Structural Origin of the Fast Polymerization Rates and Monomer Universality of Pyrazole-Based Photoiniferters

Herein, we report a combined computational and experimental investigation into the recently reported universal pyrazole-based reversible addition-fragmentation chain transfer (RAFT) agents (Z-C(=S)-S-R, where Z is 3,5-dimethyl-1<i>H</i>-pyrazol-1-yl), which can mediate controlled radical polymerization of a broad scope of monomers without the need for an additional initiator or catalyst. The results reveal that the high molar absorption coefficient and efficient photolysis kinetics of pyrazole-based chain transfer agents (CTAs) under blue light (λmax = 465 nm) enable rapid radical generation, underpinning ultrafast polymerization of acrylates, acrylamides, methacrylates, and <i>N</i>-vinylpyrrolidone (NVP). While the efficient light absorption is attributed to structural dissimilarity between the Z group and the S-R group (which breaks the local symmetry of the C=S group), the fast photolysis originates from favorable π electron donation from the Z group to the C=S group. Meanwhile, the π electron donation is still weaker than in xanthates, which explains the excellent control of a wide range of monomers, except methacrylates. This work establishes design principles for next-generation CTAs for ultrafast and monomer-universal photoiniferter RAFT polymerization.