A Kinetic Study of Ethylene and 1-Hexene Homo- and Copolymerization Catalyzed by a Silica-Supported Cr(IV) Complex: Evidence for Propagation by a Migratory Insertion Mechanism
Bibliographic record
Abstract
The surface organometallic fragment (⋮SiO) 2 Cr CHCMe 3 initiates the polymerization of ethylene and 1-hexene at room temperature in the absence of organoaluminum or other activators. The kinetics at the gas−solid interface were studied by in situ IR spectroscopy in a constant volume, variable pressure reactor. At low pressures, the reaction is first order in P(C 2 H 4 ), first order in the number of moles of chromium, and linearly dependent on the Cr loading on silica. The second-order rate constant for polymerization is (177 ± 3) s - 1 (mol Cr) - 1 at 21 °C, and is virtually unchanged upon subsequent additions of ethylene to the catalyst. The kinetic isotope effect, k (C 2 H 4 )/ k (C 2 D 4 ), is a meagre 1.29. The temperature dependence of the second-order rate constants yields Δ H ‡ obs = (30.2 ± 0.9) kJ/mol and Δ S ‡ obs = (−99 ± 3) J/(K·mol). These values are consistent with a propagation mechanism of agostically assisted migratory insertion at an alkylchromium(IV) active site. They are inconsistent with an alternating alkylidene/metallacycle mechanism requiring rate-determining C−H cleavage. The rate of polymerization is accelerated by a factor of 3.2 in the presence of 18 mol % H 2 . Homopolymerization of 1-hexene is only 4.7 times slower than that of ethylene, an effect attributed to weaker preequilibrium binding of the substituted olefin which is partially compensated by faster migratory insertion. Copolymerization of ethylene and 1-hexene is random. It results in slower incorporation of ethylene but faster incorporation of 1-hexene relative to their respective homopolymerization rates.
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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) | 0.001 | 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".