Controlled Cationic Polymerization of Sulfide-Containing Vinyl Ethers

Abstract

The living cationic polymerizations of sulfide-containing vinyl ethers, namely, alkyl-sulfide-containing 2-(ethylthio)ethyl vinyl ether (ESEVE) and arylsulfide-containing 2-(phenylthio)ethyl and 2-[4-(methylthio)phenoxy]ethyl vinyl ethers (PSEVE and MSPEVE, respectively), were examined using HCl/ZnCl₂, 1-(isobutoxy)ethyl acetate (IBEA)/Et_1.5AlCl_1.5, and IBEA /Et_1.5AlCl_1.5/SnCl₄ as initiators. The polymerization of ESEVE terminated at a 35% monomer conversion, and a high-molecular-weight polymer was not obtained; instead, the nucleophilicity of the sulfur moiety in ESEVE led to cyclization, forming five-membered ring sulfonium species. In contrast, the side reactions were suppressed when PSEVE was used, which has a less-nucleophilic sulfur moiety on its benzene ring, and polymerization proceeded to an 88% conversion, affording a high-molecular-weight polymer with a narrow molecular weight distribution (M_w = 12,500, M_w/M_n = 1.18). Furthermore, cyclic sulfonium was not produced during the polymerization of MSPEVE, in which the sulfur atom was farther from the generated carbocation; the polymerization proceeded quantitatively producing a high-molecular-weight polymer with a narrow molecular weight distribution, despite the short lifetime of the growing species. In particular, a high-molecular-weight polymer with the narrowest distribution (M_w = 13,500, M_w/M_n = 1.08) was obtained when SnCl₄ was used as the Lewis acid.