The enantioselectivity of the Novozym® 435 catalysed ring-opening of lactones is related to the conformation of the ester: cisoid lactones show S-selectivity or no selectivity while transoid lactones show a pronounced R-selectivity. The inability of Novozym 435 to polymerise 6-MeCL stems from its opposing selectivities for the alcohol and acyl moiety. By combining Novozym 435 with a racemisation catalyst, unreactive terminal alcohols in the S-configuration can be turned into reactive terminal alcohols of the R-configuration which can propagate. Combining 2 different catalysts that work together to accomplish propagation, also referred to as iterative tandem catalysis, is an elegant approach to convert a racemic monomer quantitatively into a homochiral polymer.
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