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The Effect of Counteranions on the Polymer Steric Structure in the Cationic Polymerization of α-Methylstyrene

The cationic polymerization of α-methylstyrene was carried out at −78°C by using Lewis acids and triphenylmethyl salts as initiator. The polymerization with triphenylmethyl salts was much slower than that with Lewis acids. The triad tacticity data of the polymer determined by NMR spectroscopy fitted the theoretical curve of one parameter (Bovey’s σ value). The σ value, though almost independent of the initiator used in CH2Cl2–CH3CN (σ≅0.04), increased with decreasing polarity of polymerization solvents and varied moderately (0.07–0.34) with the initiator in nonpolar solvents. In the case of triphenylmethyl salt initiators, the order of counteranions with increasing σ value (SnCl5−≳SbCl6−>AlBr4−> AlCl4−>BF4−) was the same as that observed for the isotactic propagation of isobutyl vinyl ether. These results were interpreted within the framework of the cationic propagation scheme proposed by us earlier. The steric influence of counteranions was discussed, and the unusual effect of SnCl5− was considered in connection with its coordination state. The variation of the σ value with Lewis acids (SnCl4 excluded) was much smaller than that with triphenylmethyl salts.