A combined density functional theory and molecular mechanics (QM/MM) study of single site ethylene polymerization catalyzed by [Cp{NC(t-Bu)[sub 2] }TiR[sup +] ] in the presence of the counterion (CH[sub 3] B(C[sub 6] F[sub 5] )[sub 3] [sup –] )[sup 1]

Calculations have been carried out to investigate the insertion of the ethylene monomer into the Ti–alkyl bond for the systems CpNC(t-Bu)2RTi-µ-Me-B(C6F5)3 (R = Me and Pr), using density functional theory. A validated QM/MM model was used to represent the counterion. The tertiary butyl groups in the ligands were modeled with QM/MM, with hydrogens being used as the capping atoms. Solvent effects were incorporated with single point solvent calculations done with cyclohexane (epsilon= 2.023) as the solvent. With R = Me (the initiation step), approach of the ethylene cis and trans to the -µ-Me bridge was considered. Insertion was found to be endothermic, with ΔHtot being 12.7 kcal/mol (cis) and 15.5 kcal/mol (trans). The propagation step was then studied for the contact ion pair CpNC(t-Bu)2PrTi-µ-Me-B(C6F5)3 (4). Different conformations of the propyl chain in 4 were considered by altering theta, the dihedral angle formed between the Cβ-Cα-Ti and the Cα-Ti-µC planes. The resting states were found to be at theta = –69° (4a), 177° (4b), and 53° (4c). A maxima was found near theta = 0° (4d). The cis and trans approaches of the ethylene monomer were considered for each of the four cases. The cis approach towards 4a and trans approach towards 4d led to insertion, with displacement of the counterion from the metal centre. The insertion barriers were found to be 17.8 kcal/mol, for the cis approach towards 4b and 16.4 kcal/mol for the trans approach towards 4d. The cis approaches towards conformers 4a and 4c showed common characteristics, with uptake barriers being higher than the subsequent insertion barriers in the two cases. Uptake barriers were 13.6 kcal/mol {TS(4a–7a)} and 10.7 kcal/mol {TS(4c–8a)}. The corresponding insertion barriers were 11.8 kcal/mol {TS(7a–13a)} and 8.4 kcal/mol {TS(8a–13a)}. The trans approaches towards conformers 4a and 4c led to insertion barriers of about 15.1 kcal/mol (lower than for the 4b and 4d cases). The cis approach towards 4d and trans approach towards 4b were found to lead to hydrogen transfer from the propyl chain to monomer, terminating the chain. Termination barriers were high — greater than 19.0 kcal/mol. This suggested that ethylene insertion would be favoured over termination during the propagation step. [ABSTRACT FROM AUTHOR]