pd CC coupling reactions in organic synthesis. Thus, supports with immobilized Pd for Heck, Suzuki, and Sonogashira–Hagihara-type cross-couplings are highly desirable for recyclability and automation. Frost and Mutton wrote a review in 2010 that summarized the various achievements in Pd-catalyzed reactions using polymeric monoliths prepared from free radical polymerization inside glas microreactors.115 Kirschning et al. were highlighted as having explored Heck, Suzuki, and Sonogashira-type reactions using monolithic media inside porous glass rods and glass Raschig rings.116–128 Polymerizations utilized vinyl benzyl chloride monomer and divinylbenzene cross-linker with AIBN as the initiator. Quaternization with various amines, imidazoles, or pyridines resulted in ionic media, and Pd was loaded through ion exchange using sodium tetrachloropalladate solutions, followed by reduction to Pd⁰ with sodium borohydride.115–119 The Pd nanoparticles within the media were around 7–10 nm, which results in typical Pd loadings around 0.03 wt %.120 Various Heck reactions that include the reaction between p-iodoanisole and n-butylacrylate with triethylamine as the base were implemented with short reaction times and high conversions. The authors also explored copper-free Sonogashira reactions and Suzuki–Miyaura reactions by using similar microreactors.116–128

Guijt et al. also synthesized capillary-size monoliths by using glycidyl methacrylate and ethylene dimethacrylate monomers for Suzuki–Miyaura reactions.115, 129 Phenanthroline was grafted to the remaining epoxide groups, and subsequent loading of Pd was achieved by using a solution containing PdCl₂(NCMe)₂. Conversions up to 68 % were realized by using iodobenzene and p-tolylboronic acid.115, 129 Guijt et al. also used p-chloromethylstyrene- and divinylbenzene-containing monoliths in 250 μm capillaries for Pd immobilization.130 Either 5-amino-1,10-phenanthroline or 1-methylimidazole was grafted to the monolith through reaction with benzyl chloride groups, and Pd was loaded through reaction with PdCl₂(NCMe)₂ solutions. Imidazolium-based monoliths were believed to contain both NHC-grafted Pd and [PdCl₃(NCMe)]⁻ counterions and possessed Pd loadings around 0.4 wt % compared with 0.3 wt % loadings for the phenanthroline-containing monoliths. Various Suzuki–Miyaura and Sonogashira reactions were examined and resulted in exceptional yields.130

Luis et al. also grafted imidazolium groups onto p-chloromethylstyrene-co-divinylbenzene-derived monoliths.131 Exchange of chloride counterions with [PdCl₄]⁻ was facilitated with PdCl₃ and HCl, and Pd⁰ was achieved through reduction. Also, Pd(OAc)₂ was used under basic conditions to facilitate Pd–NHCs followed by reduction. However, an important feature of these microreactors was to maintain catalyst loadings that were below the imidazolium loading in order to promote the recapture of soluble and participating catalyst under a release-and-capture mechanism. This afforded low to no observable Pd leaching. Heck reactions produced high yields with use of dimethylformamide or near-critical ethanol as the solvent and iodobenzene and methyl acrylate as substrates.131

