Non-metal with metal behavior: metal-free coordination-insertion ring-opening polymerization

The "coordination-insertion" ring-opening polymerization (ROP) mechanism has so far been the monopoly of metal catalysts. In this work, we present a metal-free "coordination-insertion" ROP of trimethylene carbonate (TMC) and epsilon-caprolactone (epsilon-CL), as well as their sequential block copolymerization, with N-trimethylsilyl-bis (trifluoromethanesulfonyl)imide (TMSNTf2) as the non-metallic initiator/catalyst. TMSNTf2 was proposed to work through an unprecedented metal-free "coordination-insertion" mechanism, which involves the coordination of monomer to the Si atom of TMSNTf2, the nucleophilic attack of the -NTf2 group on the coordinated monomer, and the cleavage of the acyl-oxygen bond of the monomer. The proposed metal-free "coordination-insertion" ROP was studied by NMR, SEC, and MALDI-TOF analyses. In addition, the TMSNTf2-mediated ROP of TMC and epsilon-CL led to linear and cyclic polymers following two-stage first-order polymerization processes, as evidenced by structural analyses and kinetics study, which further demonstrated the metal-free "coordination-insertion" mechanism.