Dependence of transfer reaction on solvent in the free radical polymerization of styrene: Application to the synthesis of PS/PEO copolymers using PEO-based transfer agents

The ability of thiols, dithiobisthioformates and unsaturated peroxyketals to induce transfer in the free radical polymerization of styrene was investigated in various solvents. An increase of the polarity of the solvent resulted in the rise of the chain transfer coefficient in the thiol and dithiobisthioformate cases, while the behavior of unsaturated peroxyacetals was unaffected by the change of solvent. The ability of the two former transfer agents to exist in a polarized transition state in polar media accounted for this result Taking advantage of these features, these transferring functions were introduced at the end of PEO chains and the resulting transfer agents were subsequently used to obtain PEO-b-PS copolymers. Among all hydrophilic transfer agents tried, methyl ether of ω-mercaptoacetate PEO with a molar mass of 2000 g · mol^-1 was found particularly interesting, exhibiting a transfer coefficient (C₁) close to the ideal value of 1 in acetonitrile/water (95/5) solution. Under these conditions and in the presence of the latter transfer agent, well-defined PS-b-PEO copolymers could be obtained by mere dispersion polymerization of styrene taken to completion.