Synthesis of thermo-sensitive polymers with super narrow molecular weight distributions: PET-RAFT polymerization of N-isopropyl acrylamide mediated by cross-linked zinc porphyrins with high active site loadings

Abstract

To overcome the aggregation of porphyrins and realize heterogeneous catalysis with high active site loadings, twisted ZnTHP–Me₂Si and layered ZnTHP–Ph₂Si are synthesized by cross-linking 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin zinc (ZnTHP) respectively with dichlorodimethylsilane and dichlorodiphenylsilane. The microstructures of both ZnTHP–Me₂Si and ZnTHP–Ph₂Si are investigated, and their active site loadings were found to be greater than 0.8 mmol ZnTHP per g ZnTHP-Me₂Si or per g ZnTHP-Ph₂Si. Furthermore they act as photocatalysts in the photoinduced electron transfer reversible addition–fragmentation chain transfer (PET-RAFT) polymerization of N-isopropyl acrylamide (NIPAm) under green light. The results indicate that both of them show more effective photocatalytic performance than ZnTHP and demonstrate the attractive features of living polymerization. Moreover, twisted ZnTHP–Me₂Si with a larger surface area and pore distribution can accelerate the polymerization of NIPAm more effectively. According to the MALDI-TOF analysis, the dispersity of the as-synthesized poly(N-isopropyl acrylamide) (PNIPAm) catalyzed by ZnTHP–Me₂Si is less than 1.100 and the minimum value can reach 1.049. The excellent recyclability of ZnTHP–Me₂Si is also confirmed by repeated PET-RAFT polymerization cycles, albeit with a slight reduction of the catalyst activity. Therefore PET-RAFT polymerization mediated by cross-linked ZnTHP is a potential method to synthesize a thermo-sensitive polymer with a super narrow molecular weight distribution.