Fundamental studies on cationic polymerizations: Molecular weights and molecular weight distributions of polyisobutylenes produced by γ‐irradiation (free ions) and chemical catalysis (ion pairs)

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Abstract
The molecular weights (M̄n, M̄v, and M̄w) and molecular weight distributions of polyisobutylenes synthesized by γ‐irradiation in the range 29 to −78°C have been determined. The plots of log M̄n, log M̄v, and log M̄w versus 1/T are linear and parallel (EM̄ = −6.36 ± 0.5), and M̄w/M̄n ≈ 3.0 ± 0.5 over this temperature range. The viscosity‐average molecular weights of polyisobutylenes obtained by γ‐irradiation were compared with those of polymers prepared by BF3, EtAlCl2, and AlCl3. It is found that, at the same polymerization temperature, the M̄v of radiation‐induced polyisobutylenes, which propagate by free carbonium ions, is significantly higher than those synthesized by the Lewis acids, which probably propagate via ion pairs. The implications of these findings are discussed, and it is concluded that the counterion in the proximity of the growing cation impedes propagation to a greater extent than it does the competing process of chain transfer to monomer.