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 saturated peroxyketals to induce transfer in the free radical polymerization of styrene was investiga ted in various solvents. An increase of the polarity of the solvent resulte d in the rise of the chain transfer coefficient in the thiol and dithiobist hioformate cases, unsaturated peroxyacetals was unaffected by the change of solvent. The ability of the two former transfer agents to exist in a polar ized transition state in polar media accounted for this result. Taking adva ntage of these features, these transferring functions were introduced at th e end of PEO chains and the resulting transfer agents were subsequently use d to obtain PEO-b-PS copolymers. Among all hydrophilic transfer agents trie d, methyl ether of omega -mercaptoacetate PEO with a molar mass of 2000 g.m ol(-1) was found particularly interesting, exhibiting a transfer coefficien t (Ct) close to the ideal value of I in acetonitrile/water (95/5) solution. Under these conditions and in the presence of the latter transfer agent, w ell-defined PS-b-PEO copolymers could be obtained by mere dispersion polyme rization of styrene taken to completion.