Tibor Nagy

Polymer-analogous reactions of polyenes in poly(vinylchloride)

ABSTRACT The primary process in the thermal degradation of PVC in an inert atmosphere and at relatively low temperatures, is the elimination of hydrogen chloride molecules and the formation of polyene sequences of different lengths. The polyenes formed are highly reactive and participate in different secondary reactions. It has been shown for PVC samples degraded both in dilute solution and in the solid state that the amount of polyenes does not correspond to the degree of dehydrochlorination: proportionality may be observed only at the initial stage of degradation and, later on, the number of double bonds in the system is much lower than the amount of hydrogen chloride molecules split off. With the increase of conversion the increase in the amount of polyenes containing three or more double bonds gradually slows down and, later on, practically ceases. This phenomenon is also quantitatively well interpretable by intramolecular cyclization (backbiting) of polyenes. The reaction leads to cyclohexadiene structures. On completing the kinetic treatment of the process, we determined the relative rate constant of cyclization. Reactive dienophilic reagents readily react with the polyenes formed in degraded PVC. Based on the changes observed in the u.v. and visible spectra, a kinetic study was made on the Diels–Alder reaction with chloromaleic anhydride. The kinetic treatment, performed on the basis of a presumed mechanism, allowed the determination of the rate constants and their temperature dependence. Depending on the experimental conditions, the polyenes formed in the course of degradation may participate in other reactions as well. Some of the possible reaction routes are: the change in molecular weight, the formation of low molecular weight aromatic compounds (e.g. benzene) and the considerable proton exchange observed in the reaction of polyenes with hydrochloric acid.

The reinitiation mechanism of HCl catalysis in PVC degradation

SummaryThe effect of HCl on the thermal degradation of PVC has been studied. Changes in UV and visible spectra and kinetics of HCl loss indicate that HCl is able to reinitiate the allyl-activated zip elimination by proton exchange with polyenes. Also polyene-consuming secondary reactions are accelerated by HCl.

Crosslinking, scission and benzene formation during PVC degradation under various conditions

SummaryThe kinetics and mechanism of crosslinking and chain scission has been studied on several PVC samples during thermal thermooxidative, dynamic, and HCl catalyzed degradation. During pure thermal degradation crosslinking at a constant rate without scission occurs, while in the presence of oxygen and/or shear scission is not negligible. In the dynamic test, under shear both processes are fast, compared to static tests. The mechanism of benzene formation without main chain scission is discussed.

Crosslinking and Scission in Thermooxidative Degradation of PVC

SummaryCrosslinking and chain scission in thermooxidative PVC degradation have been studied. Gel formation was found to give linear Charlesby-Pinner plots, consistent with the constant rate of crosslinking and scission. Similarly to thermal degradation, partial reversibility of crosslinking was observed also in oxygen.

On the most significant processes of thermooxidative PVC degradation

In the thermooxidative degradation of PVC two processes are of decisive importance: the elimination of HCl in allyl-activated steps producing polyene sequences in the polymer backbone, and the radical chain oxidation of these polyenes. The two processes are closely interrelated as the carrier radicals of the oxidation chain initiate also sequential HCl elimination. This results in an enhanced rate of HCl elimination and a low steady-state concentration of polyenes.AbstractВ термоокислительной деградации ПВХ два процесса обладают решающей важностью: выделение HCl в аллил-активированной ступени, приводящее к образованию полиеновых сегментов в полимерном скелете, и радикальное окисление этих полиенов. Два процесса тесно связаны, т. к. носители радикалов цепей окисления инициируют также последующее выделение HCl. Это приводит к повышенной скорости выделения HCl и низкой стационарной концентрации полиенов.

Heat Degradation of PVC Stabilized by Treatment with Alkylaluminum Compounds

Abstract Thermal degradation of PVC treated with alkylaluminum compounds has been studied. Four PVC samples of different molecular weights have been treated with Me3Al, and Et3A1, and the dehydrochlorination rates of the polymers were determined at 190 and 220°C under a nitrogen atmosphere. The alkylaluminum-treated low molecular weight samples show marked increase in thermal stability, i. e., slower rate of dehydrochlorination right from the beginning of degradation, whereas with the higher molecular weight samples stabilization becomes pronounced only after a few percent of dehydrochlorination. The color of R3Al-treated samples was much lighter (yellowish) than those of controls (dark brown) at 1% HCl loss. The average polyene sequence lengths formed during the early stages of dehydrochlorination are found to be much shorter with RsAl-treated PVC than with virgin samples. It appears as though polyene sequences which arose by zipping- initiation from allylic and/or tertiary chlorine sites are longer than...

Structural defects in poly(vinylchloride)—III. Degradation of vinyl chloride-phenylacetylene copolymers in solution

Abstract Vinylchloride-phenylacetylene copolymers have been prepared and characterized. A known amount of defined defect sites, viz. double bonds, has been introduced into the main chain of the polymer. Thermal degradation behaviour of the copolymers has been studied in trichlorobenzene solution. A strong dependence of the kinetics of dehydrochlorination and polyene formation on the defect concentration has been found. Rate constants and activation parameters have been determined. The effects of different structures have been compared.

Cross-linking and gel formation in the thermal degradation of PVC

Cross-linking in the thermal degradation of PVC was investigated by measuring the dependence of number-average molecular weight, gel fraction, molecular weight distribution and intrinsic viscosity on degradation time. A linear relationship was found between the cross-linking density and reaction time. Only one fifth of the polyene sequences formed participate in the cross-linking process.AbstractСшивка при термической деградации ПВХ была исследована измерением зависимости средне-числового молекулярного веса, гелевой фракции, распределения по молекулярным весам и истинной вязкости от времени деградации. Была найдена линейная зависимость между плотностью сшивки и временем реакции. Лишь одна пятая от всех образующихся полиеновых звенев принимает участие в сшивке.

Cyclization of polyenes in degraded PVC

Reactive polyenes developing during the thermal degradation of PVC isomerize in an intramolecular cyclization process with the formation of cyclohexadiene ring-structures. Therefore, the concentration of polyenes reaches a steady-state value in the course of degradation. The dependence of polyene concentration on the extent of dehydrochlorination can be readily described by the kinetic treatment performed on the basis of cyclization.AbstractРеактивные полиены, образуюЩиеся в термической деградации ПВХ, изомеризуются в дроцессе внутримолекыл ярной циклизации с образованием циклогексадиеновых кольцевых структур. Вследствие этого концентрация полиенов становится стационарной в ходе деградации. Зависимость концентрации полиенов от степени дегидрохлоринирринирования может быть описана с помоЩяю кинетической схемы, основанной на продессе циклизации.

Kinetic Evaluation of Polyethylene Autoxidation in Melt

SummaryThis work proposes a description of the kinetics of polyethylene autoxidation in melt. The model takes into consideration the effect of neighbouring groups on the hydroperoxide decomposition. Using the proposed model, reliable constants can be calculated for the autoxidation of polyethylene.