(649d) N/Al Lewis Pair Catalysts for the Synthesis of Functional Polyether Materials
AIChE Annual Meeting
Friday, November 20, 2020 - 8:30am to 8:45am
Polyethers â derived from epoxides â represent a versatile class of functional polymeric materials with unrealized potential. Epoxide ring-strain, the driving force for polymerization, depends little on the particulars of monomer structure. This fact enables true compositional control of structure-property relationships in a macromolecular platform. Unfortunately, epoxides do not polymerize to high molecular weights by any method in common usage not requiring sophisticated synthetic skill and equipment. Our group has developed a class of stable, crystalline, mono(Âµ-alkoxo)bis(alkylaluminum) (MOB) initiators that offer control of molecular weight distribution, architecture, and tolerance to chemical functionality. The initiators can be synthesized in a single step with purification afforded by direct crystallization from the reaction medium. While the MOBs have been demonstrated to be effective tools for polymer synthesis, from a mechanistic standpoint there is more to the story: The MOBs rearrange upon addition of polar monomer into a difunctional construct with sites for both chain-growth and catalysis. The catalytic moiety is a simple Lewis pair between an alkyl-aluminum and an alkyl-amine. The chain-growth and catalytic functionalities can be decoupled into distinct species. With initiation/chain-growth decoupled, we applied the emergent N-Al Lewis pair catalyst to the synthesis of block polymer materials, which revealed regimes of architectural control for the N-Al catalysts. Further fundamental mechanistic insight revealed that the nature of the polymerization mechanism was most consistent with chain-end activation following catalyst instantiation triggered by addition of polar monomer to the catalyst complex. This leads to kinetics of monomer consumption that vary from effectively zeroth order to first order. I will highlight the application of this enabling chemistry to the synthesis of membranes and polymeric cryoprotectants.