The homopolymerization of p-chloro-alpha -methylstyrene (pCl alpha MeSt) and its sequential block copolymerization with isobutylene (IB) initiated by 1,3-dimethyl-1,3-diphenyl-1-chlorobutane (Di alpha MeSt . HCl) in conjunction with BCl3 was investigated in methylcyclohexane/methyl chloride (60/40, v/v) at -80 degreesC. The homopolymers exhibited close to theoretical M-n's, and the diagnostic M-n-conversion and first-order (In[M](0)/[M]-time) plots were linear; thus, chain transfer and termination were not detected. The propagating chain end of poly(pCl alpha MeSt) was exceptionally stable; no decomposition could be detected even after 5 h under monomer-starved conditions. According to the H-1 NMR spectrum of homopoly(pCl alpha MeSt), the polymer is 94% syndiotactic; however, the precipitated polymer exhibited only a glass transition at 198 degreesC. Recrystallization from methylcyclohexane-methyl ethyl ketone solvent mixture yielded a semicrystalline polymer with T-m = 229 degreesC. Sequential block copolymerization of pCl alpha MeSt with IB was also carried out by the addition of IB to the solution of living poly(pCl alpha MeSt), followed by the addition of TiCl4. By gel permeation chromatography, the crossover efficiency was similar to 100%, and no homopolymer contamination was detected.