Copolymerization of Isothiocyanates and Nonactivated Aziridines Mediated by Zwitterionic Intermediates through Condensation and Chain Growth

Electrophilic isothiocyanates and nucleophilic aziridines have been successfully copolymerized to produce polyisothioureas through either condensation or chain growth polymerization. Aziridine-derived zwitterionic intermediates (ZWIs) present in the polymerization have been identified unambiguously by employing real-time high-resolution mass spectrometry (HRMS) and time-evolving proton nuclear magnetic resonance spectra (1H NMR), which has been revealed to play critical roles in both copolymerization processes. The spontaneous condensation–polymerization is found to proceed readily via the ZWIs free of any initiator and catalyst, involving cationic polymerization to produce polyimine content. Alternatively, transformation of the condensation–polymerization into comparably fast chain growth polymerization can be achieved by introducing an anionic nucleophile to initiate and activate the ZWI served as the monomer herein, leading to well-defined fully alternating copolymers of the isothiourea repeat unit (−NC(═N)S−) with discrete chain ends, which is supported by the density functional theory (DFT) calculations. The DFT calculations also suggest that the formation of the ambiphilic ZWIs would be favored between the pairs of the nucleophilic aziridine with a higher energy of the highest occupied molecular orbital (HOMO) and the electrophilic isothiocyanate possessing a lower energy of the lowest unoccupied molecular orbital (LUMO), and the reactivity of the ZWI in the polymerization is influenced by its electronic structure.