Variability of Chain Transfer to Monomer Step in Olefin Polymerization
Bibliographic record
Abstract
Computational studies of a variety of polymerization catalyst models have revealed an unexpected fluidity in chain termination mechanisms. For many early transition metal olefin polymerization catalysts two distinct transition states exist for β-hydrogen transfer to monomer, which differ mainly in the M−H distance and CMC angle. The transition state for the “classical” (BHT A ) path resembles a metal hydride−bis(olefin) complex, whereas the alternative BHT B path involves direct transfer of an alkyl β-hydrogen to a coordinated olefin without any metal−hydride interaction. The two transition states are separated by a second-order saddle point that is just a few kcal/mol above the highest of the two transition states, indicating a flat potential-energy surface between the two paths. Of the group IV metals, Zr (in contrast to Ti and Hf) appears to have an intrinsic preference for the “classical” BHT A path. Increasing the amount of space around the metal (e.g., in lanthanocenes) changes BHT A into a two-step path (BHT C ), showing two β-hydride elimination transition states around a hydride−bis(olefin) complex local minimum. Decreasing the amount of space by using sterically demanding ligands results in a shift toward the “new” BHT B path. However, β-hydrogen elimination becomes more favorable at the same time, and our results suggest that for most early transition metal catalysts (typically 14- e metal alkyls) either BHT A or β-hydrogen elimination will be the dominant chain-transfer pathway, whereas BHT B may be relevant for some Hf complexes of intermediate crowding. The BHT B path is expected to be more important for systems that are less unsaturated (16- e transition metal alkyls; 6- e main-group metal alkyls) and also for “hetero-olefin” derivatives (alkoxides, amides), where β-hydrogen elimination is strongly endothermic.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| 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) | 0.027 | 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 teacher head, 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".