The solvent-free enzyme-catalyzed polytransesterification of divinyl adipate and 1,4-butanediol yielding high molecular-weight polyesters was studied. This heterogeneous system is characterized by initial exothermicity, a 10(4)-fold increase in viscosity and complex kinetics involving parallel reactions and variable reaction volumes. Herein a semitheoretical analysis of solvent-free polytransesterification was developed to further refine understanding of the role of diffusion, especially how molecular weight and polydispersity evolve during solvent-free biocatalytic polytransesterification. The evolution of polydispersity observed during the polymerization was attributed to diffusion, and therefore internal diffusion limitations were assessed experimentally. The analysis demonstrated that the system is initially under weak diffusional control, which is strengthened by the initial exothermicity of the reaction. At molecular weights over 5,000 Da, the system experienced severe mass-transfer resistance due to chain entanglements. Reduced enzyme specificity with increasing chain length, enzyme deactivation, and vinyl hydrolysis dampen the diffusional constraints toward the end of the polymerization such that the system could return to slight diffusional or kinetic control on exceeding molecular weights of 20,000 Da.