Green Polymer Chemistry: Enzyme-Catalyzed Polymer

Chapter 2

Downloaded by CENTRAL MICHIGAN UNIV on September 19, 2015 | http://pubs.acs.org Publication Date (Web): June 18, 2015 | doi: 10.1021/bk-2015-1192.ch002

Green Polymer Chemistry: Enzyme-Catalyzed Polymer Functionalization Judit E. Puskas,*,1 Marcela Castano,1,2 and Attila L. Gergely1 1Department

of Chemical & Biomolecular Engineering, The University of Akron, 264 Wolf Ledges, Akron, Ohio 44325 2Present address: Avery Dennison, 15939 Industrial Parkway, Cleveland, Ohio 44135 *E-mail: [email protected].

Enzymes are “green” alternative to conventional chemical catalysts offering several advantages including high selectivity, high efficiency, ability to operate under mild conditions, recyclability and biocompatibility. This paper will first give an overview of the functionalization of natural and synthetic polymers. Subsequently our work related to the synthesis of symmetric and asymmetric telechelic polymers using Candida antarctica lipase B will be discussed. Quantitative functionalization of polyisobutylenes, polysiloxanes and poly(ethylene glycol)s (PEGs) was achieved. Multifunctional PEGs were also successfully produced. Controlled polycondensation of tetraethylene glycol and divinyl benzene yielded symmetric and asymmetric telechelic PEGs with high efficiency.

Introduction Green Chemistry is a relatively new emerging field that focuses on achieving sustainability to meet environmental and economic goals by developing chemical processes that follow the 12 basic principles of sustainable chemistry (1). Enzymes are an environmentally friendly alternative to conventional chemical catalysts (2). Specifically, lipases are widely used in esterification, transesterification, aminolysis, and Michael addition reactions in organic syntheses (2, 3). Enzymes can also catalyze polymer synthesis – this area of research has extensively © 2015 American Chemical Society In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

been reviewed (3–7). Our group has been concentrating on the quantitative end-functionalization of synthetic polymers (8–15). This paper will give an overview of our work after reviewing prior representative work related to polymer functionalization.


Functionalization of Natural Polymers Several research groups reported work related to enzyme-catalyzed functionalization of natural polymers. Some interesting examples are given below. The CALB-catalyzed acylation of cellulose acetate (16) and hydroxypropyl cellulose (17) yielded various ester side groups with high efficinecy. In the case of the acylation of cellulose acetate with lauric and oleic acids (Figure 1), the final conversion of both fatty acids was about 35% after 96 h of incubation at 50 °C. In the case of hydroxypropyl cellulose the final ester content was about 11% after 6-day incubation at 50°C.

Figure 1. CALB-catalyzed acylation of cellulose acetate.

Starch nanoparticles in microemulsions were reacted with vinyl stearate, -caprolactone, and maleic anhydride in the presence of CALB at 40 °C for 48 h to give esters with degrees of substitution (DS) of 0.8, 0.6, and 0.4, respectively. Substitution occurred regioselectively at the C-6 position of the glucose repeat units (Figure 2) (18).

Figure 2. CALB-catalyzed functionalization of starch nanoparticles. 18 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Another example is a novel regioselective strategy for the transesterification of Konjac glucomannan (KGM) with vinyl acetate using CALB in a solvent-free system. KGM is an abundant, naturally occurring polysaccharide isolated from the tubers of the Amorphophallus konjac plant. It consists of β-1,4-linked D-glucose and D-mannose units, and the molar ratio of glucose to mannose has been reported to be around 1 to 1.60. The degree of substitution (DS) was 0.34 and 0.58 at 30 and 60ºC, respectively. It was also found that the DS decreased with increasing KGM molecular weight (from 114,000 to 980,000 g/mol) (19).

Functionalization of Synthetic Polymers Enzymes have also been employed for the functionalization of synthetic polymers. CALB-catalyzed modification of pendant ester groups of a polystyrene is a good example of a clear-cut regioselective transesterification reaction (20). From the two ester groups present, only the ester group distant from the polymer backbone was involved in the reaction. It is possible that due to the proximity of the acyl (ester) group (A) to the bulk of the polymer backbone, the enzyme is incapable of coordinating to the acyl group, but the more distant group (B) is available to the enzyme (Figure 3).

