A catalytic peptide/hemin complex as an artificial enzyme for RAFT polymerization

Abstract

This work explores the potential of artificial enzymes to initiate reversible addition–fragmentation chain-transfer (RAFT) polymerization. Although natural enzymes like horseradish peroxidase (HRP) have been shown to initiate RAFT polymerization under mild conditions successfully, their activity is often hampered by high temperatures or even the sulfur-containing chain transfer agents (CTAs) required for the RAFT polymerization. Here, we synthesize a library of short peptides from histidine (H) and phenylalanine (F) by an ester–amide exchange reaction in deep eutectic solvents. The peptides then bind with hemin to form an HF/hemin complex that uses H₂O₂ as the substrate and reacts with acetylacetone to produce radicals that initiate RAFT polymerization. The HF/hemin-catalyzed RAFT polymerization not only shows good control over molecular weight distribution, but also tolerates high reaction temperatures and CTAs. A high monomer conversion can be achieved, although the reaction rate is slow at the later stages of the polymerization. In addition, glucose oxidase (GOx) and HF/hemin work together as an enzymatic cascade to initiate RAFT polymerization under air. These results imply the versatility of artificial enzymes composed of short peptides and hemin in initiating RAFT polymerization under conditions that are not possible for natural enzymes.