Unveiling the mechanisms of epoxide polymerization with N-Al adduct catalysts: a comprehensive experimental and theoretical investigation

Polyethers are a technologically and industrially important class of polymers, often made from the ring opening polymerization of functional epoxide monomers. While there are several techniques available to polymerize epoxides, no one technique can claim to be a general epoxide polymerization platform in the same way that reversible addition fragmentation with chain transfer (RAFT) polymerization can be considered for vinyl-based monomers or ring opening metathesis polymerization (ROMP) can be considered for cyclic olefins. Recent work by our group and others has demonstrated a general, tunable, and accessible epoxide polymerization platform featuring Earth-abundant aluminum-based compounds. While several epoxide polymerization methods feature aluminum, our method is distinct in feature set; providing control over kinetics, molecular weight, end group, architecture, and composition. However, the polymerization mechanism is not understood, lying between other aluminum-based epoxide polymerization techniques. In this work, we elucidate the mechanism of epoxide polymerization with aluminum-based compounds through a combination of experiment and theory. We reveal the effect of N-Al adduct catalyst chemistry on the polymerization rate and polydispersity, filling a crucial gap in the experimental understanding of the system. Our study employs density functional theory (DFT) to rationalize the full polymerization mechanism, including adduct formation, enchainment, polymerization, and catalyst reformation. Our findings are consistent with both new and previously reported experiments, providing a clear picture of the complex interactions between monomer, initiator, and catalyst. Our results pave the way for the further enhancement of catalyst and initiator design to maximize the polymerization power of the system. We provide a roadmap for future research and development, leveraging our mechanistic insight to advance this promising platform for polyether synthesis. Overall, our work enhances understanding epoxide polymerization with Earth-abundant aluminum-based compounds.