Enzymatic Functionalization of Liquid Polymers in Bulk - American

Chapter 28

Downloaded by UNIV OF ROCHESTER on August 20, 2013 | http://pubs.acs.org Publication Date (Web): August 11, 2010 | doi: 10.1021/bk-2010-1043.ch028

Green Polymer Chemistry: Enzymatic Functionalization of Liquid Polymers in Bulk Judit E. Puskas* and Mustafa Y. Sen Department of Polymer Science, The University of Akron, 170 University Ave., Akron, OH 44325-3909, USA *[email protected]

The use of enzymes as catalysts for organic synthesis has become an increasingly attractive alternative to conventional chemical catalysis. Enzymes offer several advantages including high selectivity, the ability to operate under mild conditions, catalyst recyclability and biocompatilibity. Although there are many examples involving enzymes for the synthesis of polymers, only a few are in the area of polymer functionalization and most of the examples are characterized by low conversion. In this paper, we present examples of quantitative enzyme-catalyzed methacrylation of liquid polymers. Specifically, vinyl methacrylate was transesterified with liquid α,ω-dihydroxy polyisobutylenes, α,ω-dihydroxy polydimethylsiloxane (PDMS), PDMS-mono and –dicarbinol, and low molecular weight poly(ethylene glycol) in the presence of Candida antarctica lipase B (CALB; Novozym® 435) under solventless conditions. 1H and 13C NMR spectroscopy verified the structure of the functionalized polymers.

Introduction Enzymes are nature’s catalysts that accelerate specific metabolic reactions in living cells. However, many different types of enzymes are also known to catalyze the transformation of a wide range of “unnatural” substrates in vitro (1). In the area of polymer science this feature of enzymes has been well exploited for the synthesis of polymers (2–4), but the number of examples involving enzyme-catalyzed polymer modifications is rather © 2010 American Chemical Society In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


limited (5) and generally characterized by low conversions (6, 7). As an environmentally friendly alternative to conventional chemical catalysts, enzymes offer several advantages including high selectivity, the ability to operate under mild conditions, catalyst recyclability and biocompatilibity (8). We have previously shown the quantitative chain end functionalization of poly(ethylene glycol)s (PEG)s and polyisobutylenes (PIB)s in solution by Candida antarctica lipase B (CALB)-catalyzed transesterification (9, 10). In the framework of on-going enzymatic polymer functionalization research in our laboratory, we recently reported the synthesis of α,ω-thymine-functionalized PEG via Michael addition using Amano lipase M from Mucor javanicus as the enzyme (11). Moreover, we utilized enzyme regioselectivity to methacrylate asymmetric α,ω-hydroxyl-functionalized PIBs exclusively at the ω-termini (11). In this paper, we present examples of CALB-catalyzed methacrylation of liquid hydroxyl-functionalized PIBs, α,ω-dihydroxy poly(dimethyl siloxane) (HO-PDMS-OH), hydroxyl-ethoxypropyl-terminated PDMSs (PDMS-monoand dicarbinol) and poly(ethylene glycol) (PEG) under solventless conditions via transesterification of vinyl methacrylate (VMA).

