Effect of 5-Me substituent(s) on the catalytic activity of palladium(II) 2,2′-bipyridine complexes in CO/4-tert-butylstyrene copolymerization

The coordination of two 5-substituted-2,2′-bipyridines L (L1=5-methyl-2,2′-bipyridine, L2=5,5′-dimethyl-2,2′-bipyridine) to palladium was studied. The neutral complexes [Pd(L)Cl2] and [Pd(L)(Me)Cl], and the cationic complexes obtained after chlorine abstraction [Pd(L)2][BAr′4]2 and [Pd(L)(Me)(NCMe)][BAr′4] (Ar′=3,5-(CF3)2-C6H3), respectively, were isolated and characterized by NMR and FAB mass spectroscopy. The complex [Pd(L2)(L3)][BAr′4]2 (L3=2,2′-bipyridine) bearing different ligands, was prepared for comparison purposes. The activity of the monocationic and dicationic complexes as catalytic precursors in the CO/4-tert-butylstyrene copolymerization was compared with that of related well-known catalysts containing the unsubstituted 2,2′-bipyridine as nitrogen ligand, to evaluate the influence of the substituents in 5- and 5,5′-position. The presence of one or two substituents on the nitrogen ligand has a positive effect on productivity using both types of precursors. No influence was observed on the polymer properties in terms of molecular weight and tacticity. Analysis of the reactivity of the methyl–palladium complexes towards carbon monoxide shows further differences depending on the nitrogen ligand.