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 ) was used to prepare poly(propylene oxide) with a molar mass (M_n ) >500 000 g mol^-1 , in some cases even >10⁶  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 polyethers 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. This strategy can also be extended to other epoxides, including functionalized monomers.