The living anionic polymerization of activated aziridines: a systematic study of reaction conditions and kinetics

Abstract

“A living race” – polymerization kinetics of anionic polymerizations depends strongly on the solvent polarity and reactivity of the growing chain end. Both the carb- and oxyanionic polymerization is under control at the university lab and on the industrial level, however, no information for the aza-anionic polymerization of aziridines has been reported systematically. This work studies the polymerization of two activated aziridines (2-methyl-N-mesylaziridine (MsMAz) and 2-methyl-N-tosylaziridine (TsMAz)) by real-time ¹H NMR spectroscopy. This technique allows monitoring the consumption of the monomer precisely during the polymerization under different conditions (temperature, solvent, initiator and counter-ion variation). From the experimental data, propagation rate constants (k_p) were calculated and analyzed. The polymerization of MsMAz was monitored at different temperatures (20, 50, and 100 °C). The increase of temperature increases the speed of polymerization, but keeps the living behavior. Furthermore, the influence of different solvents on the polymerization speed was examined, proving solvating solvents such as DMSO and DMF as the fastest solvents. Two different initiators, the potassium salts of N,N′-(1,4-phenylenebis(methylene))dimethanesulfonamide (BnBis(NHMs)), the first bifunctional initiator for the AROP of aziridines, and of N-benzyl-sulfonamide (BnNHMs) were compared. The variation of the counter ions Li⁺, Na⁺, K⁺, and Cs⁺ (generated from the respective bis(trimethylsilyl)amide salts) proved successful polymerization of both monomers with all counter ions. Slight variations have been detected in the order: Cs⁺ > Li⁺ > Na⁺ > K⁺, which is in strong contrast for the AROP of epoxides, shows a strong gegenion-dependent kinetic profile. This allows the use of commercially available initiators, such as BuLi for the synthesis of PAz. With these results in hand, the azaanionic polymerization can be used as a valuable tool in the family of anionic polymerization for the preparation of structurally diverse polysulfonamides and polyamines under a broad variety of conditions, while maintaining the living behavior.