Polymerization of isobutylene (IB) in supercritical carbon dioxide (SC.SO2) at 32.5-degrees-C and 140 bar by the use of 2-chloro-2,4,4-trimethyl-pentane (TMPCI) initiator in conjunction with a mixture of TiCl4/BCl3 leads to well-defined polyisobutylenes (PIB) capped by a t-Bu head group and a t-Cl tail group (tBu-PIB-Cl(t)) of M(n) approximately 1800 g/mole and M(w)/M(n) = 1.3. The TiCl4/BCl3 mixture may be viewed a new Friedel-Crafts Acid that effects rapid initiation, essentially chaintransferless propagation and reversible termination. The mechanism of IB polymerization of TiCl4/BCl3 mixtures is discussed.