The polymerization of alpha-methylstyrene was studied in the absence of a separately added cationogen (i.e., initiator) and in the presence of a proton trap, using boron halides (BX3, X = Cl, Br, and I). The effect of the monomer and BX3 concentration, solvent polarity, and temperature was examined. The polymerization rates increased with decreasing temperature and with increasing solvent polarity. The molecular weight (M-n) of the products also increased with decreasing temperature; however it was relatively independent of solvent polarity. Under identical conditions the M-n's were highest with BCl3, lower with BBr3, and lowest with BI3. Kinetic experiments support that initiation is by haloboration, but direct proof was only obtained for BI3. For polymers made with BI3 the presence of one boron per chain was confirmed by elemental analysis, while the high M-n's obtained with BCl3 and BBr3 prevented quantitative analysis of head and end groups.