The effect of counterion/ligand interplay on the activity and stereoselectivity of palladium(II)-diimine catalysts for CO/p-methylstyrene copolymerization.

The catalytic activity and stereoselectivity of complexes [Pd(h1,h2-C8H12OMe) (Ar_N=C(R’)_C(R’)=N_Ar)]X in the copolymerization of CO and p-methylstyrene have been correlated with their interionic structure in solution and in the solid state, as determined by 19F,1H-HOESY NMR spectroscopy and X-ray diffraction studies, respectively. The highest productivity is obtained with unhindered diimine ligands bearing electron-donating substituents and with the least coordinating counterion. Copolymers with a microstructure ranging from atactic to predominantly isotactic are obtained. The degree of isotacticity increases as the steric hindrance in the apical positions and the coordinating ability of the counterion increase. The counterion is located close to the diimine in both solution and the solid state but it moves toward the palladium as the steric hindrance in the apical positions decreases. When the latter is small the counterion competes with the substrate for apical coordination, and consequently it affects the productivity. In the case of ortho-dimethyl-substituted ligands the counterion is confined in the back, above the N=C(R’)_C(R’)=N moiety, and does not affect the productivity. However, it contributes to increasing the stereoregularity of the copolymer by making the aryl moieties more rigid. With R’=Me and Ar=o-Me2C6H3 an ll of 81% and 72% was obtained with X_=CF3SO3_ or BArF_, respectively. The isotacticity of the copolymers produced by ortho-monosubstituted catalysts depends greatly on the counterion and ranges from 30% to 59% with X_=BArF_ and X_= CF3SO3_, respectively, with Ar=o- EtC6H4 and R’=Me. Based on the interionic structural results, this effect can be explained by a greater reduction of the copolymerization rate of Cs-symmetric isomers with respect to their C2- symmetric counterparts.