A combined density functional theory and molecular mechanics (QM/MM) study of single site ethylene polymerization catalyzed by [Cp{NC(<i>t</i>-Bu)<sub>2</sub>}TiR<sup>+</sup>] in the presence of the counterion (CH<sub>3</sub>B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub><sup></sup>)<sup>1</sup>
Bibliographic record
Abstract
Calculations have been carried out to investigate the insertion of the ethylene monomer into the Tialkyl bond for the systems CpNC(t-Bu) 2 RTi-µ-Me-B(C 6 F 5 ) 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 (ε= 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 ΔH tot 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) 2 PrTi-µ-Me-B(C 6 F 5 ) 3 (4). Different conformations of the propyl chain in 4 were considered by altering θ, the dihedral angle formed between the C β -C α -Ti and the C α -Ti-µC planes. The resting states were found to be at θ = 69° (4a), 177° (4b), and 53° (4c). A maxima was found near θ = 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(4a7a)} and 10.7 kcal/mol {TS(4c8a)}. The corresponding insertion barriers were 11.8 kcal/mol {TS(7a13a)} and 8.4 kcal/mol {TS(8a13a)}. 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.Key words: homogeneous catalysis, counterion, initiation, propagation, termination.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.002 | 0.001 |
| Insufficient payload (model declined to judge) | 0.004 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".