Green polymer chemistry - CyberLeninka

European Polymer Journal 47 (2011) 524–534

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Feature Article

Green polymer chemistry: Precision synthesis of novel multifunctional poly(ethylene glycol)s using enzymatic catalysis Judit E. Puskas ⇑, Kwang Su Seo, Mustafa Y. Sen Department of Polymer Science, The University of Akron, Akron, OH 44325-3909, USA

a r t i c l e

i n f o

Article history: Available online 2 November 2010 Dedicated to Professor Nikos Hadjichristidis in recognition of his contribution to polymer science. Keywords: Green chemistry Enzymes Poly(ethylene glycol) Functionalization Dendrimer

a b s t r a c t This paper gives an overview about enzyme catalysis, and reports the precision synthesis of multifunctional poly(ethylene glycol)s using this green chemistry approach. Specifically, vinyl acrylate was transesterified with tetraethylene glycol (TEG) and a PEG with DPn = 23, and then (HO)2–TEG–(OH)2 and (HO)2–PEG–(OH)2 were synthesized by the Michael addition of diethanolamine to the acrylate double bonds. These structures will serve as the core of novel dendrimers designed for drug delivery applications. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction In tune of the globally increasing interest in ‘‘green or greener’’ chemistry, including the efficient use of energy, hazard reduction, waste minimization and the use of renewable resources, designed to prevent pollution and reduce resource consumptions [1,2], we have been exploring the power of enzyme catalysis in polymer functionalization. Enzymatic catalysis in organic synthesis has emerged as an attractive ‘‘green chemistry’’ alternative to conventional chemical catalysis [3]. Enzymatic catalysis has now been applied to polymer synthesis [4–6] and functionalization [7–12] with several advantages, including high efficiency, recyclability, the ability to operate under mild conditions, and environmental friendliness [3]. There are several excellent reviews on this topic [13–16]. This paper will be discussing our latest results towards the design and synthesis of novel multifunctional poly(ethylene glycol)s PEGs and PEG dendrimers for drug delivery applications. Enzyme catalysis gives unprecedented control in

⇑ Corresponding author. E-mail address: [email protected] (J.E. Puskas). 0014-3057/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2010.10.015

polymer chemistry, similarly to living anionic polymerization techniques pioneered by Professor Nikos Hadjichristidis [17–21]. This paper is dedicated to him, on the occasion of his retirement.

2. Enzymes in organic chemistry It was recognized over 30 years ago that enzymes were not limited to operate in their native aqueous media and they could efficiently act as catalysts for the biotransformation of a wide range of substrates in organic solvents. So far, about 3000 enzymes have been identified and classified by the International Union of Biochemistry and Molecular Biology into six categories according to the type of reactions they can catalyze (Fig. 1) [3]. Among these, oxidoreductases and hydrolyses are the most widely used catalysts in biotransformations. For example, in the 1987–2003 time periods about 85% of all enzyme research was performed with hydrolases and oxidoreductases (60% and 25%, respectively) [3]. Lipases belong to the group of hydrolyses, and are the most popular biocatalysts. They are widely used in esterification, transesterification, aminolysis, and Michael addition reactions in organic solvents [13].

J.E. Puskas et al. / European Polymer Journal 47 (2011) 524–534

525

Fig. 1. Classification of enzymes.

Lipase-catalyzed transesterification reactions are especially useful for the preparation of optically active compounds by asymmetrization or resolution of racemic or prochiral substrates (Fig. 2) [22]. Although both lipases and proteases (the enzymes that hydrolyse peptide bonds in vivo) have been used as transesterification catalysts, the latter found limited application due to high substrate selectivity [23–25]. Several parameters such as the structure of acyl donor, the enzyme source, the structure of alcohol and the reaction stoichiometry must be taken into account for a successful transesterification. Transesterification reactions are generally reversible. In order to change the reversible nature of the reaction into an irreversible type, the nucleophilicity of the leaving group of the acyl donor should be depleted by the introduction of electron-withdrawing groups such as trifluoroethyl or trichloroethyl into the ester [26]. Alternatively, the use of oxime esters [27], thioesters [28], and anhydrides [29] as activated acyl donors have been proposed. The use of enol esters [30], such as vinyl or isopropenyl esters appears to be the most useful since they liberate unstable enols as by-products which rapidly tautomerize to give the corresponding aldehydes or ketones (Fig. 2). Therefore, the reaction becomes completely irreversible. It was shown that acyl transfer reactions using enol esters are 100 to 1000 times faster than the reactions using non-activated esters such as ethyl acetate [30]. Vinyl esters are favored over isopropenyl esters because of less steric hindrance and thus higher reaction rates [31]. Acetaldehyde, which forms during the reactions with vinyl esters, is known to inactivate the lipases from Candida rugosa and Geotrichum candidum by forming a Schiff’s base with the lysine residues of the protein; however most lipases, including Candida antarctica lipase B (CALB), tolerate the lib-

