Buffer and Polymer Molecular Weight Affect Zinc Myoglobin-Mediated PET-RAFT Polymerizations

Protein photocatalysts provide a powerful and highly adaptable method for synthesizing and transforming small organic molecules; however, they remain underexplored for photocatalytic polymer synthesis. In particular, the effects of polymer molecular weight and protein structure are unknown in the context of photoinduced electron/energy transfer reversible-activation fragmentation chain transfer (PET-RAFT) polymerizations. Herein, we investigate the effects of buffer conditions and the molecular weights of macro chain-transfer agents (macro-CTA) on zinc myoglobin-mediated PET-RAFT chain-extension polymerizations. Chain extensions performed in tris buffer showed a 17% higher apparent rate constant than those performed in PBS. Macro-CTA molar mass also affected the rate of chain extension polymerizations, where a 22 kg/mol macro-CTA showed a chain extension apparent rate constant 40% faster than a 75 kg/mol macro-CTA. Finally, we calculated the solvent-accessible surface area (SASA) of model polymers and found that the RAFT end group becomes less accessible to the protein photocatalyst as the macro-CTA molar mass increases, corroborating the observed decrease in the apparent rate constant. Overall, this work provides insight into the use of proteins as PET-RAFT catalysts by examining how polymerization conditions affect catalyst performance.