Abstract
Study of the M w / M n parameter can help, in principle, to solve an important problem, namely, to distinguish between intermolecular and intramolecular chain propagation in the oxidation of a polymer [1]. If the oxidative destruction of polymer chains is the predominant process, long macromolecules are the first to be cleaved and the polymer polydispersity decreases during aging. Conversely, if cross-linking predominates during aging, M w increases and the M w / M n ratio also increases. In the former case, the M w and M n values approach each other (i.e., the MWD narrows), whereas in the latter case, M w and M n disperse and the MWD is broadened (depending on the polymer chemical structure and thermooxidative destruction conditions, the MWD can become both narrower and broader [2‐4]). This is true, first of all, for the structural transformations of polymers at relatively low degrees of conversion when heat-treated samples contain no gel fraction. For high conversions (high content of the gel), the relationship between the destruction mechanism and the pattern of M w / M n variation is not unambiguous. Thus, it was found that the kinetic curves for the variation of M w / M n and M w and the number of branching points in macromolecules for poly(terphenylene phthalide) (PTP) aging in air pass through a maximum. The MWD first widens and shifts toward higher MWs; then, it starts to narrow and shifts in the opposite direction. It was shown that the nonmonotonic pattern of variation of the molecular-weight characteristics of poly(terphenylene phthalide) is due to gelation, which mainly involves the highest molecular-weight and branched portion of the polymer sol fraction, rather than to the predominance of intermolecular cross-linking at initial stages of aging and the destruction of macromolecules at high degrees of conversion. The M w and M w / M n values of the polymer were determined by GPC, and the molecular weight of the branches ( M b ) was found from the dependence of the
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