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.

Recent progress on NHC-stabilized early transition metal (group 3–7) complexes: Synthesis and applications

“…In a related study by Luo and co-workers, the reactivity of styrene with the Y(III) cation 19 (Figure 2) was DFT computed showing that the styrene first insertion product was stable but that subsequent chain propagation was disfavored for steric reasons, accounting for the inactivity of 19 in styrene polymerization catalysis. 24 The bidentate fluorenyl NHC Sc(III) species 20 (Figure 2) was also used by Hou and coworkers as an ethylene/1,6-heptadiene cyclopolymerization catalyst to afford new cyclopolymers containing primarily with six-membered rings (Mn = 1.2 x 10 4 -5.1 x 10 4 g.mol - An Y(III) amido complex bearing two tridentate amidate-functionalized NHC ligand (25, Scheme 13) was recently prepared by Wang and co-workers through a one-step reaction. 29 The rather long Y-NHC bond distances in species 25 (2.640 and 2.800 Å) presumably reflects steric hindrance at the Y(III) center.…”

Syndioselective Polymerization of Vinylnaphthalene

“…This mainly attributes to the increasing steric hindrance and the reduced Lewis acidity of active species, that helps to inhibit the side‐reactions between the active species and the naphthalene ring. According to the polymerization behaviors of 2VN and MVN, we can deduce that these monomers are polymerized into syndiotactic products via the same pathway to styrene and its derivatives …”

A Theoretical and Experimental Study for Screening Inhibitors for Styrene Polymerization

“…where E LUMO denotes electron energies of the lowest unoccupied molecular orbital (i.e., LUMO) and E HOMO indicates electron energies of the highest occupied molecular orbital for a neutral component (i.e., HOMO). Higher HOMO energy corresponds to stronger electron-donating ability of the molecule [41][42][43][44][45][46]. The FT-IR spectra were recorded for UP seed ( Figure 5).…”