The Influence of Aluminum-Containing Lewis Acids on Polyisobutylene, Isobutylene-lsoprene Copolymers (Butyl Rubber), and Chlorinated Isobutylene-lsoprene Copolymer (Chlorobutyl)

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Abstract
The influence of aluminum-containing Lewis acids, e.g., AlCl3, AlEtCl2, AlEt2Cl, AlEt3, and AliBu3, on polyisobutylene, isobutylene-isoprene copolymer (butyl rubber), and chlorinated butyl rubber has been investigated in nonpolar and polar solvents at various Lewis acid concentrations in the temperature range -10 to -78°C. Polyisobutylene does not degrade even under the most aggressive conditions employed (AlEtCl2, -10°C). Butyl rubber degrades rapidly in the presence of AlEtCl2 in the range -10 to -50°C. In contrast, no degradation occurs with the milder Lewis acid AlEt2Cl; however, in conjunction with small amounts of a suitable Bronsted acid (i.e., HCl) immediate and extensive degradation takes place with AlEt2Cl as well. Chlorobutyl rubber severely and very rapidly degrades in the presence of AlCl3 and AlEtCl2. With the less acidic AlEt2Cl and AlEt3, molecular weight breakdown can be prevented only when employing milder conditions, i.e., at low Lewis acid concentrations in nonpolar solvents at lowest temperatures. A comprehensive mechanism involving carbonium ions of these degradation processes is proposed.

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