The Interfacial Polycarbonate Reaction: Modeling the Kinetics of Carbamate Side Reactions

Interfacial phosgenation of bisphenols to form polycarbonates is catalyzed by trialkylamines. Trialkylamines also react with chloroformate-terminated intermediates and form acyl ammonium salts. These salts are important intermediates in the polymerization but also may undergo decomposition to chain-terminating carbamates that contaminate the polymer. These side reactions are modelled by the use of phenyl chloroformate and triethylamine which leads to the formation of N,N-diethyl phenyl carbamate. Teh reaction is exothermic but has been measured under isothermal conditions in refluxing dichloromethane (at 39 deg C). The initial salt-forming step is quickly established at near equilibrium conditions; its apparent equilibrium constant is 21 plus/minus 2 L/mol and its rate constat is estimated to be at least an order of magnitude greater than 30 L/mol/min. Subsequent decomposition of the salt appears to follow first order kinetics in the salt concentration. Its rate constant is 1.3 plus/minus 0.2 min exp --1 under the experimental conditions. The various definitively to exclude base abstraction as an ancillary mechanism. NMR showed that unreacted triethylamine is loosely associated with species within the reaction and does not exist as free amine. Graphs. 10 ref.--AA