erated acetaldehyde [32]. Yadav and Trivedi [33] compared the catalytic activity of various commercially available lipases in transesterification of vinyl acetate with n-octanol. CALB, Mucor meihei lipase and Pseudomonas lipase were immobilized on macroporous polyacrylic resin beads, anionic resin and diatomite, respectively, whereas Candida rugosa lipase and porcine pancreatic lipase were in free form. It was observed that CALB was the most efficient lipase. The structure of the alcohol is another important parameter affecting the initial rate and overall conversions in enzymatic transesterification. It was observed that straight-chain alcohols gave better conversion compared to aromatic and branched-chain alcohols in the CALB-catalyzed transesterification of vinyl acetate due to less steric hindrance around the hydroxyl group (Fig. 3) [33]. Furthermore, aromatic alcohols with saturated shorter side chains (e.g. benzyl alcohol) were more reactive than those with longer but unsaturated side chains (e.g. cinnamyl alcohol); however, no explanation about this finding was given. Although an increase in ester concentration was shown to increase the rate and conversion of transesterification [33,34], an increase in alcohol concentration might cause reduced rates and conversions due to competitive inhibition by the alcohol which can bind reversibly to the enzyme active site and prevent the binding of the ester substrate [33,35]. The driving force for alcohol binding might be the high polarity of the region around the active serine site of the enzyme [33]. For example, Yadav and Trivedi [33] varied the concentration of n-octanol while keeping the amount of vinyl acetate (1 mol/L) constant in CALB-catalyzed transesterification. They observed an increase in the reaction rate when n-octanol concentration was increased from 0.25 to 1 mol/L; however, upon further increase the rate decreased. Comparison of the catalytic activities of CALB and distannoxane, a conventional tin-based catalyst revealed that

Fig. 2. Asymmetrization of a prochiral diol by CALB-catalyzed enantioselective transesterification to produce an intermediate for antifungal agents.

Fig. 3. The reactivity of various alcohols in CALB-catalyzed transesterification of vinyl acetate.

2.1. Lipase-catalyzed transesterification

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the transesterification of vinyl acetate with 2-phenyl-1propanol (Fig. 4) catalyzed by CALB was complete in 2 h, while the traditional tin-based catalyst yielded 95% conversion in 12 h [36]. The reaction was irreversible because the vinyl alcohol product instantaneously tautomerized into acetaldehyde, which was easily removed from the system due to its low volatility. However, when the methyl group was replaced by a polyisobutylene chain, no reaction occurred due to the steric hindrance (Fig. 5) [37]. The tertiary chloride chain end of PIB–OH (Mn = 8200 g/ mol, Mw/Mn = 1.13) was removed by dehydrochlorination using t-BuOK [38]; and the resulting polymer was reacted with vinyl methacrylate in the presence of CALB. 1H NMR spectroscopy revealed that the polymer remained intact as the spectrum of the reaction product was identical to that of the starting material. Another example of steric effect on the transesterification was demonstrated using primary hydroxy-functionalized polystyrenes, which were prepared by end-capping of poly(styryl)lithium with ethylene oxide (Mn = 2100 g/mol, Mw/Mn = 1.07) and (CH3)2SiClH followed by hydrosilation with allyl alcohol (Mn = 2600 g/mol, Mw/Mn = 1.06) [39], respectively. The ethylene oxide end-capped PS–OH did not react while a PS–OH with a (CH3)2Si–CH2 spacer was quantitatively methacrylated by transesterification of vinyl methacrylate within 48 h [10]. 2.2. Lipase-catalyzed Michael addition The Michael addition, one of the most fundamental reactions in organic synthesis, is the 1,4-addition of a nucleophile to an activated a,b-unsaturated carbonyl compound. The first enzymatic Michael addition was reported in 1986, in which 2-aminophenol was reacted with 2-(trifluoromethyl) propenoic acid in a phosphate buffer in the presence of the lipase from Candida cylindracea, reaching 83% conversion [40]. However, only a few groups have focused on enzyme-catalyzed Michael-type additions since

