A theoretical study of the insertion barrier of MAO (methylaluminoxane)-activated, Cp2ZrMe2-catalyzed ethylene polymerization: further evidence for the structural assignment of active and dormant species
Bibliographic record
Abstract
Density functional theory has been used to calculate the ethylene insertion barrier into the Zr-C bonds of the [Cp2ZrMe](+)[AlMe3Me(A-MAO)]- and [Cp2ZrMe](+)[Me(A-MAO)]- ion pairs where (AlOMe)6 was used to model the active forms of MAO (A-MAO). The results support the proposal that the former is the active and the latter the dormant species in MAO-activated olefin polymerization. For the first insertion, with the active species the trans approach has a lower barrier to insertion than the cis approach in solution, due to the larger ion pair separation. It is likely that the separated ion pairs recombine between subsequent insertions. For the second insertion, we have considered frontside, backside and combined insertion mechanisms showing that the total barrier to insertion is approximately the same in all cases ranging from 13.8 kcal mol(-1)-14.8 kcal mol(-1). For the frontside and combined insertion this barrier is a result of the rotation of the propyl chain: for the backside mechanism it is stems from the ethylene insertion barrier. Comparison of the results for the ion pair and for the naked cation show that in the former case olefin complexation is an endothermic process whereas for the latter it is exothermic. In general, the large ion pair separation prevents the orientation of the anion from exerting much influence on the geometry or energy of the cation. However, when structures with alpha-agostic interactions are considered the ion pair separation is small enough so that the orientation has some influence on the reaction mechanism. For the second insertion, dissociated transition states were found to have lower barriers than associated ones.
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
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 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.001 | 0.001 |
| 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".