The copolymerization of the isobutylene (IB)-isoprene (IP) monomer pair by the cumyl acetate (CumOAc)/TiCl4, cumyl propionate (CumOPr)/TiCl4, and dicumyl methyl ether (DiCumOMe)/TiCl4 initiating systems has been studied using CH3Cl/n-C6H14 (40 vol %/60 vol %) mixed solvent in the -30 to -80-degrees-C range. Irreversible termination was absent in these systems, and the extent of chain transfer decreased with decreasing temperature. Cyclization of polyisoprene sequences is not detected even with 70 mol % IP in the charge. In contrast, with the CumOAc/BCl3 initiating system irreversible termination was found at -40-degrees-C and cyclization occurred beyond 20 mol % IP in the charge. IB-IP copolymer compositions determined by H-1 NMR spectroscopy are in good agreement with values calculated by the conventional reactivity ratios, r(IB) = 2.5 and r(IP) = 0.4. In line with the mechanism proposed for living IB polymerizations, it is suggested that CumOAc, CumOPr, or DiCumOMe in the presence of excess TiCl4 give cumyl chloride (CumCl) or dicumyl chloride (DiCumCl), respectively, plus the corresponding in situ electron pair donors (ED) TiCl3OAc, TiCl3OPr, and TiCl3OMe, and that the copolymerizations are induced by the Cum+ or +DiCum+ carbocations arising from CumCl or DiCumCl plus excess TiCl4. The propagating centers are in rapid equilibrium with dormant polymeric chlorides. The very similar apparent activation enthalpy differences of chain transfer and propagation calculated from parallel Arrhenius plots of the highest theoretically obtainable molecular weights-ln (1/M(n,lim)BAR) versus reciprocal temperatures-for tert-ester (CumOAc and CumOPr)/TiCl4 and tert-ether (DiCumOMe)/TiCl4 initiated IB-IP copolymerizations (DELTA(DELTA-H) = 5.1 and 4.6 kcal/mol, respectively) indicate that initiation and propagation occur by a common species, while the different apparent activation entropy differences (DELTA(DELTA-S) = 3.2 and 1.6 cal/K.mol) are most likely due to the different in situ EDs arising from the different initiators. Living copolymerization leading to soluble random copolymers has been obtained with the IB/2,4-dimethyl-1,3-pentadiene (DMPD) system. Internal strain and steric hindrance by the three methyl groups of the conjugated diene facilitate propagation and prevent chain transfer. In contrast to IB-IP copolymerizations, rate deceleration was not observed with IB-DMPD systems and IB-DMPD copolymers were found to be richer in DMPD than charged at incomplete conversions.
