Spontaneous Zwitterionic Polycondensation for N,S,P-Heteroatom Polymers and Ring Formation

Abstract

N,S,P-heteroatom polymers have been synthesized through spontaneous zwitterionic polycondensation of nucleophilic nonactivated aziridines and electrophilic Lawesson’s reagent at high concentrations. The evolution of zwitterionic linear polymers and neutral macrocycles during the copolymerization process has been unveiled through an efficient end-functionalization strategy coupled with a robust Montmorillonite clay-based purification protocol for selective isolation of cyclic compounds, combined with characterizations by high-resolution mass spectrometry, time-resolved 1H NMR, and absolute molecular weight SEC. The intramolecular ring closure can be a dominant process in the polycondensation to yield macrocycles up to 90% in a highly concentrated reactant solution. By comparing with the kinetics model developed specifically for the irreversible AB-type condensation polymerization, which incorporates the kinetics of the linear growth and the cyclization under a Gaussian statistics assumption, the propensity of cyclization has been ascribed to the synergistic seeding-like effects from the inherent cyclic conformation of the zwitterionic oligomers at the early stage of polymerization, reinforced by the electrostatic interaction in the zwitterionic system. DFT calculations also reveal the correlation of cyclization tendency with the conformational structures of the oligomers and the substituents. The solvent, temperature, and Lewis acid additive have been utilized to regulate ionic interactions, association of the zwitterionic intermediates, and the polycondensation process, thus tuning the composition and topology of the products. The fabricated transparent polymer film exhibits a high refractive index of nD25 ∼ 1.73.