The polymerization of isobutylene has been investigated by the use of the steady, slow, continuous monomer addition technique in the presence of a variety of initiating systems, i.e., “H2O”/TiCl4, “H2O”/AlCl3, C6H5C(CH3)2Cl/TiCl4, p-ClCH2C6(CH3)4* CH2Cl/AlCl3at -50°C. Quasiliving polymerizations have been obtained with the “H2O” and C6H5(CH3)2Cl/TiC14systems in 60/40 v/v n-hexane/methylene chloride solvent mixtures with very slow monomer input. After a brief “flash” polymerization, theMnof PIB increased linearly with the cumulative amount of monomer added (consumed); however, the number of polymer molecules formed also increased, indicating the presence of chain transfer to monomer. With the “H2O”/TiCl4initiating system,Mn,maxwas 56,000 andMw/Mn< 2.0. By the use of the C6H5C(CH3)2CL/TiCl4initiating system, quasiliving polymerization has been achieved and chain transfer could virtually be eliminated.