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Influence of tacticity on thermal degradation of PVC. V. Relation between the nature of labile conformations and the polyene distribution in the degraded polymer
Author(s) -
Martinez G.,
Mijangos C.,
Millan J.
Publication year - 1983
Publication title -
journal of applied polymer science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.575
H-Index - 166
eISSN - 1097-4628
pISSN - 0021-8995
DOI - 10.1002/app.1983.070280104
Subject(s) - polyene , tacticity , polymer , intrinsic viscosity , degradation (telecommunications) , solvent , polymer chemistry , molar mass distribution , acetone , chemistry , vinyl chloride , polyvinyl chloride , double bond , raman spectroscopy , materials science , organic chemistry , polymerization , telecommunications , physics , computer science , optics , copolymer
Four poly(vinyl chloride) (PVC) samples, 1, 2, 3, and 4, were prepared in bulk using 2,2′‐azodiisobutyronitrile at 90°C and 60°C and the same initiator together with UV irradiation at 0°C and −50°C, respectively. Fractions were obtained from samples 2,3, and 4 by extracting with acetone, sample 1 being completely soluble in this solvent. The whole PVC's as well as their acetone soluble and insoluble fractions, were characterized by determining the intrinsic viscosity, the osmometric molecular weight, and the tacticity; then they were thermally degraded up to conversion of 0.3% in powder state. The values of degradation rate for both the insoluble fractions and the whole PVC's were in agreement with some prior results on the influence of syndiotactic sequences on propagation step. The soluble fractions proved to be very unstable in comparison with the insoluble, which, despite the lower molecular weight of the former, seems to obey their higher content of isotactic conformations. The fine polyene distribution in the degraded polymers was carried out by UV–visible spectroscopy. The results reveal the occurrence of two different polyene distributions, and allow for the peculiar one of the soluble fractions to be related to polyenes consisting of two sequences of trans conjugated double bonds separated by a single cis double bond. The results clearly shows that there are two mechanisms for initiation of the PVC degradation, depending on whether it occurs by random unstable structures or by the normal GTTG isotactic or TTTG heterotactic triads. Moreover, these two initiation processes are proved to give rise to different types of polyenes, which accounts for the occurrence of two unlike mechanisms of propagation.