Ruthenium-polymerized ROMP-based monoliths can also be used for monoliths to immobilize Pd, but using pyridyl ligands then becomes challenging because they tend to coordinate to the ruthenium moieties.2 Therefore, a presynthesized monomer complex was employed, N,N-dipyrid-2-yl-7-oxanorborn-2-en-5-ylcarbamido palladium dichloride, along with NBE to graft palladium catalysts onto the polymeric monoliths and to prevent coordination of the dipyridylamide ligands with the living ruthenium groups on the monolith.2, 132 Pd loadings up to 0.07 % were achieved, and turnover numbers of 1.2–1.6 s⁻¹ resulted for styrene and iodobenzene model reactions.2, 132 This result is comparatively greater than that of polymer-supported catalysts from ROMP-based precipitation polymerization.2 Palladium leaching in the products was less than 2.2 %. Moreover, an additional coating procedure was developed to replace the grafting-from approach. Monoliths could also be coated with poly(N,N-di(pyrid-2-yl)norborn-2-ene-5-yl-carbamide) and treated with H₂PdCl₄.2, 20, 22, 81, 105 These supports contained greater amounts of Pd, up to 0.33 wt %, and this method was further used for HTS cartridges under continuous flow conditions. Continuous flow reactions of styrene with iodobenzene in tributylamine showed production of stilbene over 3 h. However, turnover numbers were comparable to those of supports from ROMP-based precipitation polymerization.132, 133 Buchmeiser et al. also introduced the formation of palladium nanoparticles in the pores of monolithic media for Heck, Suzuki, and Sonogashira–Hagihara-type cross-couplings,11, 41 which is discussed further below.

3.3. Metal nanoparticles

Nanoparticle-filled monoliths have been synthesized for both localized organometallic catalysis11, 41 and for stimulation of tissue growth.92 Although nanoparticle-based filler aided in the reinforcement of polymer matrices, one of the key issues that underlie their use in monoliths was Ostwald ripening or deformity of their placement inside the monolith over time. Kirschning et al. earlier explored the use of Pd-based nanoparticle immobilization for microreactor technology.115–128 However, recent advances include selective placement of the nanoparticles inside the microporous regions of the polymeric support to avoid displacement.41

Buchmeiser et al. selectively formed palladium nanoparticles in the small pores of monolithic media for Heck, Suzuki, and Sonogashira–Hagihara-type cross-couplings (Figures 10 and 11).11, 41 Electron beam-triggered initiation was used to polymerize glycidyl methacrylate and trimethylolpropane triacrylate with the solvents 2-propanol, toluene, and 1-dodecanol. The authors used pore-size-selective functionalization to locate Pd-based nanoparticles only in the small pores to maintain a high life span of the particles within the monolith. The epoxy groups in the large pores were hydrolyzed with poly(styrene sulfonic acid), and any remaining epoxy moieties in the small pores were reacted with norborn-5-en-2-ylmethylamine and a first generation Grubbs initiator to graft various NBE-based dicarboxylic anhydride monomers, carboxylic amides, and phosphonates. H₂PdCl₄ was used to immobilize Pd through the di-2-pyridylamide ligand with loadings of 2.7 mg Pd g⁻¹, and reduction with NaBH₄ produced nanoparticles less than 2 nm in diameter. The subsequent use of these Pd-nanoparticle-loaded monoliths resulted in high turnover numbers of around 167 000 and 63 000 for Heck- and Suzuki-based coupling reactions.41

Figure 10. Representative pore-size-selective functionalization of monolithic media synthesized through electron beam-triggered free radical polymerization.41

Figure 11. Selective immobilization of metal nanoparticles within the small pores of monolithic media synthesized through electron beam-triggered free radical polymerization.41

Buchmeiser et al. also synthesized COE-based monoliths with Pd nanoparticles for Sonogashira–Hagihara- and Suzuki-type couplings.11 The monomers (Z)-9-oxabicyclo[6.1.0]non-4-ene and tris(cyclooct-4-en-1-yloxy)methylsilane were used along with a second generation Grubbs catalyst and both 2-propanol and toluene were used as porogenic solvents to generate monolithic structures (Figures 12 and 13).

Figure 12. Pore-size-selective functionalization of COE-based monolithic media.11 Copyright 2011 ARKAT USA, Inc.

Figure 13. Immobilization of palladium within the small pores of cis-COE-based monolithic media.11 Copyright 2011 ARKAT USA, Inc.