HO n

Cl

t-BuOK HO

O O

O O +

n

CALB, Hexane 50 oC, 24 h

n

O H

Fig. 5. Unsuccessful transesterification of vinyl methacrylate with dehydrochlorinated PIB–OH prepared from the a-MSE/TiCl4 initiator system.

then. Hydrolases, specifically lipases and proteases, have been used as Michael addition catalysts. Cai et al. [41] investigated the Michael addition of pyrimidine derivatives to acrylates using an alkaline protease from Bacillus subtilis (Fig. 6). Among several organic solvents with differing polarity, DMSO was found to be the most effective solvent. Higher yields were obtained when the pyrimidine had a more electron-withdrawing group and smaller substituent and the acrylate had a shorter alcohol chain, but no explanation for these results were given. The same researchers carried out a systematic study of the Michael-type addition of imidazoles to acrylates using several commercially available serine hydrolases [42]. In the reaction of imidazole with methyl acrylate, Amano lipase M from Mucor javanicus showed the highest catalytic activity (96% conversion). In addition, higher conversions were achieved in less polar solvents. However, in the reaction of imidazole derivatives containing a nitro group, this trend was not observed due to their poor solubility in nonpolar solvents. Donors with a more electron-withdrawing group and acceptors with a shorter alcohol chain gave better yields. When the acceptor had the same chain length, the yields with acrylates were higher than with methacrylates. In another study, the initial reaction rate of the Michael addition of 4-nitro-imidazole to methyl acrylate in DMSO in the presence of Amano acylase from Aspergillus oryzae, a protease containing a zinc ion in the active site, was found to be 638-fold faster than the reaction in the

Fig. 4. Transesterification of vinyl acetate: comparison of CALB and a tin-based catalyst.


R O

NH N H

O

R: H, CH3, F, Br

+

O O

R'

Protease from Bacillus subtilis

R': CH3 CH2CH3 CH2CH2CH2CH3

50

oC

O

R O

O

N N H

527

R'

O

Fig. 6. Enzymatic Michael addition of uracil and its derivatives to acrylates.

absence of a biocatalyst [43]. This enzyme was also shown to effectively catalyze the addition of a variety of N-heterocycles to a,b-unsaturated compounds. The addition of 1,3-dicarbonyl compounds to a,b-unsaturated compounds was also reported (Fig. 7) [44]. Comparison of several hydrolases revealed that D-aminoacylase from Escherichia coli, which is another zinc-dependent protease, was the best catalyst for the addition of ethyl acetoacetate to methyl vinyl ketone in 2-methyl-2-butanol. When the reaction was carried out in different organic solvents, no correlation between the enzyme activity and the partition coefficient (logP) of the solvent was observed. However, this enzymatic behavior could not be explained. 3. Enzymes for the functionalization of synthetic polymers Before our group has started working on the functionalization of preformed synthetic polymers, only a few examples were available in the literature. When polybutadiene was reacted with hydrogen peroxide and a catalytic amount of acetic acid in the presence of CALB, only about 60% of the units in 1,4-enchainment (cis and trans) were epoxidized, without any change to the pendant vinyl groups [9]. The inability to reach higher conversion was ascribed to the conformational changes in the partially epoxidized polymer. Battistel et al. [45] reported the hydrolysis of the pendant nitrile groups of polyacrylonitrile fibers into the corresponding amides using a nitrile hydratase enzyme with 16% conversion. The CALB-catalyzed esterification of poly(acrylic acid) with various polyols showed that the reactions were highly regioselective and that only one of the alcohol groups of the polyol reacted with a pendant acrylic acid group [46] (Fig. 8). Furthermore, the enzyme did not catalyze intermolecular reactions between the free hydroxyl groups of the esterified polyol and the carboxylic acid groups of another poly(acrylic acid) chain. Based on 1 H NMR spectroscopy, it was found that the maximum esterification with glycerol was 32%. Heise and coworkers [47] reported the grafting of vinyl acetate onto poly[styrene-co-(p-vinyl-2-phenylethanol)] containing 45% secondary OH groups by CALB-catalyzed

