“Controlled” High-Speed Anionic Polymerization of Propylene Oxide Initiated by Alkali Metal Alkoxide/Trialkylaluminum Systems

The anionic polymerization of propylene oxide initiated by alkali metal alkoxide suffers from several drawbacks such as a slow polymerization rate in nonpolar solvents and an important chain transfer reaction to monomer. We found that the addition of trialkylaluminum to the alkali metal alkoxide/propylene oxide system in hydrocarbon media strongly enhances the polymerization rate and strongly reduces the transfer reactions, thus allowing the controlled synthesis of poly(propylene oxide) with relatively high molar masses (up to 20 000 g/mol). At constant monomer and alkali metal alkoxide concentrations the polymerization rate increases with increasing trialkylaluminum concentration. Kinetic data and 1H NMR studies indicate that the trialkylaluminum derivative is involved in the formation of two distinct complexes, one with the alkali metal alkoxide and another with the PO monomer. The strong electron-withdrawing on PO α-carbons associated with AlR3 complexation makes the monomer much more susceptible to ring opening. Moreover, since the withdrawing effect is much less pronounced on the PO methyl group, the complexation also results in a higher selectivity of the nucleophilic species toward the ring-opening reaction to the detriment of the proton abstraction process yielding transfer to monomer.