Probing water-temperature relationships for lipase-catalyzed lactone ring-opening polymerizations

Polymerizations of epsilon-CL catalyzed by Novozyme-435 (immobilized Lipase B from Candida antarctica) were studied at temperatures between 20 and 108 degreesC. The monomer conversion to polymer was remarkably rapid at ambient temperature. At 20 degreesC by 7 h, epsilon-CL conversion and product M-n were >97% and 17 800, respectively. Contrary to previous reports, the number of chains formed, as well as the product molecular weight, was almost identical for polymerizations at constant enzyme water content between 60 and 108 degreesC. Thus, differences in reaction temperature over a 48 degreesC range did not free water from bound states so that it could function for chain initiation. At 60 degreesC, variation in the enzyme water content from 0.6 to 1.9% increased the number of chains formed but did not change the polymerization propagation kinetics. Therefore, the enzyme water content and not the reaction temperature regulated the product molecular weight. In contrast, at 108 degreesC, an increase in the reaction water content from 0.6 to 1.8% increased both the number of chains and the polymerization propagation kinetics. Explanations for these differences in behavior as a function of temperature and water contents are discussed.