Fig. 7. Enzyme-catalyzed Michael addition of 1,3-dicarbonyl compounds to a,b-unsaturated compounds.

Fig. 8. CALB-catalyzed esterification of poly(acrylic acid) with various polyols in bulk.

transesterification in toluene (Fig. 9(A)). The enzyme stereoselectively catalyzed the reaction with vinyl acetate in R configuration. When a backbone containing 100% primary hydroxy groups was used, 75% of the OH groups reacted with vinyl acetate and changing the reaction parameters did not increase the conversion significantly. Similarly, when a copolymer of styrene and 4-vinylbenzyl alcohol with 10% primary hydroxyl functionality was used, vinyl acetate grafting was 95% as determined by 1H NMR spectroscopy [48]. It was also observed that when [poly(styrene-co-methyl-2-(4-styryl) acetate)] was reacted with benzyl alcohol, there was no reaction at all [49]. This problem was overcome by introducing a spacer in the esterfunctionalized polymer (Fig. 9(B)), and up to 78% conversion was reached [49]. Poly[N-(2-hydroxypropyl)-11-methacryloylaminoundecanamide-co-styrene], a comb-like polymer, and its corresponding monomer was acylated with vinyl acetate, phenyl acetate, 4-fluorophenyl acetate and phenyl stearate in the presence of a lipase from Pseudomonas fluorescens [50]. The copolymer was acylated with about 40% conversion when phenyl acetate was used as the acyl donor. The higher reactivity of the monomer compared to the copolymer indicated the effect of steric hindrance on the reaction kinetics. b-Galactosidase from Aspergillus oryzae was employed as the catalyst for the functionalization of PEG with galactosylate in water using lactose as the donor [51]. 1H NMR analysis revealed 30–50% conversion. Gross and coworkers [52] prepared organosilicon carbohydrate conjugates by CALB-catalyzed regioselective esterification of siloxanes having diacid end groups with a,b-ethyl glycoside under vacuum in bulk (Fig. 10). The organosilicon reacted with the primary hydroxyl group of the a,b-ethyl glycoside; and the extent of esterification was determined by electrospray ionization mass spectrometry as 99%. Recently, the first examples of quantitative functionalization of synthetic polymers using CALB-catalyzed reactions with and without organic solvents has been reported by our group [10–12,36,37]. The primary hydroxyl groups of hydroxyl-functionalized polyisobutylenes (PIBs) were quantitatively methacrylated by transesterification of vinyl methacrylate in the presence of CALB within 24 h in hexane and 2 h in bulk, respectively (Fig. 11) [12,37]. Specifically, asymmetric methacrylation of a,x-hydroxy functionalized PIB was achieved by the regioselective transesterification of vinyl methacrylate using CALB in hexane within 24 h, leaving the sterically hindered hydroxyl group intact (Fig. 11) [10,37]. Commercially available polydimethylsiloxanes (PDMSs): HO–PDMS–OH (HO–[Si(CH3)2–O]n–Si(CH3)2–OH, Mn = 3200 g/mol); PDMS-monocarbinol (HO–(CH2)2–O–(CH2)3–[Si(CH3)2

528


(A) n

X

m

O

+

O

CALB Toluene

OH

X

(B) m

+

O

+

H

O

ROH

CALB

n

O

O

OH ,

m

O

O

O ROH:

m

O

X: H or CH3

n

n

O

+

CH3OH O O

R

OH

Fig. 9. CALB-catalyzed transesterifications: (A) transesterification of vinyl acetate with poly(styrene-co-4-vinyl alcohol)s and (B) transesterification of poly[methyl-6-(2-(4-vinylphenyl)acetoxyl) hexanoate-co-styrene with benzyl alcohol and 1-phenylethanol.

