Enzyme-catalyzed polymerization and degradation will play an important role in both the synthesis and chemical recycling of green and sustainable polyurethane. This minireview covers the new synthetic routes to polyurethane without using diisocyanate, the biodegradation of polyurethane, and the enzymatic synthesis and the chemical recycling of poly(ester-urethane) (PEU) and poly(carbonate-urethane) (PCU). The lipase-catalyzed polymerization of low molecular weight and biodegradable urethanediols with short-chain dialkyl carbonate and alkanedioates produced PCU and PEU, respectively. They were readily degraded in an organic solvent into the repolymerizable cyclic oligomers by lipase as a novel chemical recycling. These results will be applicable for the production strategies of green and sustainable polyurethanes.

Diethyl l-aspartate was polymerized by a bacterial protease from Bacillus subtilis (BS) in organic solvent at a temperature between 30 and 50 degrees C to yield alpha-linked poly(ethyl l-aspartate) having an M(w) of up to 3700 and a maximum polymer yield of 85%. The best polymerization conditions were the 40 degrees C polymerization of diethyl l-aspartate using 30% protease BS containing 4.5 vol % water in acetonitrile for 2 days. Poly(ethyl l-aspartate) was readily depolymerized by the enzyme into the oligomeric and monomeric l-aspartate in aqueous acetonitrile. Poly(sodium aspartate) prepared by the saponification of poly(ethyl l-aspartate) was readily biodegradable by activated sludge obtained from the municipal sewage treatment plant. Also, poly(sodium aspartate) was depolymerized by the hydrolase enzyme into the monomeric aspartate. These results may indicate the sustainable chemical recycling and biorecycling of this polymer.

A series of enzymatically recyclable poly(ester-urethane)s consisting of a biodegradable diurethane moiety as a hard segment and an ester moiety as an enzymatically cleavable linkage was chemo-enzymatically prepared by two routes. The poly(ester-urethane) was prepared by a) the ring-opening polymerization of a cyclic ester-urethane monomer synthesized via the transesterification reaction of biodegradable diurethanediol and dicarboxylate ester using lipase and b) the direct polycondensation of a diurethanediol and a dicarboxylate ester. A significantly higher molecular-weight poly(ester-urethane) having the highest molecular weight (Mw) of 101,000 was produced by the ring-opening polymerization of the cyclic ester-urethane monomer when compared with that produced by the polycondensation of the dicarboxylate ester with diurethanediol. The poly(ester-urethane) was readily degraded by lipase into the corresponding cyclic oligomers; the oligomers were readily repolymerized by the ring-opening polymerization using lipase for chemical recycling.

Oxiranes, such as glycidyl phenyl ether, benzyl glycidate, glycidyl methyl ether, and styrene oxide, were copolymerized with dicarboxylic anhydrides, such as succinic anhydride, phthalic anhydride, and maleic anhydride, by the action of an enzyme in a stepwise reaction to produce the corresponding polyesters containing some ether linkages having a maximum Mw of 13 500. Oxiranes, such as glycidol and glycidyl phenyl ether, were also homopolymerized and copolymerized with other oxiranes by the enzyme to produce the corresponding polyethers.Enzymatic polymerization of oxiranes and dicarboxylic anhydrides.magnified imageEnzymatic polymerization of oxiranes and dicarboxylic anhydrides.

Novel enzymatically recyclable poly(carbonate-urethane) consisting of a diurethane moiety as a hard segment and a carbonate linkage as an enzymatically cleavable unit was prepared by the polycondensation of biodegradable diurethanediol and diethyl carbonate using lipase. The produced poly(carbonate-urethane) was readily transformed by lipase into the corresponding cyclic oligomers which were more easily repolymerized by lipase to produce a higher molecular weight poly(carbonate-urethane) than that of the parent poly(carbonate-urethane).