Figure 3. CALB-catalyzed modification of pendant ester groups of a polystyrene.

Jarvie, Overton and Pourçain showed that CALB was able to catalyze the selective epoxidation of polybutadiene (Mn=1,300 g/mol) (35% trans, 20% cis, 45% vinyl) in organic solvents in the presence of hydrogen peroxide and catalytic quantities of acetic acid (Figure 4). The cis and trans alkene bonds of the backbone were epoxidised in yields of up to 60% while the pendant vinyl groups remained untouched (21). 19 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 4. CALB-catalyzed epoxidation of polybutadiene.

The enantioselective enzymatic transesterification of copolymers of R-isomers and S-isomers of styrene and p-vinylphenylethanol by CALB using vinyl acetate was investigated (Figure 5) (22). When a backbone contained 100% S groups no acetylation was detected. By contrast, when the backbone contained 100% R groups, 75% of the alcohol groups was converted into ester groups within 24 h.

Figure 5. Enzymatic transesterification of vinyl acetate with copolymers of Rand S-isomers of styrene and p-vinlyphenylethanol by CALB.

CALB-catalyzed acylation of comb-like methacrylamide polymers was induced by reacting the OH groups in the side chains with vinyl acetate, phenyl acetate, 4-fluorophenyl acetate, and phenyl stearate in THF at an ambient temperature for 6 days (Figure 6). Conversions varied from 20% to 93%, depending on the acylating agent (23).

Figure 6. CALB-catalyzed acylation of comb-like methacrylamide polymers. 20 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Telechelic carboxylic acid functionalized poly(dimethyl siloxanes) (PDMSs) were reacted with α,β-ethylglucoside at 70 °C under vacuum for 34 hours in the presence of CALB. The product was a mixture of mono- and difunctional esters (Figure 7) (24). These examples demonstrate chemo- and regioselectivity, but tend to be less than quantitative.

Figure 7. Esterification of telechelic carboxylic acid functionalized PDMSs with α,β-ethylglucoside in the presence of CALB.

Quantitative End-Functionalization of Synthetic Polymers Despite all the advantages that the enzymatic catalysis offers, the area of end-functionalization of pre-formed synthetic symmetric or asymmetric telechelic polymers has not been fully developed yet. Our group reported the first examples of quantitative functionalization of synthetic polymers using CALB-catalyzed reactions with and without organic solvents (8–10). For example, Figure 8 shows the quantitative methacrylation by transesterification of vinyl methacrylate with hydroxy-functionalized polyisobutylenes (PIBs) in the presence of CALB within 24h in hexane and 2h in bulk, respectively. The last example demonstrates regioselective transesterification, leaving the sterically hindered hydroxyl group intact (9). Additionally, commercially available polydimethylsiloxanes (PDMS), PDMS-monocarbinol and PDMS-dicarbinols were also methacrylated with vinyl methacrylate under solventless conditions within 2 hours in the presence of CALB (10, 11). Primary hydroxy-functionalized polystyrene (PS-(CH3)2Si-CH2-OH, Mn=2600 g/mol; Mw/Mn =1.06) was quantitatively methacrylated by transesterification of vinyl methacrylate within 48 hours (11). PEGs were also effectively functionalized (8, 10–15). PEG is a particularly important polymer since it is non-toxic, hydrophilic polymer that is used widely for biomedical applications (25, 26). One applications is to enhance the circulation time and blood half-life for cell imaging, drug delivery, and antibody-based therapy (27–31). However, the HO-end groups that are available for chemical derivatization are only a small fraction of the molecular mass of the polymer, and chemistries utilized for end-group modification must be of high fidelity in nature and leave few or ideally no residuals (32–34). Since 21 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


CALB is immobilized on a resin, it can conveniently be separated from the product, yielding very pure compounds for potential pre-clinical and clinical applications (35–37).

Figure 8. CALB-catalyzed methacrylation of PIB-OHs. PIB-OH (Mn=5200 g/mol; Mw/Mn=1.09), Glissopal-OH (Mn=3600 g/mol; Mw/Mn=1.34), and asymmetric telechelic HO-PIB-OH (Mn=7200 g/mol; Mw/Mn=1.04).

