The polymerization of a-methylstyrene (alpha MeSt) was studied in conjunction Viith the (CH3)(3)-c CCH2C(CH3)(2)CH2C(Ph)(2)Cl (1,1-diphenylethylene-capped 2,4,4-trimethyl-2-chloropentane)/SnBr4 or SnCl4 initiating systems. Using SnBr4, living polymerization was obtained in methyl chloride (MeCl)/hexane (Hex) 60/40 to 40/60 v/v at -80 or-60 degrees C. The polymers exhibited theoretical molecular weights and narrow molecular weight distributions (M(w)/M(n) similar to 1.1). The low polymerization rate obtained in MeCl/ Hex 40/60 v/v more than doubled when the solvent polarity was increased to 60/40 v/v. A similar ca. twofold increase in rate was also observed when the temperature was raised from -80 to -60 degrees C. Living polymerization and high rates were obtained using SnCl4; however, theoretical molecular weight and therefore 100% initiator efficiency were only achieved at [SnCl4] similar to 0.5[chain end]. Mechanistic studies indicated that the initiator is fully ionized at this ratio, giving rise to SnCl62- counteranions. The polymer chain end, however, is not fully ionized, and propagation involves SnCl5- counteranions.