Effects 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-2,20-bipyridines L (L1 ¼5-methyl-2,20-bipyridine, L2 ¼5,50-dimethyl-2,20-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
0
4]2 and [Pd(L)(Me)(NCMe)][BAr
0
4] (Ar
0 ¼3,5-(CF3)2-C6H3), respectively, were isolated and characterized
by NMR and FAB mass spectroscopy. The complex [Pd(L2)(L3)][BAr
0
4]2 (L3 ¼2,20-bipyridine) bearing different ligands, was prepared
for comparison purposes. The activity of the monocationic and dicationic complexes as catalytic precursors in the CO/4-tertbutylstyrene
copolymerization was compared with that of related well-known catalysts containing the unsubstituted 2,20-bipyridine
as nitrogen ligand, to evaluate the influence of the substituents in 5- and 5,50-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.