DFT Studies on Styrene Polymerization Catalyzed by Cationic Rare-Earth-Metal Complexes: Origin of Ligand-Dependent Activities

Abstract

The mechanism of styrene polymerization catalyzed by five analogous cationic rare-earth-metal complexes [(RCH2–Py)Y(CH2SiMe3)]+ (R = C5Me4 (Cp′), 1+; R = C9H6 (Ind), 2+; R = C13H8 (Flu), 3+), [(Flu–Py)Y(CH2SiMe3)]+ (4+), and [(Flu–CH2CH2–NHC)Y(CH2SiMe3)]+ (5+) has been studied through DFT calculations. Having achieved an agreement between theory and experiment in the activity discrepancy and selectivity, it is found that styrene polymerization kinetically prefers 2,1-insertion to 1,2-insertion. The free energy profiles for the insertion of a second monomer molecule have been computed for both migratory and stationary insertion manners, and the former resulting in a syndiotactic enchainment indicates obvious kinetic preference. The current results suggest that the coordination of styrene to the active metal center could play an important role in the observed activity difference. Interestingly, the charge on central metal of the cationic species accounts for the activities of 1+, 2+, and 3+: the higher the charge on the central metal, the higher the activity. The coordination of a THF molecule to the central metal and more difficult generation of the active species could be responsible for the low activity of 4+. For species 5+, the resulting product of the first styrene insertion is quite stable, and the ancillary ligand and styryl group hamper the insertion of the incoming styrene molecule. This could be responsible for the absolute inertness of 5+ toward styrene polymerization. The calculated results also suggest that a longer alkyl chain of the side arm of the ancillary ligand could deter monomer coordination and thus decrease the polymerization activity.