Lipase-catalyzed ring-opening polymerizations of 4-substituted ε-caprolactones:: Mechanistic considerations

Lipase-catalyzed ring-opening polymerizations of 4-substituted epsilon-caprolactones employing Novozym 435 as the biocatalyst demonstrate dramatic differences in polymerization rates and selectivity depending on the size of the substituent. Quantification of the reaction rates shows that the polymerization rate decreases by a factor of 2 upon the introduction of a Me substituent at the 4-position. Moreover, 4-EtCL polymerizes 5 times slower than 4-MeCl and 4-PrCL is even 70 times slower. The decrease in polymerization rate is accompanied by a strong decrease in enantioselectivity: while the E-ratio of 4-MeCL polymerization is 16.9, the E-ratios of 4-EtCL and 4-PrCL are 7.1 and 2.0, respectively. Interestingly, Novozym 435 displays S-selectivity for 4-MeCL and 4-EtCL in the polymerization reaction, but the enantioselectivity is changed to the (R)-enantiomer in the case of the 4-PrCL. The nature of these differences was investigated by hydrolyzing all monomers in water/diisopropyl ether mixtures employing Novozym 435 as the catalyst. In the hydrolysis reactions, the rates are only moderately affected upon increasing the substituent size, and the enantioselectivity is S in all cases, also for 4-PrCL. Again, a steady decrease of the E-ratio was observed upon increasing the substituent size, but this was less pronounced than in the polymerization reactions: the E-ratios were 17.6, 12.4, and 4.6, going from 4-MeCL to 4-PrCL. For 4-substituted epsilon-caprolactones, the results obtained are a clear indication that the chirality of the propagating alcohol chain end is important in the catalytic cycle and that-in contrast to unsubstituted lactones-the rate-determining step is not necessarily the formation of the acyl-enzyme intermediate but more likely the deacylation of the acyl-enzyme intermediate by the propagating alcohol chain end.