Fig. 10. CALB-catalyzed esterification of diacid end-blocked siloxanes with a,b-ethyl glycoside.

–O]n–Si(CH3)2–C4H9, Mn = 5000 g/mol) and PDMS-dicarbinol (HO–(CH 2 ) 2 –O–(CH 2 ) 3 –Si(CH 3 ) 2 –O–[Si(CH 3 ) 2 –O] n – Si(CH3)2–(CH2)3–O–(CH2)2–OH, Mn = 4500 and 1000 g/mol) were quantitatively functionalized by transesterification of 1.5 eq. of vinyl methacrylate under solventless conditions within 2 h in the presence of CALB (10 wt.% relative to the total weight of reactants) [10,36]. PEGs with various molecular weights and molecular weight distributions were quantitatively methacrylated or acylated by CALB-catalyzed transesterification in THF within 24 h (Fig. 12) [11]. Thymine-functionalized PEG was successfully prepared by the Amano lipase M-catalyzed Michael addition of thymine to PEG diacrylate within 72 h [10]. The PEG-diacry-

late was prepared by the transesterification of vinyl acrylate with HO–PEG–OH (Mn = 2000 g/mol, Mw/Mw = 1.91) in the presence of CALB. The CALB-catalyzed Michael addition of an amino terminated poly(ethylene glycol) methyl ether (Mn = 2000 g/mol, Mw/Mn = 1.05) to 1,3,5-triacryloylhexahydro-1,3,5-triazine yielded a monofunctional product with two acrylate groups available for further conjugation with other functional groups [10]. Recently, we were able to functionalize PEGs under solvent-free conditions within 4 h, by dissolving low molecular weight HO–PEG–OH (Mn = 1050 and 2000 g/mol) in the corresponding acyl donors at 50 °C (Fig. 13). 1H and 13C NMR along with MALDI-ToF confirmed quantitative conversion with the expected structures.

Fig. 11. CALB-catalyzed methacrylation of PIB–OHs. PIB–OH (Mn = 5200 g/mol, Mw/Mn = 1.09) made from PIB-allyl [53], Glissopal–OH (Mn = 3600 g/mol, Mw/Mn = 1.34) made from GlissopalÒ2300 [53], and asymmetric telechelic HO–PIB–OH (Mn = 7200 g/mol, Mw/Mn = 1.04).

529


Fig. 12. CALB-catalyzed acylation and methacrylation of PEGs.

O O

O

O

(5 eq. per -OH)

O

O O

n

O HO

O

O

H n

O (5 eq. per -OH)

O

CALB (10 wt%) 50 oC,

O O (5 eq. per -OH)

O

4h

O

O O

n

O H

O O

O

O O

n

Fig. 13. Quantitative functionalization of PEGs in bulk within 4 h.

Based on these results, we set out to design and construct PEG dendrimers using enzyme catalysis. 4. Dendrimer synthesis Dendrimers have extensively been used in novel drug delivery systems, due to multivalent surfaces, and high drug carrying capacity [54–58]. However, the synthesis of dendrimers remains an expensive and difficult process with a complex mixture of products, resulting in stochastic conjugation of both targeting ligands and drugs [59–61].

PEG-based dendrimers [62–68] are outstanding candidates for targeted drug delivery, because of a unique combination of the properties of PEG including water solubility, non-toxicity and limited recognition by the immune system [69–72]. Gnanou and et al. reported the synthesis of multifunctional PEGs prepared by an iterative divergent approach combining anionic ring-opening polymerization (AROP) of ethylene oxide from multi-hydroxylated precursors and subsequent branching of the PEO chain ends (Fig. 14) [66–68]. PEGG1(OH)3 (90% conversion) was prepared by

Fig. 14. Synthetic processes for PEG dendrimers by anionic ring-opening polymerization.

530


Fig. 15. Dendrimer design for targeted drug delivery.