The epoxy sites in the large pores were selectively hydrolyzed with poly(styrene sulfonic acid), and subsequent reaction with N,N-dipyrid-2-ylamine functionalized the small pores that were less than 6 nm in diameter with loadings around 6–7 μmol g⁻¹ of monolith, and loadings of 1 mg g⁻¹ of Pd²⁺ were accomplished by using procedures similar to those used in the previous work with the acrylate-based monoliths. Reduction led to Pd-based nanoparticles that were less than 4 nm in diameter and that were later used in both Suzuki and Sonogashira–Hagihara couplings with low metal leaching. Comparing the use of the acrylate-derived versus ROMP-derived monoliths in Pd-based Suzuki coupling reactions, we found that the acrylate-based monoliths had a higher number of small pores, higher Pd loadings, and therefore better turnover numbers than did the ROMP-based materials.11

Palladium-based nanoparticles find direct application in continuous catalysis, but a noteworthy advancement in nanoparticle-containing monoliths also involves the development of CaCO₃ nanoparticle-reinforced monolithic materials to support and sustain tissue growth.88 There is an increasing demand for the use of porous monolithic media in biocatalysis and as biosupports due to the simplicity of their preparation, their high throughput, and their ability to withstand higher flow rates or pressures. Their large pores also facilitate the in-growth of cells into their structure and even support cell differentiation.63 In this nanoparticle study, the CaCO₃ nanoparticles were synthesized with CaCl₂ and anhydrous Na₂CO₃ in water and then surface-functionalized with exo,endo-norborn-5-ene-2-ylphosphonic acid. Covalent attachment of the nanoparticles to the polymer matrix occurred during synthesis of the monolith in the presence of porogenic solvents. NBE was used along with a 7-oxanorborn-2-ene cross-linking reagent and a third generation Grubbs initiator [RuCl₂(Py)₂(IMesH₂)(CHPh)]. Inclusion of the CaCO₃ nanoparticles was homogeneous and reinforced the polymer matrix, which provides monoliths with greater mechanical integrity.88

Additional monoliths were prepared by using the polar monomer cis-5-cyclooctene-trans-1,2-diol and a biocompatible 7-oxanorborne-2-ene-based cross-linking reagent with 12 wt % or less of either CaCO₃ or calcium hydroxyapatite (HAp) nanoparticles.134 These nanoparticles were of size 50–100 nm obtained by SEM and less than 20 nm platelets obtained by X-ray diffraction. HAp nanoparticles were synthesized from CaCl₂⋅H₂O, H₃PO₄, and NH₄OH. SEM images confirmed that increasing the amount of nanoparticles also led to an increase in the pore sizes of the monolith, and the inclusion of HAp resulted in the largest pore sizes due to its greater polarity than CaCO₃. Although incorporation of a filler improved the hardness of the monoliths, their porosity also increased with increasing filler to a point at which porosity usurped the dominating role. Therefore, the hardness of the monolithic materials increased to a maximum and then decreased as more and more filler was added. Human adipose tissue-derived stromal cells were used to examine biocompatibility, and cell growth was observed to 30 times the original value over 4 days. Cell growth continued through 8 days of cultivation.134

3.4. Enzymes

Fréchet, Svec et al. were among the first to optimize the acrylate-based monoliths for high-throughput biocatalysis in the 1990s.10, 43 Their main goal was to immobilize enzymes such as trypsin. Trypsin, which breaks down peptides or proteins into smaller units for absorption is found in the digestive system, and its main role is to hydrolyze proteins through a process called proteolysis or trypsinization. Svec, Fréchet et al. synthesized poly(2-vinyl-4,4-dimethlazlactone-co-acrylamide-co-ethylene dimethacrylate) monoliths to covalently immobilize trypsin by using peripheral azlactone functionalities. The immobilized catalysts showed high activity in the hydrolysis of L-benzoyl arginine ethyl ester and casein under flow rates up to 180 cm min⁻¹. Activities around 810 mmol min⁻¹ mL⁻¹ were achieved at flow rates of 102 cm min⁻¹ at 75 °C.43

