The influence of the counterion B(C6F5)3CH3– and solvent effects on the propagation and termination steps of ethylene polymerization catalyzed by Cp2ZrR+ (R = Me,Pr). A density functional study
Bibliographic record
Abstract
Cp 2 Zr(CH 3 )R and its derivatives can serve as powerful olefin polymerization catalysts after activation by a Lewis acid A to form the ion-pair [Cp 2 ZrR] + [CH 3 A] – ( I ) which is held together by a Zr-μ-CH 3 A – bridge. It is generally assumed that the cation Cp 2 ZrR + ( II ) of I is the active species whereas the influence of the anion CH 3 A – ( III ) is less well understood. We have conducted an extensive study based on density functional theory (DFT) of ethylene polymerization catalyzed by both I and II in order to probe the influence of the anion CH 3 A – for the case where A = B(C 6 F 5 ) 3 . The reaction between ethylene and the cation II leads in the first place to a π−complex in a highly exothermic and exogonic reaction without any (uptake) barrier. Even the subsequent insertion process has a transition state that is lower in energy than II and C 2 H 4 at full separation. The only (internal) barrier is the modest energy required to proceed from the π-complex to the insertion transition state. For the reaction between the ion-pair and ethylene the monomer can approach cis or trans to the Zr-μ-CH 3 A – bridge. In addition with R = Pr, cis and trans approaches are possible for 4 different orientations of the propyl chain. We find for all of these approaches that the rate determining step is the (partial) displacement of the anion CH 3 A – . For the first insertion (R = Me) the total insertion barrier is 13.0 kcal/mol for the most favorable (trans) approach. The second (R = Pr) insertion (which likely also is a good model for subsequent propagation steps) prefer a cis-approach in which the ethylene uptake barrier of 9.5 kcal/mol is rate determining whereas the barrier for the subsequent insertion process only is 6.8 kcal/mol. Displacement of the anion was found to be more pronounced for R = Pr than R = Me and larger for the insertion transition state than the uptake transition state. Solvation effects were seen to stabilize anion displacement and thus reduce especially insertion barriers. Thus for the favored cis-path of the second propagation, the insertion was the rate determining step in the gas-phase with a barrier of 10.0 kcal/mol whereas the corresponding uptake process became the rate determining step in solution with a barrier of 8.6 kcal/mol. After the insertion, the ion-pair was found to recombine completely so that ethylene will have to displace the anion in the next propagation step. Considerations were also given to the chain termination step by transfer of hydrogen to the monomer. Here the rate determining step is again cis-uptake off ethylene with a barrier of 12.5 kcal/mol whereas the subsequent hydrogen transfer barrier only is 10.6 kcal/mol. .
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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 | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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".