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Lewis Pair Polymerization of Epoxides via Zwitterionic Species as a Route to High-Molar Mass Polyethers.

A dual catalytic setup based on N-heterocyclic olefins (NHOs) and magnesium bis(hexamethyldisilazide) (Mg(HMDS)2) enables the preparation of poly(propylene oxide) with a molar mass (Mn) > 500.000 g mol-1, in some cases even > 10^6 g mol-1, as determined by GPC/light scattering. This is achieved by combining the rapid polymerization characteristics of a zwitterionic, Lewis Pair-type mechanism with the efficient epoxide activation by the Mg(II) species. Transfer-to-monomer, traditionally frustrating attempts at synthesizing polyether with a high degree of polymerization, is practically removed as a limiting factor by this approach. NMR and MALDI-ToF MS experiments reveal key aspects of the proposed mechanism, whereby the polymerization is initiated via nucleophilic attack by the NHO on the activated monomer, generating a zwitterionic species. It is further demonstrated that this strategy can be extended to other epoxides, including functionalized monomers.