DFT Insights Into Non-Catalytic Aminolysis of Polycarbonates

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<middle dot>mol-1. The same reaction with methylamine dimer is characterized by activation and reaction free energies equal to 147.1 and -77.7 kJ<middle dot>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<middle dot>10-16 at 298 K and 1.20<middle dot>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.