Role of diffusion in biocatalytic polytransesterification

The solvent-free enzyme-catalyzed polytransesterification of divinyl adipat e 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 react ions and variable reaction volumes. Herein a semitheoretical analysis of so lvent-free polytransesterification was developed to further refine understa nding of the role of diffusion, especially how molecular weight and polydis persity evolve during solvent-free biocatalytic polytransesterification. Th e evolution of polydispersity observed during the polymerization was attrib uted to diffusion, and therefore internal diffusion limitations were assess ed experimentally. The analysis demonstrated that the system is initially u nder weak diffusional control, which is strengthened by the initial exother micity of the reaction. At molecular weights over 5,000 Da, the system expe rienced 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 p olymerization such that the system could return to slight diffusional or ki netic control on exceeding molecular weights of 20,000 Da.