PEGs were effectively functionalized under solvent-free conditions within 4 h by dissolving low molecular weight liquified HO–PEG–OH (Mn = 1050 and 2000 g/mol) in the corresponding acyl donors (vinyl methacrylate, vinyl acrylate and vinyl crotonate) at 50 ºC (Figure 9). 1H and 13C NMR along with MALDI-ToF confirmed quantitative conversion with the expected structures (14).

Figure 9. Functionalized PEGs via CALB-catalyzed transesterification. 22 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Multifunctional structures such as (HO)2–TEG–(OH)2 and (HO)2–PEG–(OH)2 were also successfully synthesized using sequential CALB-catalyzed transesterification and Michael addition of diethanolamine to the acrylate double bonds (Figure 10) (12). These structures will be used as cores of novel dendrimers. Both transesterification and Michael addition reactions were successful as quantitative conversions were reached within 24 h and 2 h, respectively.

Figure 10. Synthesis of (HO)2–PEG–(OH)2 via sequential CALB-catalyzed transesterification and Michael addition.

Subsequently (HO)4–TEG–(OH)4 and (HO)4–PEG–(OH)4 have also been made by repeating the reaction sequence shown in Figure 10.

Synthesis of Asymmetric and Symmetric Vinyl-Functionalized PEG Oliogomers In order to be able to synthesize asymmetric and symmetric telechelic PEGs we investigated the kinetics of the transesterification of divinyl adipate (DVA) with tetraethylene glycol (TEG). We found conditions under which polycondensation was minimized and symmetric and asymmetric telechelic TEGs were obtained with Mn = 700 – 1700 g/mol and Mw/Mn = 1.1-1.3. Specifically, at DVA/TEG=1.5 molar ratio 100% of the oligomers had divinyl end groups after 20 minutes reaction time. At DVA/TEG = 3 only vinyl end groups were detected after 5 minutes. HO-(TEG)n-Vinyl was maximized at 70% at DVA/TEG 1/1.5 molar ratio at 10 minutes reaction time (Figure 11), while ~90% HO-(TEG)n-OH was obtained in 20 minutes with TEG excess (15). 23 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 11. Kinetics of the polycondensation of TEG with DVA. Thus with the judicious selection of reaction conditions and time symmetric and asymmetric telechelic oligomers can be synthesized with high efficiency.

Conclusion In summary, CALB was shown to be an effective catalyst for selective endfunctionalization of polymers. The end functionalization is still an ongoing work in our laboratories and there are many opportunities for new applications.

References Anastas, P.; Eghbali, N. Chem. Soc. Rev. 2010, 39, 301–312. Faber, K. Biotransformations in Organic Chemistry, 5th ed.; SpringerVerlag: New York, 2004. 3. Kobayashi, S.; Makino, A. Chem. Rev. 2009, 109, 5288–5353. 4. Gross, R.; Kumar, A.; Kalra, B. Chem. Rev. 2001, 101, 2097–2124. 5. Uyama, H.; Kobayashi, S. Adv. Polym. Sci. 2006, 194, 133–158. 6. Kobayashi, S.; Uyama, H.; Kimura, S. Chem. Rev. 2001, 101, 3793–3818. 7. Varma, I. K.; Albertsson, A.-C.; Rajkhowa, R.; Srivastava, R. K. Prog. Polym. Sci. 2005, 30, 949–981. 8. Puskas, J. E.; Sen, M. Y.; Kasper, J. R. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 324–327. 9. Sen, M. Y.; Puskas, J. E.; Ummadisetty, S.; Kennedy, J. P. Macromol. Rapid Comm. 2008, 29, 1598–1602. 10. Puskas, J. E.; Sen, M. Process of Preparing Functionalized Polymers via Enzyme Catalysis. U.S. Patent 12/732,467, September 23, 2010. 11. Puskas, J. E.; Sen, M. Y.; Seo, K. S. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 2959–2976. 12. Puskas, J. E.; Seo, K. S.; Sen, M. Y. Eur. Polym. J. 2011, 47, 524–534. 1. 2.

