Novel, efficient, palladium-based system for the polymerization of norbornene derivatives: Scope and mechanism

[(1,5-Cyclooctadiene)(CH₃)Pd(Cl)], when reacted in situ with 1 equiv of a monodentate phosphine ligand and 1 equiv of the complex Na⁺[3,5-(CF₃)₂C₆H₃]₄ B^- was found to catalyze the vinyl addition polymerization of norbornene derivatives, including those with pendant oxygen functionalities. For norbornene, a polymerization rate of 1000 tons norbornene/mol Pd·h was observed at 25 °C. For several norbornene derivatives, the molecular weight of the polymer was found to decrease with increasing amounts of added 2-propanol. Mechanistic data confirm a vinyl insertion mechanism for these polymerizations. The polymerization rate was found to decrease dramatically for norbornene derivatives with pendant oxygen functionalities. The effect of coordinating solvents and the uptake of endo vs exo isomers for functionalized norbornenes was tested. Experiments show that (a) the endo isomer reacts more slowly than the exo isomer and (b) both isomers react much more slowly compared to norbornene derivatives lacking coordinating functionalities. Reaction of 5-norbornene carboxylic acid ethyl ester with the [(Et₃P)₂Pt(H)]⁺ fragment yields the endo-inserted product exhibiting intramolecular coordination of the ester functionality to the platinum center. The formation of chelates, both upon the coordination of the endo-functionalized norbornene and in the endo-inserted product, appears to be responsible, in part, for the observed decrease in polymerization rate for functional norbornene derivatives. A further reason for the diminution of activity of both the endo- and the exo-functionalized isomers is simply the coordination of the functionality. Of the two factors, the latter is the dominant one.