A method to regulate copolymer structure using lipase catalyzed transesterification reactions, by selecting a first reactant from the group consisting of polymers and monomers, selecting a second reactant from the group consisting of polymers and monomers, and combining the first reactant, the second reactant, and a lipase in a reaction vessel and allowing the ensuing transesterification reaction to proceed resulting in a polymer. The method allows one to regulate the copolymer repeat unit sequence distribution, and may take place with chain-length specificity such that activation of chains for exchange reactions with other chains or monomers can occur preferentially at certain locations along the chain.