24 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


13. Castano, M.; Seo, K. S.; Kim, G.; Becker, M. L.; Puskas, J. E. Macromol. Rapid Commun. 2013, 34, 1375–1380. 14. Seo, K. S.; Castano, M.; Casiano, M; Wesdemiotis, C.; Becker, M. L.; Puskas, J. E. RSC Adv. 2014, 4, 1683–1688. 15. Castano, M.; Seo, K. S.; Guo, K.; Becker, M. L.; Wesdemiotis, C.; Puskas, J. E. Polym. Chem. 2015 DOI:0.1039/C4PY01223B. 16. Sereti, V.; Stamatis, H.; Koukios, E.; Kolisis, F. J. Biotechnol. 1998, 66, 219–223. 17. Sereti, V.; Stamatis, H.; Pappas, C.; Polissiou, M.; Kolisis, F. Biotechnol. Bioeng. 2001, 72, 495–500. 18. Chakraborty, S.; Sahoo, B.; Teraoka, I.; Miller, L. M.; Gross, R. A. Macromolecules 2005, 61–68. 19. Chen, Z.-G.; Zong, M.-H.; Li, G.-J. J. Appl. Polym. Sci. 2006, 102, 1335–1340. 20. Padovani, M.; Hilker, I.; Duxbury, C. J.; Heise, A. Macromolecules 2008, 41, 2439–2444. 21. Jarvie, A. W. P.; Overton, N.; Pourçain, C. B. S. J. Chem. Soc., Perkin Trans. 1999, 1, 2171–2176. 22. Duxbury, C. J.; Hilker, I.; Wildeman, S. M.; Heise, A. Angew. Chem., Int. Ed. 2007, 46, 8452–8454. 23. Pavel, K.; Ritter, H. Makromol. Chem. 1992, 328, 323–328. 24. Sahoo, B.; Brandstadt, K. F.; Lane, T. H.; Gross, R. Org. Lett. 2005, 7, 3857–3860. 25. Kukula, H.; Schlaad, H.; Antonietti, M.; Förster, S. J. Am. Chem. Soc. 2002, 124, 1658–1663. 26. Lin, L. Y.; Karwa, A.; Kostelc, J. G.; Lee, N. S.; Dorshow, R. B.; Wooley, K. L. Mol. Pharm. 2012, 9, 2248–2255. 27. Shrestha, R.; Shen, Y.; Pollack, K.; Taylor, J.-S.; Wooley, K. L. Bioconjugate Chem. 2012, 23, 574–585. 28. Sun, X.; Rossin, R.; Turner, J. L.; Becker, M. L.; Joralemon, M. J.; Welch, M. J.; Wooley, K. L. Biomacromolecules 2005, 6, 2541–2554. 29. Duncan, R.; Vicent, M. J. Adv. Drug Delivery Rev. 2013, 65, 60–70. 30. Marcano, D. C.; Bitner, B. R.; Berlin, J. M.; Jarjour, J.; Lee, J. M.; Jacob, A.; Fabian, R. H.; Kent, T. A.; Tour, J. M. J. Neurotrauma 2013, 30, 789–796. 31. Muthiah, M.; Vu-Quang, H.; Kim, Y. K.; Rhee, J. H.; Kang, S. H.; Jun, S. Y.; Choi, Y. J.; Jeong, Y. Y.; Cho, C. S.; Park, I. K. Carbohydr. Polym. 2013, 92, 1586–1595. 32. Jevsevar, S.; Kunstelj, M.; Porekar, V. G. Biotechnol. J. 2010, 5, 113–128. 33. Jacob, M. K.; Leena, S.; Kumar, K. S. Biopolymers 2008, 90, 512–525. 34. Kochling, J. D.; Miao, H.; Young, C. R.; Looker, A. R.; Shannon, M.; Montgomery, E. R. J. Pharm. Biomed. Anal. 2007, 43, 1638–1646. 35. Leung, M. K. M.; Hagemeyer, C. E.; Johnston, A. P. R.; Gonzales, C.; Kamphuis, M. M. J.; Ardipradja, K.; Such, G. K.; Peter, K.; Caruso, F. Angew. Chem., Int. Ed. 2012, 51, 7132–7136. 36. Fang, Y.; Xu, W.; Wu, J.; Xu, Z.-K. Chem. Commun. (Cambridge, U.K.) 2012, 48, 11208–11210. 37. Fang, Y.; Huang, X.-J.; Chen, P.-C.; Xu, Z.-K. BMB Rep. 2011, 44, 87–95. 25 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Green Polymer Chemistry: Enzyme-Catalyzed Polymer

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