Enzyme biocatalyst for removal of residual monomer fractions in waterborne polymers

Abstract

This study explores a sustainable post-polymerization approach for waterborne methyl methacrylate/butyl acrylate (MMA/BA) polymer dispersions (latexes) using horseradish peroxidase (HRP) in the presence of hydrogen peroxide and acetylacetone to catalyze radical initiation at extremely low monomer concentrations (ppm level). In the heterogeneous latex system, where residual monomers are primarily confined within polymer particles, the initial monomer conversion was hindered by aqueous-phase radical generation via the enzymatic catalyst, limited monomer diffusion, and restricted penetration of growing oligomers into the particles. Extending the post-polymerization reaction time and supplying an additional fraction of initiator successfully overcame these constraints, enabling very high conversion at 22 °C (>95 % for MMA and ∼ 90 % for BA). As a result, the biocatalytic latex treatment at 22 °C reduced the residual monomer content in the latex from 2345 to 114 ppm. Polymer films prepared from the treated latexes contained MMA/BA oligomers formed during post-polymerization and traces of the enzyme, which reduced mechanical strength but did not affect water uptake. However, the films exhibited improved thermal stability, likely due to enhanced char formation from residual HRP that delayed thermal degradation. Overall, this work demonstrates that HRP-mediated post-polymerization provides a greener strategy for eliminating residual monomers in waterborne polymers, with minimal adverse impact on polymer performance. Traditional polymerization methods, especially chemical ones, often require relatively high monomer concentrations to drive chain propagation efficiently. Demonstrating enzymatic polymerization in very dilute systems reveals previously unexplored operational regimes, providing enhanced mechanistic understanding under these conditions.