AROP of ethylene oxide with 1,1,1-tris(hydroxymethyl) ethane as an initiator, in the presence of diphenylmethylpotassium. The nucleophilic substitution of allyl chloride with PEGG1(OH)3 followed by the oxidation of the allyl groups in the resulting polymer using OsO4 and subsequent polymerization with ethylene oxide yielded PEGG1(OH)6. PEG dendrimers up to Mn = 900000 g/mol (Mw/ Mn = 1.28) with theoretically 384 outer hydroxy functional groups were obtained, but the overall efficiency was less than 20%. Preliminary to the precision synthesis of PEG dendrimers for targeted drug delivery applications (the design is shown in Fig. 15: the dendrimer should carry multiple targeting, therapeutic, imaging and diagnostic agents), here we report the precision synthesis of four-functional TEG and PEG using enzymatic catalysis. 4.1. (HO)2–TEG–(OH)2 A four-functional tetraethylene glycol (TEG) was prepared by sequential transesterification of vinyl acrylate

followed by the Michael addition of diethanolamine catalyzed by CALB (Fig. 16(B); details are given in the Supporting Information). Monitoring the reaction with thin layer chromatography (TLC) (Fig. 16(C)) revealed that both transesterification and Michael addition reactions were successful as quantitative conversions were reached within 24 and 2 h, respectively. Fig. 17 shows the 1H NMR spectra of the products in each step. In the spectrum of the TEG diacrylate, the integration ratios of the vinyl [d = 5.8 (3), d = 6.2 (2), and d = 6.3 (1)], and methylene protons [d = 4.2 (4)] were 1:1:1:2 as expected (Fig. 17(A)). After the Michael addition, the vinyl protons from the acrylate groups disappeared and new signals corresponding to the protons of the OH end groups and that of newly formed chain end appeared with the expected integration ratios of 1:1:1:2:2 at d = 4.2 ppm and d = 2.7 ppm (a), d = 2.4 ppm (b), d = 2.5 ppm (c), and d = 3.3 ppm (d), respectively. (Fig. 17(B)). The 13C NMR spectrum of the TEG acrylate (A) and (HO)2–TEG–(OH)2 (B) also confirmed the expected structures (Fig. 18). The carbons attached to the hydroxyl group in the starting material at d = 60.13 ppm shifted downfield to d = 63.41 ppm (d) after the reaction and the carbon resonances of the acrylate group appeared at d = 165.47 ppm (c), d = 131.77 ppm (b) and d = 128.08 ppm (a) corresponding to carbonyl carbon, a-carbon and the vinyl carbons connected to the a-carbon, respectively (Fig. 18(A)). Upon Michael addition of diethanolamine to the TEG diacrylate (2 h) the carbon resonances of the acrylate groups at d = 172.23 ppm (c), d = 32.11 ppm (b) and d = 50.02 ppm (a) disappeared, and new signals appeared at d = 59.56 ppm (1) and d = 56.30 ppm (2), confirming the successful synthesis of the TEG dendrimer (Fig. 18(B)). 4.2. (HO)2–PEG–(OH)2 Using a similar synthetic strategy, a four-functional PEG was also synthesized. a,x-hydroxy PEG (HO–PEG–OH, Mn = 1050 g/mol, Mw/Mn = 1.08) was reacted with vinyl acrylate (3.0 equivalent per –OH of PEG) in the presence

Fig. 16. Enzyme-catalyzed synthesis of (HO)2–TEG–(OH)2.


Fig. 17. 1H NMR spectra of TEG acrylate (A) and (HO)2–TEG–(OH)2 (B).

Fig. 18.

13

C NMR spectra of TEG acrylate (A) and (HO)2–TEG–(OH)2 (B).

531

532


Fig. 19. 1H and

13

C NMR spectra of (HO)2–PEG–(OH)2 in DMSO-d6.

