Dual role for alkali metal cations in enhancing the low-temperature radical polymerization of N,N-dimethylacrylamide

Abstract

The radical polymerization of N,N-dimethylacrylamide (DMAAm) has been investigated in the presence of several alkali metal salts, including lithium bis(trifluoromethanesulfonyl)imide (LiNTf₂). The addition of an alkali metal salt led to a significant increase in the yield and molecular weight of the resulting polymer. NMR analysis of mixtures of DMAAm and LiNTf₂ suggested that DMAAm was being activated by the coordination of Li⁺ to its CO group. Electron spin resonance analysis of the DMAAm polymerization in the presence of LiNTf₂ suggested that the propagating radical was being stabilized by Li⁺ through a single-electron lithium bond, because a signal for the propagating radical of the acrylamide derivatives was observed for the first time in solution when LiNTf₂ was added. Based on these results, we have proposed a mechanism for this polymerization, where the propagation steps occur between a lithium ion-stabilized propagating radical and a lithium ion-activated incoming monomer. Furthermore, polymers with a wide range of stereoregularities, such as isotactic, syndiotactic and heterotactic systems, were successfully prepared using this method by carefully selecting the appropriate combination of solvent and alkali metal salt.