The lipase-catalyzed polytransesterification of divinyl adipate and 1,4-butanediol is a dynamic process controlled by the relative magnitudes of the rates of transesterification and hydrolysis as the polymerization proceeds. A mathematical model that describes the kinetics of the biocatalytic polytransesterification process is presented. Initial rate studies with model substrates in transesterification, hydrolysis, and esterification were used to provide the initial kinetic parameters that, when combined with the effect of enzyme specificity, were used to predict product compositions under different reaction conditions. A comparison of experimental data and model predictions shows that the product composition is strongly influenced by the selective nature of enzyme catalysis, and this model effectively predicts the results of a step condensation polymerization where reactivity depends on molecular weight.