A high-molecular-mass protein is difficult to hydrolyze into many smaller units. A high loading of catalyst must be achieved, and the protein must readily diffuse into the pores of the catalytic support. Large proteins, such as casein, can also form viscous solutions. Monoliths possess both small pores to achieve a large amount of functional surface area for catalysis and large flow-through pores that prevent high back pressures with viscous solutions. Therefore, trypsin supported on a monolithic column is a facile way to lyse even large proteins. The authors found an effectiveness of 33.8 % for the hydrolysis of casein on an acrylate-cross-linked monolithic column functionalized with trypsin by using 1 mg mL⁻¹ of casein solution and a flow rate of 12.7 cm min⁻¹ at 37 °C.43

The authors also investigated the catalytic conditions in depth, which included the column activity over time, the activity with different flow rates, the effect of temperature, the effect of pH, and the products of trypsinization. The activity of the trypsin-functionalized column toward the hydrolysis of casein was maintained for 24 h but started to deactivate within 48 h of operation. The flow rate of casein through the column as adjusted with temperature also affected the catalysis, and fast flow rates up to 102 cm min⁻¹ still produced sufficient activities around 750 mmol min⁻¹ mL⁻¹. A pH of 8 was ideal for catalysis with both unbound and bound trypsin, but the effect of changing the pH was much less dramatic if supported trypsin was used; activities of 30–62 % still remained at pH 6 and 10 for immobilized trypsin.43 Svec and Fréchet et al. also used MALDI–TOF and ESI–TOF mass spectrometers to monitor the digestion of proteins after trypsinization to map the wide range of amino acid products found.30

Furthermore, Svec and Fréchet et al.31 prepared microfluidic devices that were able to perform both solid-phase extraction and enzyme-based catalysis by using porous poly(butyl methacrylate-co-ethylene dimethacrylate) capillary-size monoliths. The columns were used to nanoelectrospray analytes into a mass spectrometer. The authors selectively functionalized 20 mm of the 25 mm capillaries by UV-initiated photografting of poly(2-vinyl-4,4-dimethylazlactone) and post-functionalization with trypsin. The trypsin-functionalized portion functioned as an enzymatic reactor, whereas the nonfunctionalized section of the capillaries functioned as a solid-phase extractor. Absorption and digestion of myoglobin with various solution concentrations were monitored in both flow directions. The use of the enzymatic reactor in the solid-phase extraction direction allowed for enhanced separation of undigested components and indicated the potential use of these columns in proteomic separations.31

Buchmeiser et al. also recently synthesized capillary-based monolithic biocatalysts by using immobilized trypsin (Figure 14).40 However, electron beam-triggered polymerization was used with EMA and TMPTA along with toluene and 2-propanol as porogenic solvents to make the core monolithic structure. Post-synthetic grafting was achieved by using NBE- or acrylate-based monomers that contain N-hydroxysuccinimidyl esters through electron beam-triggered polymerization or ROMP, respectively. These N-hydroxysuccinimidyl esters allowed for covalent attachment of trypsin through aminolysis. The ROMP-grafted polymers performed as the best supports for trypsin and promoted protein adhesion, which facilitated 100 % hydrolysis of N-a-benzoyl-L-arginine ethyl ester (BAEE) to benzoyl-L-arginine. Furthermore, their principal applicability as supports for peptide synthesis was studied through acyl transfer reactions of BAEE to amino acid amides by using immobilized and succinylated trypsin.40 These studies demonstrated the combination of biocatalysis, specification or functionalization of products, and separation of analytes using a single monolithic support.

