Tacticity Changes during Controlled Degradation of Polypropylene
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Bibliographic record
Abstract
The controlled reactive degradation of polypropylene (CPP) in a twin-screw extruder has been investigated from the perspective of tacticity. CPP is initiated by the decomposition of a peroxide producing radicals that abstract hydrogen from the chain backbone creating tertiary radicals. Since C atoms with tertiary radicals have no longer sp3 configurations, they temporarily lose their stereospecificity. The C atoms resume stereospecificity upon termination, whereby methyl groups may assume a different orientation than before, causing a tacticity change. Degradation experiments in a twin-screw extruder of isotactic PP samples have been performed and tacticity changes are observed by C-13 NMR measurements revealing a decrease of isotactic <i>mmmm</i> pentads and increases of pentads with <i>racemic</i> sequence pairs. Statistical analysis of NMR data shows that the change of tacticity is nonrandom, i.e., units on the chain backbone are more likely to change orientation if they are close together. We attribute the nonrandomness to isomerization reactions, where through cyclic intermediates, a radical from one tertiary C carbon is transferred to another—radical chain walking. To validate this chain-walking hypothesis, a kinetic Monte Carlo simulation algorithm has been developed that predicts the change of the pentad distribution from the reactions taking place: initiation of tertiary radicals, transfer to polymer, chain walking, and termination by disproportionation. PP chains simulated consist of around 10<sup>9</sup> units with orientations of methyl groups inferred from the pentad distribution measured by NMR. Simulated pentad changes are compared to those measured from NMR, which reveal a relatively high racemic content. kMC simulations show that this can only be explained by assuming chain walking. Also, this isomerization via ring-shaped intermediates influences the patterns of changes in pentad distribution. These findings demonstrate that molecular weight changes are accompanied by tacticity changes. The kMC models confirmed by NMR data show that these tacticity changes are clustered locally along PP chains and that tacticity changes happen within the length scale of individual pentads. Chain-walking isomerization as a main cause of the tacticity changes is supported by predictions from the kMC model.
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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) | 0.000 | 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 it