Block Copolymer Sequence Inversion through Photoiniferter Polymerization.

Block copolymers prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization are often restricted to a specific comonomer blocking sequence that is dictated by intermediate radical stability and relative radical leaving group abilities. Techniques that provide alternative pathways for reinitiation of thiocarbonylthio-terminated polymers could allow access to block copolymer sequences currently unobtainable through the RAFT process. We report a method for preparing "inverted" block copolymers, whereby the traditional order of monomer addition has been reversed through the use of photoiniferter-mediated radical polymerization. Specifically, thiocarbonylthio photolysis of xanthate- and dithiocarbamate-functional macromolecular chain transfer agents (macro-CTAs) led to the direct formation of leaving group macroradicals otherwise unaffordable by an addition-fragmentation mechanism. We believe this method could provide a route to synthesize multiblock copolymers of synthetically challenging comonomer sequences.