On the mechanism of polymerization of methyl substituted 7‐oxabicyclo‐[2.2.1]‐heptanes

The polymerization of endo‐ and exo‐2‐methyl‐7‐oxabicyclo‐[2.2.1]‐heptane with phosphorus pentafluoride catalyst in methylene dichloride and nitrobenzene has been found to proceed with increasing yield and molecular weight with decreasing temperature in the range +25°C to −30°C, but at −78°C there was no polymerization. The ring‐opening appears to proceed exclusively through nucleophilic attack at C‐1 in the alkyloxonium ion, leading to only one type of structural unit in each type of polymer. The polymers of the exo‐2‐methyl isomer are completely soluble in THF, whilst those of the endo‐2‐methyl isomer are insoluble. The solubility appears to be connected with flexibility of the chain molecule associated with the possibility for interconversion between different chair conformations for the cyclohexane ring‐structures. By ring‐closure of 2.6‐dimethylcyclohexane‐(1.4)‐diol two isomers have been prepared and identified. The endo,exo‐2.6‐dimethyl‐7‐oxabicyclo‐[2.2.1]‐heptane gives insoluble, highly crystalline polymers with phosphorus pentafluoride catalyst in methylene dichloride. The failure of the corresponding exo,exo‐2.6‐dimethyl isomer to polymerize under identical conditions is explained by conformational analysis.