Highly efficient vinylic addition polymerization of 5-vinyl-2-norbornene using benzylic palladium complexes as precatalysts

In contrast with the parent norbornene, the vinylic addition polymerization of substituted norbornene derivatives is difficult to achieve. We have found that benzylic palladium complexes are suitable precatalysts in the challenging vinylic addition polymerization of 5-vinyl-2-norbornene (VNB). A mixture of [Pd2(μ-Br)2(η3-C6H5CHCH2C6F5)2] (1), PCy3 and NaBArF4 leads to a very active catalyst that homopolymerizes VNB leading to high molecular weight polymers where the exocyclic pendant double bond is preserved. Only 2 ppm of palladium are needed to obtain VA-PVNB in high yield. The order of mixing of the components of the precatalyst mixture does not affect the degree of polymerization but it influences the initiation step which occurs by insertion of VNB: (a) into a Pd–H bond generated from the neutral complex 1 or (b) into a Pd–C(benzyl) bond when the cationic [Pd(η3-C6H5CHCH2C6F5)(PCy3)(L)]BArF4 is preformed. In either case, the easy transformation from η3- to a σ-benzyl opens a coordination site on the metal that favors the initiation. The nature of the counterion is important in the propagation of the polymerization and, among several low-coordinating anions tested, BArF4− is the most efficient.