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• 1955-1959  (3)
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• 1
Electronic Resource
Hoboken, NJ : Wiley-Blackwell
Journal of Polymer Science 19 (1956), S. 583-585
ISSN: 0022-3832
Keywords: Chemistry ; Polymer and Materials Science
Source: Wiley InterScience Backfile Collection 1832-2000
Topics: Chemistry and Pharmacology , Physics
Type of Medium: Electronic Resource
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• 2
Electronic Resource
Hoboken, NJ : Wiley-Blackwell
Journal of Polymer Science 18 (1955), S. 586-588
ISSN: 0022-3832
Keywords: Chemistry ; Polymer and Materials Science
Source: Wiley InterScience Backfile Collection 1832-2000
Topics: Chemistry and Pharmacology , Physics
Additional Material: 1 Ill.
Type of Medium: Electronic Resource
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• 3
Electronic Resource
Hoboken, NJ : Wiley-Blackwell
Journal of Polymer Science 16 (1955), S. 491-504
ISSN: 0022-3832
Keywords: Chemistry ; Polymer and Materials Science
Source: Wiley InterScience Backfile Collection 1832-2000
Topics: Chemistry and Pharmacology , Physics
Description / Table of Contents: An attempt is made to apply the usual kinetic scheme (initiation, propagation, mutual termination) to polymerizations in precipitating media, by simply assuming that the stationary state hypothesis cannot be applied in this case, because of a strong decrease of the termination rate. The integration of the set of differential equations is carried out for two cases: (a) concentration of the catalyst is kept constant all through the polymerization (eq. 5a); (b) the catalyst disappears by a first order reaction (eq. 5b). The resulting equations can be easily handled only in the first case. An approximation procedure is proposed to take into account the disappearance of the catalyst and the accumulation of the catalyst in the monomer phase which becomes important at higher degrees of conversion if the catalyst is insoluble in the polymer. The equations obtained are used to analyze the experiments of Bengough and Norrish and of Schindler and Breitenbach at various temperatures and catalyst concentrations. The results are highly satisfactory (Figs. 1 and 2) and the log kp/kt and log (kaA0kt)-1/2 vs (1/T) plots lead to straight lines, corresponding to Ep≠ - Et≠ = 5.2 kcal./mole; Ea≠ + Et≠ = 29.5 kcal./mole (Fig. 3). These values indicate that the activation energy for the termination step cannot be very high. The slowing down of the reaction is hence due to a decrease of the frequency factor. The constants determined from experiments at low conversion rates are then used to make a priori calculation with the help of the approximation method indicated above, of the conversion rates as a function of time under the conditions of temperature and catalyst concentration used by Schindler and Breitenbach and by Prat. The theoretical curves are compared with the experimental data in Figures 4 to 6. Here again the agreement can be considered as satisfactory. It is also possible to represent with the same formula the data of Bengough and Norrish on BPO catalyzed polymerization of vinylidene chloride at 47° and data of Burnett and Melville on the photochemical polymerization of the same compound at 25°C. The ratio of kp/kt is smaller in this case than for vinylchloride. Combining the results of Bengough and Norrish on the two monomers and assuming that the decomposition rate of BPO is the same in the two monomers one obtains: \documentclass{article}\pagestyle{empty}\begin{document}$\begin{array}{*{20}c} {\frac{{k_p ({\rm vinylidene})}}{{k_p ({\rm vinyl})}} = 2.6;} & {\frac{{k_t ({\rm vinylidene})}}{{k_t ({\rm vinyl})}} = 0.7.} \\ \end{array}$\end{document} Using the measurements of Burnett and Melville of the initiation rate one can also calculate the absolute values of kp and kt for vinylidene chloride at 25°C. However, the combination of the results at 25 and 47°C. leads to an abnormally high activation energy Ep≠ - Et≠ = 26 kcal., which suggests that some other phenomena take place. These difficulties, as well as some disagreements in details concerning the PVC polymerization, together with the observation that other polymerizations in precipitating media cannot be described with the same formula, lead us to believe that although the general approach is sound, the real situation is more complex. Until the objections are overcome equation (5a) is to be considered as a semiempirical formula, applicable to PVC polymerization rather than completely representing the reality.
Notes: Une tentative est faite d'utiliser pour la description de la polymérisation en milieu précipitant le schéma cinétique habituel (amorçage, propagation, terminaison mutuelle), mais en admettant que l'état quasi-stationnaire n'est pas atteint au début de la réaction. Le système d'équations différentielles pu être résolu pour certains cas: concentration constante de catalyseur, concentration décroissante selon une loi cinétique de premier ordre, etc…. Une méthode d'approximation est proposée pour traiter les cas où il y a accumulation du catalyseur dans la phase monomère par suite de son insolubilité dans le polymère. Les formules obtenues permettent de rendre compte de la polymérisation en masse du chlorure de vinyle, à différentes températures et à différentes concentrations du catalyseur (peroxyde de benzoyle), et ceci aussi bien pour des expériences à faibles degrés de conversion que pour des expériences poussées à des degrés de conversion élevés. Les constantes kp/kt et (kaA0kt)-1/2, déterminées à différenctes températures correspondent à des énergies d'activation Ep≠ - Et≠ = 5,2 kcal., Ea≠ + Et≠ = 29,6 kcal. Cette dernière valeur indique que l'énergie d'activation de terminaison n'est pas très élevée et que le ralentissement de terminaison observé est dû à une diminution du facteur de fréquence. Les mêmes formules ont été utilisées pour le cas de polymérisation du chlorure de vinylidène. Dans ce cas il a été possible de calculer les valeurs absolues des constantes de vitesse de terminaison et de propagation qui sont très sensiblement plus faibles que dans le cas de polymérisation en milieu homogène. Certaines difficultés persistent, qui amènent à penser que la théorie n'est pas complète.
Additional Material: 7 Ill.
Type of Medium: Electronic Resource
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