Living Free Radical Polymerization Initiated By Bithioxanthydrol

In this study, the possibility of utilizing reversible coupling and reactivation mechanism of semipinacol radicals to initiate living radical polymerization was explored. The thermal/light initiator,9,9'-bithioxanthydrol (BITX), was synthesised by reductive coupling of two thioxanthone molecules. Because of the carbon-carbon weak bond of BITX, it can undergo homolytic cleavage to give two same semipinacol radicals. BITX-semipinacol radicals, unlike triphenylmethyl radicals that scarcely initiate vinyl polymerization, can serve as both initiators and terminators for the polymerization of methyl methacrylate (MMA) and styrene (St).The resulting short-chain polymers with BITX-semipinacol groups at both ends could further initiate the polymerization due to the thermal cleveage of semipinacol end groups. As a result, it was found that the monomer conversion and molecular weight of polymers increased linearly with the polymerization time. Moreover, the influence of several factors, including reaction temperature, the concentration of BiTX, side groups of BiTX, polarity and activity of monomers and the AIBN/BiTX polymerization system, on the polymerization process was investigated.The polymers with semipinacol groups at chain ends could induce further radical polymerization of second monomers under thermal condition to give block copolymers. Based on these results, it could be concluded that the polymerization system with BiTX as thermo/light initiator showed living feature. Therefore, we first demonstrated that the radical polymerization process could be controlled by the reversible coupling/reactivation mechanism of weak C-C bond.