Palladium Catalysts for Norbornene Polymerization. A Study by NMR and Calorimetric Methods

Neutral trans-[Pd(C6F5)XL2] (X = Cl, Br) and cationic trans-[Pd(C6F5)L2(NCMe)]BF4 (L = SbPh3, AsPh3, As(C6Cl2F3)Ph2, AsCyPh2, AsMePh2, PPh3) complexes have been studied and tested for norbornene (NB) polymerization and copolymerization with 5-norbornene-2-carboxaldehyde (NB-CHO). The neutral complexes are almost inactive and produce only small amounts of oligomers, but the cationic complexes with arsines and stibines are very good for palladium-catalyzed norbornene polymerization. External coordinating molecules (e.g., ligands or monomers with O-donor groups) compete with the olefin function for the coordination sites on Pd and inhibit the reaction. In spite of this, copolymerization of norbornene and 5-norbornene-2-carboxaldehyde could be achieved, with lower yields and higher incorporations of the functionalized norbornene as the NB:NB-CHO ratio decreases. The complex with the fluorinated arsine AsPh2(C6Cl2F3) is the most active catalyst and allows for easy spectroscopic study of the reacting systems by 19F NMR. Calorimetry provides straightforward monitoring and detection of activation and deactivation processes in polymerization reactions that give insoluble products and cannot be followed by NMR spectroscopy.