To gain increased insight into the mechanism of living polymerization of isobutylene (IB) and specifically into the effect of the structure of the initiator on the rate, we have investigated the polymerization of IB initiated by eight 2,4,4-trimethylpentyl (TMP) esters RCOO-C(CH3)2C(CH3)3where R = -CCl3, -CHCl2, -CH2C6H5, -CH3-CH(CH3)2, -C(CH3)3, -C6H5, and -CH=CHC6H5in conjunction with BCl3coinitiator using CH3Cl diluent at -30°C. The rates decreased along the sequence of these substituents from very high values (with R = -CCl3, -CHCl2) to very low values (R = -C(CH3)3, -C6H5, -CH=CHC6H5). The trend of decreasing rates was interpreted in terms of inductive effects. According to conversion-time curves obtained with the five esters R = -CH2C6H5, -CH(CH3)2, -C(CH3)3-C6H5, and -CH=CHC6H5, propagation is first order in monomer and the apparent rate constants of propagation decrease along the above sequence, suggesting the presence of inductive effects and the absence of resonance effects. With highly electron-donating substituents, i.e., with R = -CH(CH3)2-C(CH3)3, -C6H5, and -CH=CHC6H5, chain transfer to monomer is operational, the rates of which were found to be monomolecular (zero order in monomer). Chain transfer to monomer can be avoided by increasing the polarization of the C-O bond by using slightly electron-donating or strongly withdrawing substituents (R = -CH2C6H3, -CH3or -CHCl2, -CCl3) or by the use of CH2Cl2; both measures also result in enhanced propagation rate constants. Solvent polarity critically affects the stability of the growing chain end. By decreasing the polarity of the solvent, the decomposition temperature of the growing site decreases, leading to termination. Quenching studies have been carried out with model compounds as well as with polymerization systems and both kinds of experiments indicated the exclusion formation of t-chloro endgroups.