Step Toward Cationic RAFT Polymerization of Less Reactive Monomers: Dithiobenzoates Bearing Electron-Withdrawing Groups as Chain-Transfer Agents

Here, we report a new strategy for cationic RAFT polymerization of less reactive monomers, such as p-methylstyrene and styrene. This strategy implies that the addition of electron-withdrawing substituents to the stabilizing group of cumyl dithiobenzoate results in a decrease in the stability of cationic intermediates, thus facilitating their fragmentation and cationic RAFT polymerization. Particularly, it was demonstrated that both cumyl dithiobenzoate and cumyl dithiobenzoate, with a trifluoromethyl substituent in the Z group, activated by a small amount of SnCl4, induced living cationic RAFT polymerization of p-methoxystyrene, affording polymers with molecular weights of up to 80,000 and 30,000 g mol–1, respectively. In contrast to the unsubstituted counterpart, cumyl dithiobenzoate with a trifluoromethyl substituent in the Z group was also an efficient chain-transfer agent for conducting the living cationic RAFT polymerization of p-methylstyrene, giving polymers with Mn up to 10,000 g mol–1 and moderate dispersity (Đ = 1.3–1.8), while polymerization was terminated at incomplete conversion when cumyl dithiobenzoate was used. Cumyl dithiobenzoate with a trifluoromethyl substituent in conjunction with SnCl4 also induced cationic RAFT polymerization of styrene, affording well-defined polystyrenes with Mn up to 5000 g mol–1. Finally, block copolymers of p-methylstyrene with styrene and methyl methacrylate were successfully synthesized via a mechanistic transformation from cationic to radical RAFT polymerization.