Rates of Elementary Reactions in Cationic Polymerization of Tetrahydrofuran Catalyzed by Trialkyloxonium Salts

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Abstract
Kinetic studies are reported on the polymerization of tetrahy drofuran by trialkyloxonium salts. Triethyloxonium tetrafluoroborate (Et3 OBF4) and -tetrachloroaluminate (Et3OAlC14) were employed as the initiator. At various times the polymerization system was treated with excess sodium phenoxide. The propagating species of polymer and the remaining triethyloxonium of initiator were converted quantitatively into the corresponding phenyl ethers. Phenetole from the remaining catalyst was successfully separated from the polymer phenyl ether by vacuum distillation. The concentrations of propagating species and the remaining initiator were determined separately by UV analysis of the corresponding phenyl ethers. In the polymerization by Et3OBF4 a slow termination occurs to destroy the propagating species, i.e., this polymerization is not strictly a living system. The polymerization by Et3OAlC14, on the other hand, is characterized by a very rapid termination. On the basis of the time-[P*] and time-conversion data in the polymerization by Et30BF4, the propagation rate constant kp was calculated kp according to where [M]t's and [M]e are the instantaneous and the equilibrium monomer concentrations, respectively. In addition, the rate constants of initiation (ki) and termination (kt) for the two initiators were roughly estimated on the basis of the above kinetic data. The kt values for them were very much different from each other, whereas the ki values were almost the same.