Mechanism of Isobutylene Polymerization: Quantum Chemical Insight into AlCl₃/H₂O-Catalyzed Reactions

Abstract

The production of polyisobutylene with Lewis acid catalysts has been in widespread use for over 60 years, but no validated molecular-level understanding of the reaction mechanism exists. We have computed initiation and propagation reaction pathways for isobutylene polymerization under industrially relevant conditions with an AlCl3/H2O initiator from density functional theory calculations. The initiator/catalyst complex we identified is fundamentally different from the putative complex identified in the literature, which typically assumes that the AlCl3OH2 complex is the active catalyst. We found that the reaction pathway with the AlCl3OH2 complex is infeasible due to unreasonably high energy barriers. Our calculations indicate that a minimum of two AlCl3 groups and one H2O molecule is required to initiate the reaction and that the complex must produce a highly acidic proton. It is the extreme acidity of the complex that is crucial for successful initiation of the reaction. The active catalyst moiety we identified produces low-energy-barrier pathways for both initiation and propagation steps. This complex was identified using the growing-string method to identify possible reaction pathways with various AlCl3/H2O complexes. The initiation reaction with our proposed complex was observed to occur naturally in an ab initio molecular dynamics simulation under typical operating conditions, confirming the activity of the complex.