Triphenylmethyl salts which contain metal chlorides or bromides as the anion were prepared systematically, and employed as initiator for the cationic polymerization of α-methylstyrene. The isotactic unit content of the polymer was found to correlate with the anion size in a relatively simple way except for pentacoordinate anions. This was explicable by considering the steric effect of the counteranion and the coulombic interaction of the growing ion pair. Pentacoordinated counteranions gave rise to greater amounts of the isotactic unit than expected from their sizes, and, in the case of Ph3SnCl4Br, the polymer steric structure was dependent on the initiator concentration. These results were attributed to the peculiar structural characteristics of these anions.
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