of CALB for 24 h at 50 °C. The resulting PEG diacrylate was then reacted with diethanolamine (1.0 equivalent per acrylate group of PEG) in DMSO at 50 °C for 24 h via CALB-catalyzed Michael addition to obtain (HO)2–PEG–(OH)2 (99.5% yield by 1H NMR). Fig. 19 shows the 1H and 13C NMR spectra of the (HO)2–PEG–(OH)2 product. The integral ratio of the chain end hydroxyl protons (8) to the methylene protons next to the ester linkage (3) was 1:1 confirming the expected structure. The 13C NMR spectrum of (HO)2– PEG–(OH)2 (Fig. 19(B)) was identical to that of (HO)2– TEG–(OH)2 (Fig. 18(B)). 5. Summary In summary, we successfully synthesized (HO)2–TEG– (OH)2 and (OH)2–PEG–(OH)2 as a core of novel dendrimers using sequential CALB-catalyzed transesterification and Michael addition. The use of enzymes provides us with unprecedented control in polymer functionalization and building of novel architectures. Specifically, the design and synthesis of novel polymeric architectures by utilizing enzyme selectivity has the potential of revolutionizing polymer science. Acknowledgements This work is supported by National Science Foundation under DMR-509687 and DMR-0804878. We wish to thank The Ohio Board of Regents and The National Science Foundation for funds used to purchase the NMR (CHE-0341701 and DMR-0414599) instruments used in this work.

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Dr. Puskas received a PhD in plastics and rubber technology in 1985, and an M. E. Sc in organic and biochemical engineering in 1977, from the Technical University of Budapest, Hungary. Her advisors were Professors Ferenc Tüdös and Tibor Kelen of Hungary, and Professor Joseph P. Kennedy at the University of Akron, Ohio, USA, in the framework of collaboration between the National Science Foundation of the USA and the Hungarian Academy of Sciences. She started her academic career in 1996. Before that she was involved in polymer research and development in the microelectronic, paint and rubber industries. Her present interests include green polymer chemistry, biomimetic processes and biomaterials, living/controlled polymerizations, polymerization mechanisms and kinetics, thermoplastic elastomers and polymer structure/property relationships, and probing the polymer-bio interface. She is one of the editors of the new Interdisciplinary Reviews in Nanomedicine and NanoBiotechnology WIRE, Published by Wiley-Blackwell, and a member of the Advisory Board, of the European Polymer Journal. Until July of 2008 she was one of the two regional Editors with the highest citation index. She was also member of the IUPAC Working Party IV.2.1 ‘‘Structure-property relationships of commercial polymers’’. Puskas has been published in more than 350

publications, including technical reports, is an inventor or co-inventor of 26 U.S. patents and applications, and has been Chair or organizer of a number of international conferences. Puskas has been awarded her fourth NSF and first NIH grant (the first ever in the Department of Polymer Science) in 2010. She is the recipient of several awards, including the 1999 PEO (Professional Engineers of Ontario, Canada) Medal in Research&Development, a 2000 Premier’s Research Excellence Award, the 2004 Mercator Professorship Award from the DFG (Deutschen Forschungsgemeinschaft, German Research Foundation), the LANXESS (previously Bayer) Industrial Chair 1998-2008, and the 2009 ‘‘Chemistry of Thermoplastic Elastomers’’ Award of the Rubber Division of the American Chemical Society. She was elected Fellow of the American Institute of Medical and Biological Engineering AIMBE in 2010. As a coinventor of the polymer used on the TaxusÒ coronary stent, Puskas helped the University of Akron generate more than $5 million in license fees. With her business partner they founded Biopolymers International Inc. to supply research quantities of functionalized polymers using enzyme catalysis under license from the University of Akron.

Kwang Su Seo studied polymer chemistry at the Chonbuk National University (South Korea), where he received his BS degree in 2003 and MS degree in 2005 under the supervision of Dr. Gilson Khang, Dr. Hai Bang Lee and Dr. Moon Suk Kim. He is currently a Ph.D. student in the Puskas group in the Department of Polymer Science at The University of Akron, where he works on the synthesis of PEG-based dendrimers using enzyme catalysis for targeted multivalent cancer drug delivery.

Dr. Sen obtained his BS degree in Chemistry at Bogazici University, Turkey in 2002 and MS degree in Polymer Science at University of Akron in 2005 under the supervision of Dr. Roderic P. Quirk. He then joined the Puskas group in December of 2005 and received his PhD in 2009. He was the first student working on the functionalization of polymers using enzymatic catalysis, a new research direction in the Puskas group. His research interests included the enzymatic functionalization of biomaterials and getting a better fundamental understanding of enzyme catalysis. He currently works at Kordsa Global R&D Center in Turkey as a Project Leader.