DFT Insights Into Non-Catalytic Aminolysis of Polycarbonates

Abstract

The problem of environmental pollution by plastic is becoming more and more obvious. In this study, the non-catalytic reaction of diphenyl carbonate with methylamine as a model reaction for the depolymerization of polycarbonate was studied at the B3LYP/6-31++G(d,p) and M062X/6-31++G(d,p) levels. The reaction can proceed by the nucleophilic substitution and by the “addition–elimination” pathways. Calculations at the B3LYP/6-31++G(d,p) level indicate that the reaction of diphenyl carbonate with methylamine monomer leading to the formation of N-methyl-O-phenylcarbamate via the nucleophilic substitution pathway at 298 K is characterized by activation and reaction free energies equal to 174.0 and −57.1 kJ·mol⁻¹. The same reaction with methylamine dimer is characterized by activation and reaction free energies equal to 147.1 and −77.7 kJ·mol⁻¹, respectively. Thermodynamic and kinetic preference is also observed in the reaction of N-methyl-O-phenylcarbamate with the monomer and dimer of methylamine. Reactions with the formation of tetrahedral intermediates are unlikely. Their formation is endothermic and occurs with a decrease in entropy. This leads to small values of equilibrium constants. The equilibrium constant of the reaction of diphenyl carbonate with methylamine monomer to form a tetrahedral intermediate is 1.64·10⁻¹⁶ at 298 K and 1.20·10⁻¹⁴ at 423 K. The same trends are observed in the reactions of N-methyl-O-phenylcarbamate. The reactions of diphenyl carbonate, N-methyl-O-phenylcarbamate with methylamine dimer via the “addition–elimination” pathway are also characterized by small values of equilibrium constants. In all cases, interactions involving the methylamine dimer are more favorable than reactions involving the methylamine monomer.