Abstract
The polymerization of 6,8-dioxabicyclo[3.2.1]octane (DBO) was carried out in toluene, methylene chloride, and 1-nitropropane over the wide range of temperatures from −78 to 30°C. Boron trifluoride etherate was used as an initiator. The stereoregularity of the polymer obtained was evaluated from the relative intensities of the two NMR peaks appearing at τ 5.15 and 5.60, which were due to the equatorial and axial acetal protons of the tetrahydropyran ring, respectively. The EQ—H% of the polymer, defined as the percent of the equatorial acetal protons to the total acetal protons, decreases from nearly 100 to 70–80 depending on the solvents used with the rise in the polymerization temperature. On the basis of these results, it is concluded that the cationic polymerization of DBO proceeds at lower temperatures through an SN2 mechanism involving the reverse side attachment of the monomer to the acetal carbon atom of a propagating cyclic trialkyloxonium ion to afford a stereoregular polymer. At higher temperatures and in polar solvents, the participation of a car-boxonium ion derived from the cyclic trialkyloxonium ion in the propagation reaction and/or the competitive oxonium ion exchange reaction at the propagating chain end becomes significant, thus causing the loss of stereospecificity of the polymerization.
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Okada, M., Sumitomo, H. & Hibino, Y. Polymerization of Bicyclic Acetals. II. Steric Course in the Cationic Polymerization of 6,8-Dioxabicyclo[3.2.1]octane. Polym J 6, 256–263 (1974). https://doi.org/10.1295/polymj.6.256
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DOI: https://doi.org/10.1295/polymj.6.256