Kinetic Studies on the Cationic Ring-Opening Polymerization of Tetrahydrofuran Initiated by Superacid Esters

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Abstract The present paper descibes kinetic studies on the cationic ring-opening polymerization of tetrahydrofuran (THF) initiated by superacid esters such as ethyl fluorosulfate (EtOSO2F), chlorosulfate (EtOSO2Cl), and trifluoromethanesulfonate (EtOSO2CF3). The phenoxyl end-capping method was found to de applicable to kinetic analyses of the THF polymerization initiated by superacid esters. Also, the rate constants of propagation (kp) and of initiation (ki) were determined directly by NMR spectroscopic method. Rate constants determined by these two methods were in good agreement within experimental error. As a polymerization mechanism it was established that the propagation proceeds via cyclic oxonium species. The kp values of superacid esters varied depending upon the nature of the counter anion, OSO2X−, e.g., kp at 0°C in CH2Cl2: 0.66×10−3 (X=F), 1.4×10−3 (X=Cl), and 1.7×10−3 (X=CF3) 1/mol·sec, respectively. These magnitudes, however, were at least lower than one half of those of typical Lewis acid initiators. Initiation by superacid ester is a dipole-diple reaction, and ki values were about 1/10–1/20 of the corresponding value of triethyloxonium tetrafluoroborate (Et3O+BF4−). Activation parameters were calculated for both the propagation and initiation reactions. In order to compare the present systems with those of superacid initiators, model reactions were examined by NMR spectroscopy.