Syndiotactic- and heterotactic-specific radical polymerization of N-n-propylmethacrylamide complexed with alkali metal ions

Abstract

We investigated the radical polymerization of N-n-propylmethacrylamide (NNPMAAm) in the presence of alkali metal bis(trifluoromethanesulfonyl)imides (MNTf₂), in particular LiNTf₂. The addition of MNTf₂ led to a significant improvement in the yield and molecular weight of the resulting poly(NNPMAAm)s. Furthermore, the solvent employed influenced stereospecificity in the presence of LiNTf₂. The stoichiometry of the NNPMAAm–Li⁺ complex appeared to be critical for determining the stereospecificity in the NNPMAAm polymerization. The 1 : 1-complexed monomer in protic polar solvents provided syndiotactic-rich polymers, whereas the 2 : 1-complexed monomer in aprotic solvents gave heterotactic-rich polymers. Stereochemical analyses revealed that m-addition by an r-ended radical was the key step in the induction of heterotactic specificity in the aprotic solvents. Spectroscopic analyses suggested that the Li⁺ cation played a dual role in the polymerization process, with Li⁺ stabilizing the propagating radical species and also activating the incoming monomer. Kinetic studies with the aid of electron spin resonance spectroscopy revealed that the addition of LiNTf₂ caused a significant increase in the k_p value and a decrease in the k_t value. The stereoregularity of poly(NNPMAAm)s was found to influence the phase transition behavior of their aqueous solutions. In a series of syndiotactic-rich polymers, the phase-transition temperature decreased gradually with increase in the rr triad content. Furthermore, heterotactic-rich poly(NNPMAAm) exhibited high hysteresis, which increased in magnitude with increasing mr triad content.