The counterion effect on the steric course of the cationic polymerization of tert-butyl vinyl ether

The cationic polymerization of tert-butyl vinyl ether was carried out at ?76°C with a variety of triphenylmethyl salts as initiator. The microtacticity of the polymer was determined by ¹H NMR spectroscopy after conversion to poly(vinyl alcohol). The fraction of isotactic units, P_m, increased with decreasing polarity of the polymerization medium, in consistence with the published data for metal halide initiators. The P_m value decreased gradually from 0,85 to 0,72–0,75 in 7 : 3 (v/v) CH₂Cl₂-toluene by increasing the counteranion radius from 2,4 Å (BF^?₄) to ca. 4 Å (AlCl^?₄). A further increase in the counteranion size, however, did not notably influence the steric structure of the polymer. These structure variations are discussed in terms of the steric course of cationic propagation. In the case of pentacoordinate counteranions (SnCl^?₅ and ZrCl^?₅), the steric structure depended also on the counteranion concentration, as already reported for other monomers.