From-syndiotactic-to-isotactic stereogradient methacrylic polymers by RAFT copolymerization of methacrylic acid and its bulky esters

Abstract

The synthesis of stereogradient polymers with tacticities that vary from predominantly syndiotactic to highly isotactic was investigated by reversible addition–fragmentation chain transfer (RAFT) copolymerization of bulky methacrylates, such as triphenylmethyl methacrylate (TrMA) and 1-phenyldibezosuberyl methacrylate (PDBSMA) and methacrylic acid (MAA) in both non-polar and polar solvents. The MAA monomer showed increased reactivity in toluene because of hydrogen bonding and was consumed slightly faster than TrMA or PDBSMA. However, the RAFT copolymerization of TrMA and MAA in 1,4-dioxane resulted in consumption of both monomers at the same rate. The copolymers can be easily converted to homopoly(MAA) by the acid hydrolysis of the bulky group and converted further to poly(methyl methacrylate) by methyl esterification using trimethylsilyldiazomethane to analyze the molecular weights and tacticity. The molecular weights of the polymers obtained in both solvents increased with monomer conversion, which indicates that controlled/living radical copolymerization proceeded irrespective of the solvents. ¹³C NMR analyses of the polymers revealed that stereogradient polymers were produced in toluene, in which the tacticity changed from mm = 11% to nearly 100%, whereas the copolymers obtained in 1,4-dioxane resulted in nearly atactic enchainment (rr/mr/mm ≈ 38/49/13), independent of monomer conversion. A similar stereogradient copolymer was also obtained by RAFT copolymerization of PDBSMA and MAA in toluene, where the isotacticity changed more gradually from mm = 14% to nearly 100%.