The modification of natural or synthetic polymers with enzymes is an environmentally friendly alternative to classical chemical modification reactions which generally require harsh reaction conditions. Despite the advantage of using enzymes in functionalization reactions, i.e. milder reaction conditions and highly specific transformations, only a few examples have been reported in the literature involving either natural or synthetic polymers.
Mushroom tyrosinase has been used to introduce phenolic functionalities into chitosan. The synthetic pathway involved enzyme-catalyzed oxidation of phenol to an o-quinone which dissociated from the enzyme and freely diffused to the nucleophilic amine site of the chitosan polymer. In addition to the incorporation of quinone into the chitosan, subsequent polymerization of quinone into oligomeric phenols was also observed as determined by UV-Vis spectroscopy. The effectiveness of the reactions was not discussed. Polysaccharides have been functionalized with fatty acids by transesterification using ion-paired subtilisin Carlsberg protease in organic solvents. It has been observed that the enzyme selectively acylated the primary hydroxyl groups on polysaccharides and that the degrees of substitution per glucose moiety in amylose and β-cyclodextrin were 0.185 and 0.250, respectively. Lactose was attached to the hydroxyl groups of hydroxyethylcellulose (HEC) by transglycosylation in sodium acetate buffer using Aspergillus oryzae. Although the number of sites available in each HEC unit is 3, the maximum degree of substitution obtained by this method was only 0.033. The same natural polymer, HEC, has recently been modified by enzymatic transesterification with both vinyl propionate and vinyl acrylate in the presence of subtilisin Carlsberg in anhydrous pyridine. These biotransformations of cellulose were promising; however, low conversions limited their viability. It has been reported that the phosphorylation of cotton cellulose using bakers' yeast hexokinase as the enzyme and ATP as the phosphoryl donor. The phosphorylation of 0.03% of the glycopyranose units in the cellulose resulted in improved dyeability and flame resistance. All of the examples listed here were hindered by low conversion.
Even fewer examples have been reported for synthetic polymer functionalization using enzymatic catalysis, which are also characterized by low conversion. For instance, 16% of the pendant nitrile groups of polyacrylonitrile fibers were converted to the corresponding amides by a nitrile hydratase enzyme, resulting in a significant increase in the hydrophilicity of the fiber surface. Mushroom tyrosinase has been utilized to graft poly(4-hydroxystyrene) (PHS) onto chitosan. In this approach, first 1-2% of the phenolic moieties of PHS were enzymatically oxidized and then the resulting polymer was reacted with the amine groups of chitosan. Lipase-catalyzed acylation of poly[N-(2-hydroxypropyl)-11-methacryloylaminoundecanamide-co-styrene] and the corresponding monomer with vinyl acetate, phenyl acetate, 4-fluorophenyl acetate and phenyl stearate has been reported. 1H NMR results revealed that the reactivity of the monomer was higher than that of the copolymer and that the copolymer could be acylated with up to 40% conversion. In addition, it was found that the reactivity of copolymer was dependent on copolymer composition which indicated the effect of steric hindrance and hydrophobicity on reaction kinetics. The synthesis of organosilicon carbohydrate macromers by Candida antarctica lipase B catalyzed esterification has been reported. Diacid-endblocked siloxanes were reacted with α,β-ethyl glucoside under vacuum in bulk. Esterification occurred with high regioselectivity (>98%) at the primary hydroxyl (C6) of the glucoside, but electrospray ionization mass spectrometry (ESI MS) showed the presence of a mixture of mono- and diesters.
The preparation of polyolefin-based telechelic polymers, such as methacrylate-terminated polyisobutylenes (PIB-MAs), has been shown in the prior art by a variety of strategies. In one example, PIB-Clt prepared by the inifer technique was first converted to the corresponding exo-olefin by dehydrochlorination with t-BuOK, followed by hydroboration/oxidation to yield PIB-OH which was subsequently converted to PIB-MA by acylation with methacryloyl chloride. In another process, PIB-OH was synthesized by hydroboration/oxidation of allyl-terminated PIB, prepared by end-quenching of living PIB+ with allyltrimethylsilane, and the latter was converted to PIB-MA with methacryloyl chloride. A further process describes how PIB-MA was synthesized by nucleophilic substitution of PIB-CH2—CH2—CH2—Br (prepared by anti-Markovnikov hydrobromination of allyl-terminated PIB) with sodium methacrylate. This same method was also applied to Glissopal®2300, a commercially available PIB with olefinic end groups. Quantitative syntheses of primary hydroxyl terminated PIB and Glissopal-OH has also been described.
Notwithstanding the state of the art as described herein, there is a need for further improvements in the preparation of functionalized polymers (monofunctional or difunctional (telechelic) polymers) via enzymatic catalysis which is an environmentally friendly method when compared with toxic acylating agents such as methacryloyl chloride, and catalysts; and is useful in biomaterials prepared from these functionalized polymers.