Searching for More Effective Agents and Conditions for the RAFT Polymerization of MMA:  Influence of Dithioester Substituents, Solvent, and Temperature

A series of tertiary dithiobenzoates differently substituted on the phenyl group (Z) were synthesized in order to investigate the RAFT-mediated polymerization of MMA. The chain-transfer agent 2-cyanoprop-2-yl dithiobenzoate, although an excellent RAFT agent for polymerization of MMA, does not yield a very narrow polydispersed polymer at low conversion. The introduction of electron-withdrawing substituents on the dithiobenzoate aromatic ring improves the activity of the chain-transfer agent narrowing the molecular weight distribution, especially in the first 2 h of the process; conversely, electron-donating substituents deteriorate the performance of the process in the early stages. Substituents on the dithiobenzoate that, for reasons of steric hindrance reduce the conjugation of the phenyl with the CS double bond of the dithiogroup, diminish the effectiveness of the chain-transfer agent. The nature of the leaving group (R) also has a strong influence on the process: the substitution of 2-cyanoprop-2-yl with the bulkier 2-cyano-4-methylpent-2-yl as leaving group improves remarkably the MMA polymerization, especially in the early stages. The solvent effect on MMA RAFT polymerization using benzene, acetonitrile, and DMF was also investigated. The lower propagation rate constant of MMA in benzene gives rise to reduced rates of polymerization but narrower polydispersities of the polymer, especially at low conversions. The RAFT polymerizations of MMA were carried out at 60 and 90 °C to assess the temperature dependence of the process. The higher temperature increases both the rate of polymerization and the transfer constant of the RAFT agent, resulting in lower polydispersities at a given conversion. Ab initio calculations confirmed the observed experimental results.