Functionalization of Polyisobutylenes in Bulk The quantitative enzymatic functionalization of hydroxyl-terminated polyisobutylenes (PIB-OHs) with various chain-end structures in hexane via CALB-catalyzed transesterification of VMA was previously achieved (10). Transesterification was the preferred synthetic pathway as some enzymes, i.e. lipases, are well-known to catalyze acyl transfer reactions of small molecules (8). Among several lipases used in transesterification, CALB was preferred due to its high stability and reactivity (12). VMA, which is an enolate ester, was chosen as the acyl donor. Enolate esters liberate unstable enols as by-products which instantly tautomerize to give the corresponding aldehydes or ketones and thus render the transesterification irreversible (13). Hexane solvent was used since the catalytically active conformation of the enzyme is best maintained in low polarity solvents (8). Following the same synthetic pathway, liquid hydroxyl-functionalized PIBs, i.e. PIB-CH2-C(CH3)-CH2-OH (Mn=1500 g/mol, Mw/Mn=1.29) and Glissopal-OH (Mn=3600 g/mol, Mw/Mn=1.34), were enzymatically methacrylated in the absence of solvent. PIB-CH2-C(CH3)-CH2-OH (Mn=1500 g/mol, Mw/Mn=1.29) was prepared by dehydrochlorination of the terminal –C(CH3)2Cl of a PIB-Cl (14), followed by hydroboration/oxidation of the resulting olefinic chain end (15). The PIB-Cl was synthesized by the 2-chloro-2,4,4-trimethylpentane (TMPCl)/BCl3 initiated carbocationic polymerization of isobutylene (16, 17). Glissopal-OH, which also has the chain end structure -CH2-C(CH3)-CH2-OH, was obtained by hydroboration/oxidation of Glissopal®2300 (BASF), a commercially available PIB with ~82% exo and ~18% endo terminal double bonds (18). Figure 1 shows the conditions we developed for the transesterification of VMA with these hydroxyl-functionalized PIBs in bulk. 418 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 1. Transesterification of VMA with hydroxyl-functionalized PIBs in bulk.

Figure 2. 1H NMR spectrum of the methacrylation product of a Glissopal-OH (Mn=3600 g/mol, Mw/Mn=1.34) in bulk (top) and its corresponding 13C NMR (bottom) (NMR solvent: CDCl3). Both PIB-CH2-C(CH3)-CH2-OHs derived from PIB-Cl and Glissopal®2300 reacted quantitatively. The 1H and 13C NMR spectra of the methacrylation product of Glissopal-OH are shown in Figure 2. The methylene protons adjacent to the hydroxyl group at δ=3.31-3.51 ppm shifted downfield to δ=3.84-4.02 ppm after methacrylation and the vinylidene [δ=5.56 (e) and 6.13 ppm (d)] and methyl [δ=1.97 ppm (c)] protons of the newly formed methacrylate-end appeared at the expected positions. The 13C NMR also confirmed the structure of the 419 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 3. Methacrylated PDMS products.

Figure 4. 1H NMR spectrum of the methacrylation product of a PDMS-monocarbinol (Mn=5000 g/mol) in bulk (NMR solvent: CDCl3). methacrylated polymer where the carbonyl carbon was observed at δ=167.73 ppm (e), and resonances of the vinyl (CH2=C(CH3)-), methyl (CH2=C(CH3)-), and the alpha carbon (CH2=C(CH3)-) of the methacrylate group were observed at δ=125.38 ppm (h), δ=18.60 ppm (f) and δ=136.71 ppm (g), respectively. The 1H NMR shows the presence of PIB with endo-olefin terminus (δ= 5.16 ppm) from the starting material which was not converted into PIB-OH.

Functionalization of Polydimethylsiloxanes in Bulk The enzymatic methacrylation of commercially available liquid PDMSs in bulk was also quantitative. The list of methacrylated PDMS products is presented in Figure 3. Figure 4 shows the 1H NMR spectrum of the methacrylation product of PDMS-monocarbinol. The methylene protons adjacent to the hydroxyl group in the starting material at δ=3.74 ppm shifted downfield to δ=4.31 ppm (p) upon methacrylation; and vinylidene (l and m) and methyl protons (k) of the methacrylate chain-end appeared at δ=6.15 ppm (l), δ=5.58 ppm (m) and δ=1.97 420 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 5. 13C NMR spectrum of the methacrylation product of a PDMS-monocarbinol (Mn=5000 g/mol) in bulk (NMR solvent: CDCl3). ppm (k), respectively, with relative integral ratios of 2:1:1:3 demonstrating quantitative functionalization in 24 hours. Figure 5 shows the 13C NMR spectrum of the bulk methacrylation product of PDMS-monocarbinol. Compared to the 13C NMR of the starting material, new peaks appeared at δ=18.54 ppm (m), δ=125.92 ppm (n), δ=136.49 ppm (l) and δ=167.60 ppm (k) corresponding to the carbons of the methacrylate chain-end. The peaks (p) and (r) shifted upfield and downfield after methacrylation and appeared at δ=68.70 ppm and δ=64.25 ppm, respectively. The progress of the reactions was monitored using 1H NMR. Figure 6 is given as an example where PDMS-dicarbinol was methacrylated. It was observed that the protons of the two methylene groups next to the hydroxyl group in the starting material at δ=3.74 ppm (b) and δ=3.55 ppm (c) shifted downfield to δ=4.31 (r) and 3.69 ppm (s), respectively, within only 2 hours when 1.5 eq. of VMA was used. The relative integration ratio of these protons to the methacrylate-end protons confirmed quantitative conversion.

