Polymerization of Isobutylene by AlCl3/Ether Complexes in Nonpolar Solvent

• Published in: Macromolecules 2012-04, Vol.45 (8), p.3318-3325
• Main Authors: Dimitrov, Philip, Emert, Jack, Faust, Rudolf
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Polymerization; Preparation, kinetics, thermodynamics, mechanism and catalysts
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma3003856

The carbocationic polymerization of isobutylene (IB), co-initiated by AlCl3/ether complexes, has been reexamined and extended to different dialkyl ethers. In the absence of a proton trap, 2,6-di-tert-butylpyridine (DTBP), the polymerization of IB by the cumyl alcohol (CumOH)/AlCl3·nBu2O initiator/co-initiator system in dichloromethane/hexanes (80/20 v/v) at −40 °C gave high conversion to polyisobutylene (PIB) comprising exo-olefins with high selectivity, similar to that reported before by Vasilenko et al. , However, in the presence of DTBP, polymerization was absent, suggesting that CumOH is not an initiator in conjunction with AlCl3·Bu2O, and the true initiator is adventitious water. Similarly, in the presence of DTBP in hexanes at 0 °C, polymerizations were absent not only with CumOH but with CumCl, tert-butanol, and 2-chloro-2,4,4-trimethylpentane. The polymerization of IB could be initiated only with adventitious water in the absence of DTBP, but monomer conversions and exo-olefin content (60–70%) were much lower than in a polar solvent and the PIBs exhibited M n = 700–4200 with high polydispersities (PDI ∼ 3–5). The separate addition of ether followed by AlCl3 to the polymerization mixture resulted in conventional PIB with high trisubstituted olefinic content. The previously proposed mechanism is inadequate, as it cannot explain all the observations. Mechanistic studies suggest that the reaction of water with AlCl3·R2O yields H+AlCl3OH–, which initiates the polymerization, and free ether, which abstracts a β-proton from the growing chain end before it diffuses from the immediate vicinity of the polymer cation. Accordingly, the role of the complex is to deliver the ether to close proximity of the propagating end.