| Popis: |
Industrial and laboratory samples of PVC and PVA contain 5 × 10 −3 mole/mole polymer of unsaturated end groups, however, PVA, unlike PVC is practically free from internal 〉C=C〈 bonds. It was established that at the initial stages overall-deacetylation of PVA only takes place as a result of statistical elimination of CH 3 COOH from macromolecules. During thermal degradation of PVC structural groups undergo conversion to form polyconjugated sequences. Statistical elimination of CH 3 COOH from PVA, in the same way as that of HCl from PVC k s = 0·8 × 10 −7 sec −1 , 175°) is independent of the type of polymer sample and takes place with a rate constant of k s = 0·7 × 10 −8 sec −1 (175°). Crosslinking of PVA macromolecules, in contrast with PVC, occurs readily at the initial stages of polymer degradation, when no long polyene sequences are formed and occurs by mechanisms different from those of PVC: the process takes place as the result of terminal and internal 〉C=C〈 bonds formed by thermal degradation of the polymer, or as the result of intermolecular deacetylation, etc. |
