Ion and Ion-Pair Contributions to the Polymerization of Tetrahydrofuran

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Abstract
The cationic polymerization of tetrahydrofuran is generally considered to be effected by an oxonium chain end in the form of an ion pair. Consideration of expected ionic dissociation constants of salts in typical solvents for these reactions suggests that appreciable concentration of free oxonium ions would be present under polymerization conditions. The dissociation constants of such salts have been measured in methylene chloride solution and shown to be near 4 × 10−6, sufficiently high that a contribution to the polymerization by free ions could be expected. Rate constants for the propagation reaction of the polymerization initiated by triethylox-onium tetrafluoroborate have been measured, in both the presence and absence of the common-ion salt tetrabutylam-monium tetrafluoroborate. From the dependence of the rate constant on the initiator concentration and from its reduction by varying concentration of the free common ions, it is possible to calculate the ion-pair rate constant to be 1.4 × 10−3 M sec−1 and the free oxonium ion rate constant to be 1.0× 10−2 M sec−1.