Polar copolymerization by a palladium-diimine-based catalyst. Influence of the catalyst charge and polar substituent on catalyst poisoning and polymerization activity. A density functional theory study

摘要

Combined gradient-corrected density functional theory and molecular mechanics (QM/ MM) has been used to investigate the copolymerization of ethylene with the CH2 = CHX alpha-olefins, where X = -H, -Me, -CN, -COOMe, -OC(O)-Me, -Cl. The cationic N<^>N-Pd(II) diimine complex and its modified neutral and anionic derivatives have been used as catalysts, where N<^>N = -NR"(CRCRNR)-C-2-N-3"-, R" = aryl group, and R-2, R-3 = BH3-, H. The consecutive insertion steps of CH2 = CHX into the Pd-CH3 bond and of ethylene into the Pd-C(X)HCH2CH3 bond have been investigated. Focus has been put on the role of the X functional groups and the effect of the cationic, neutral, and anionic environments on the Pd(II)-diimine system. Calculations have been performed on the CH2 = CHX monomers, model catalysts, precursor pi-complexes, and sigma-complexes of the monomers, as well as the chelate and H-agostic insertion products. The transition state of the insertion reaction and the corresponding activation energy was determined for both investigated insertion steps. The results show that the X group has only a minor effect on the insertion of the CH2 = CHX monomers into the Pd-CH3 bond. On the other hand, the barrier for insertion of ethylene into the Pd-CHXR bond revealed an increase with the electron-withdrawing ability of X. We predict that the application of neutral and anionic catalysts leads to a preference for pi-complexation over sigma-complexation of the polar monomers. Unfortunately, for an anionic model system the barriers for the first and second insertion are significantly increased for ethylene, whereas the first insertion barrier for the polar monomers only is moderately increased. Thus, while anionic catalysts are highly tolerant toward polar monomers, they are nearly inactive toward ethylene insertion.