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.