Functionalization of Poly(ethylene glycol) in Bulk Poly(ethylene glycol) (PEG)-dimethacrylate was prepared by the transesterification of VMA with HO-PEG-OH (Mn=1000 g/mol and Mw/Mn=1.08) in the presence of CALB (Figure 7). The low molecular weight PEG became liquid when warmed to 50 °C and was miscible with VMA. Monitoring the reaction with 1H NMR revealed that the reaction was quantitative within 4 hours when 5 eq. of VMA per OH group in the HO-PEG-OH was used. 421 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 6. 1H NMR spectrum of a PDMS-dicarbinol (Mn=4500 g/mol) (bottom) and its methacrylation product in bulk after 2 hours of reaction time (top) (NMR solvent: CDCl3). Figure 8 shows the 1H NMR spectrum of the PEG-dimethacrylate. The hydroxyl protons at δ=4.55 ppm from the HO-PEG-OH completely disappeared and the peak corresponding to the methylene protons adjacent to hydroxyl group shifted downfield from δ=3.50 to δ=4.42 ppm (c) after the reaction. The new peaks corresponding to the methyl [δ=1.73 ppm (e)] and vinyl [δ=6.07 ppm (f) and δ=5.81 ppm (g)] protons of the methacrylate group were observed at the expected positions with integral values of 2:3:1:1 [(c):(e):(f):(g)] confirming successful functionalization. The 13C NMR spectrum of the methacrylation product also confirmed the structure of the polymer (Figure 9). The carbons connected to the hydroxyl group in the starting material at δ=60.13 ppm shifted downfield to δ=63.89 ppm (c) after the reaction and the carbon resonances of the methacrylate group appeared at δ=166.97 ppm (d), δ=18.12 ppm (e), δ=136.24 ppm (f) and δ=126.11 ppm 422 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 7. Transesterification of VMA with HO-PEG-OH in bulk.

Figure 8. 1H NMR spectrum of the methacrylation product of a HO-PEG-OH (Mn=1000 g/mol, Mw/Mn=1.08) in bulk after 4 hours of reaction time (NMR solvent: DMSO-d6).

Figure 9. 13C NMR spectrum of the methacrylation product of a HO-PEG-OH (Mn=1000 g/mol, Mw/Mn=1.08) in bulk after 4 hours of reaction time (NMR solvent: DMSO-d6). (g) corresponding to carbonyl carbon, methyl carbon, alpha carbon and the vinyl carbon connected to the alpha carbon, respectively.

Conclusion In conclusion, we were able to functionalize liquid PDMSs and HO-PIB-OH, and low molecular weight PEGs very effectively under solventless conditions via CALB-catalyzed transesterification of vinyl methacrylate. Both 1H and 13C NMR spectroscopy verified the structure of the products. The absence of solvent in these transformations renders this approach attractive as it provides a cost-effective and environmentally benign way of producing telechelic polymers for biomedical applications. 423 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Acknowledgments This material is based upon work supported by The National Science Foundation under DMR-0509687 and #0804878. We wish to thank The Ohio Board of Regents and The National Science Foundation (CHE-0341701 and DMR-0414599) for funds used to purchase the NMR instrument used in this work. We are also grateful for the contribution of Kwang Su Seo and Dr. Serap Hayat-Soytas.


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424 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Enzymatic Functionalization of Liquid Polymers in Bulk - American

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