Ring-opening polymerization of propylene carbonate: Microstructural analysis of the polymer and selectivity of polymerization by 2D-NMR techniques

Abstract

Ring-opening polymerization (ROP) of five-membered cyclic carbonates suffers from fractional decarboxylation during polymerization. The phenomenon is well-established for polymerization of ethylene carbonate. Propylene carbonate has similar five-membered ring structure, however, there is an additional methyl group on the ring. This methyl group might influence the polymerization rates and extent of decaroboxylation during polymerization. Herein, we report on polymerization of propylene carbonate and detailed microstructral analysis of the polymer by advanced one-dimensional (1H and 13C, DEPT-90, DEPT-135) and two-dimensional NMR techniques (TOCSY, HSQC and HMBC). The detailed structural characterization by advanced NMR techniques revealed the co-existence of carbonate and ether linkages in the polymers. A possible mechanism of ROP of propylene carbonate is proposed. The formation of carbonate linkage majorly proceeds through primary alcoholic end-group whereas ether linkages are formed primarily by attack on methylene carbon instead of methine carbon. The preferred attacking sites resulted in head to head regioesletive structure of the polymer. The quantification of ether and carbonate linkages in the polymer is realized by 1H NMR spectra. Furthermore, selectivity of polymerization with regard to polymerization speed, increase in molar mass and relative carbonate to ether content in the polymer as a function of polymerization conditions (such as temperature, monomer to initiator ratio, polymerization time and catalyst concentration) are evaluated.