Figure 14. Protein functionalization of electron beam-derived polyacrylate-based monoliths through a) active esters and subsequent aminolysis or b) grafting of an NBE-containing acrylate, production of active esters, and subsequent aminolysis of the proteins.40

Further work led to the development of immobilized trypsin for continuous reactions on ROMP-based monolithic supports.44 Synthesis of ROMP-based monoliths proceeded by using NBE and COE with Grubbs-type initiators, and postreaction with oxygen terminated the living polymer ends with aldehyde end groups. Approximately 80 % of the end groups were converted to aldehydes on semitelechelic poly(NBE) and poly(COE) derivatives. Aldehyde functionality was determined by NMR analysis of hydrazones formed from conversion of polymer-based aldehyde end groups with 2,4-dinitrophenylhydrazine. The same approach was used to create aldehyde-terminated monoliths from NBE, DMN-H6, and (NBE-CH₂O)₃SiCH₃. The aldehyde end groups were then used to immobilize trypsin on the solid support, and the polymer-bound biocatalyst was analyzed under continuous flow conditions for 3 h. Although the bioactivity gradually decreased over time, this decrease was less than 20 %.44

4. Summary

Although acrylate- and ROMP-based monoliths continue to assume their role as the most popular organic polymeric monoliths, significant enhancements in the definition of grafting architecture and well-defined surface modifications have emerged in the last few years. Recent achievements include the attachment or grafting of organometallic compounds, such as Grubbs, Schrock, palladium, and copper catalysts. In addition, the introduction of pore-size-selective modifications has led to the selective immobilization of metal nanoparticles within the small pores of monolithic supports. Monolithic media continue to contribute to advancements in biocatalysts and separation chemistry, and the focus of current research efforts is on the realization of automated and continuous flow systems.

Acknowledgements

This work was supported by the Deutsche Forschungsgemeinschaft (DFG, BU 2174/4-1, BU 2174/2-1) and the State of Baden-Württemberg, Germany.

Biographical Information

Michael R. Buchmeiser received his Ph.D. in Organometallic Chemistry at the University of Innsbruck, Austria, working on early- and late-transition-metal metallocenes. He was awarded an “Erwin Schrödinger Fellowship” and spent one year at the Massachusetts Institute of Technology (Cambridge, MA) working on poly(metallocenylacetylene)s and fluorinated organomolybdenum compounds. In 2009, he chose to accept a full professorship in Macromolecular Chemistry at the University of Stuttgart, Germany. Since then, he has also been the Director of the Institute of Textile Chemistry and Chemical Fibers (ITCF), Denkendorf, Germany. His research interests include transition-metal-catalyzed polymerizations, chemical and physical surface modifications, porous polymeric supports and their applications in the areas of heterogeneous catalysis, separation, and life sciences. More recently, he also became involved in various aspects of fiber chemistry, including conductive fibers and high-performance polymeric and inorganic fibers. He is a member of the International Advisory Board of Macromolecular Rapid Communications, Macromolecular Chemistry and Physics, and Macromolecular Materials and Engineering. He has published more than 200 research papers and holds 20 patents. He received the “1998 Professor Ernst Brandl Research Award,” the “START Award-2001,” the “Novartis Award 2001,” and the “Otto Roelen Medal 2010.”

Biographical Information

Emily Baird Anderson acquired her BS in Chemistry in 2005 from the University of North Carolina at Chapel Hill, where she worked as an undergraduate researcher for Prof. Michael Rubinstein. While pursuing her Ph.D. at Virginia Tech, she studied under Dr. Douglas Robello in a 2007 summer internship at Eastman Kodak. In 2008, she was presented with an Excellence in Graduate Polymer Research Award and was selected to deliver an oral presentation in a special ACS symposium. The following year, Emily chaired the Polymers Gordon Research Conference (GRC) Graduate Student Seminar (GRS) for graduate students and postdoctoral researchers. In 2010, Emily completed her Ph.D. in Polymer Chemistry under the guidance of Prof. Timothy E. Long at Virginia Tech in Blacksburg, VA. Currently, she is a postdoctoral researcher with Prof. Michael R. Buchmeiser at the University of Stuttgart in Germany.