The Mechanism of Isobutylene Polymerization: New Insight into Proton-Catalyzed Polymerizations

Low molecular weight polyisobutylenes (PIBs) are crucial additives for lubricants, adhesives, and fuels. Industrial processes for PIB polymerization have existed for over 50 years, but very little is known about the atomic-level details of the reaction mechanism. “Conventional PIB”, which contains internal double bonds (trisusbtituted or tetrasubstituted), is routinely produced by cationic polymerization co-initiated by AlCl₃ and adventitious water. Although many different PIB catalysts have been reported in the literature, there is virtually no atomic scale understanding of these reaction mechanisms and therefore limited fundamental insight on how to model and control the reaction process. In this presentation we report atomic scale reaction mechanisms for PIB polymerization with AlCl₃ discovered from quantum chemistry modeling. Contrary to textbook mechanisms for this process, which assumes initiation by a single AlCl₃ and a single water molecule, we found that the conventional cationic polymerization involves at least two AlCl₃ molecules and a single water molecule. We have explored potential initiation, propagation, and termination steps for PIB polymerization and have mapped out potential reaction pathways and barriers.