Thermodynamics of ring-opening polymerization of cyclic propylene oxalate: keeping the polymerizability/recyclability balance

Thermodynamics is of crucial importance for the polymerizability of novel monomers undergoing ring-opening polymerization (ROP) as well as the recyclability of resulting polymers. We investigated the equilibrium between propylene oxalate polymerization and depolymerization, along with cyclic oligomers formation, in bulk using stannous octoate as a catalyst. To this end, the ROP was launched at the temperature ranging from 70 to 110 °C. Activation energy of the examined polymerization was similar to those of other six-membered cycles and amounted to 67 ± 3 kJ∙mol^− 1. The equilibrium monomer concentration rose with temperature, while the content of cyclic oligomers in the polymerization mixture was stable, following common regularities for ROP systems. The thermodynamic functions of propylene oxalate polymerization were calculated from the dependence of the equilibrium degree of conversion on temperature. The enthalpy of bulk polymerization calculated relative to the liquid monomer as a standard state was − 17.1 ± 0.5 kJ∙mol^− 1 and the entropy of polymerization under these conditions was − 19.4 ± 1.4 J∙mol^− 1∙K^− 1. Polymerization/depolymerization thermodynamics indicated that propylene oxalate is a thermodynamic favored monomer (ceiling temperature of 608 °C) and provides the possibility of poly(propylene oxalate) recycling. Additionally, it was shown that destruction of poly(propylene oxalate) proceeded predominantly via depolymerization pathway.