The effect of high pressure on the propagating and terminating steps of free-radical polymerization was summarized from our experimental data which were already described on the six polymerization systems: styrene, methyl methacrylate, butyl methacrylate, butyl acrylate, vinyl acetate, and octyl methacrylate. Using these data, the functional dependence of the rate constants for individual steps on pressure was investigated with a view to obtaining reliable values under high pressure. In practice, a linear function is the best description of the pressure dependence of the logarithmic propagation rate constant regardless of the polymerization type. It can be concluded, therefore, that the activation volume can be used as a pressure-independent parameter for the propagating step in our pressure range (1-1 000 bar). The empirical relation to estimate the pressure dependence of the propagating step for other polymerization systems was obtained from thecorrelation for the activation volumes and the volume contractions of polymerization. The effect of pressure on the termination step is also discussed. A quadratic polynomial is generally the best representation of the pressure dependence of the logarithmic termination rate constant. Introduct ior?The free-radical polymerization at high pressures is a reaction of practical and theoretical interest. In commercial practice, the most important problems encountered in operating a polymerization reactor are to control the conversion and the product quality, i.e., the reaction rate, the molecular weight and its distribution. The observed polymerization rates are determined by contributions from the rates of several different simultaneous processes such as initia-*I Present address:
The propagation and termination rate constants for butyl acrylate polymerization under high pressure at 30°C were measured by the rotating sector method.The effect of pressure on the individual rate constant was determined and the activation volumes for the propagation and the termination were obtained as -22,5 cm3/mol and 20,8cm3/mol, respectively, and the overall activation volume was -26,3 cm3/mol. The rate constant and its dependence on pressure were compared with data previously obtained on methyl methacrylate and butyl methacrylate polymerizations, and the problem concerning the differences of the pressure effect between these reactions were discussed. ZUSAMMENFASSUNG:Die Wachstums-und Abbruchsgeschwindigkeitskonstanten der Polymerisation von Butylacrylat unter hohem Druck bei 30°C wurden mit der Methode des rotierenden Sektors gemessen.Der EinfluD des Drucks auf die Geschwindigkeitskonstanten der Elementarreaktionen wurde bestimmt, und die Aktivierungsvolumina fur die Wachstums-und die Abbruchsreaktion wurden zu -223 b m . 20,8 cm3/mol erhalten, wahrend das Aktivierungsvolumen der Gesamtreaktion -26,3 cm3/mol betrug. Die Geschwindigkeitskonstante und ihre Druckabhangigkeit wurden mit Daten verglichen, die friiher bei Polymerisationen von Methylmethacrylat und Butylmethacrylat erhalten worden sind. Das Problem der Unterschiede des Druckeffekts zwischen diesen Reaktionen wird diskutiert.
By use of the rotating sector technique under high pressure, the changes which occur in the rate constants of the propagation and the termination reactions in the polymerization of vinyl acetate, photosensitized with azocyclohexane-1,l'-dicarbonitrile were studied at 30°C over a pressure range of 1 to 1 OOO bar. The effects of pressure on the individual rate constants were determined and the activation volumes of propagation and termina-*) Present address: Eidai Co. Ltd., Hirabayashi-Minami, Sumiyoshiku, Osaka 558, Japan.
Photo-sensitized Polymerization of Styrene under High PressureYOSHIAKI Oco, MASARU YOKAWA, and TATSUYA IMOTO (Date of receipt: January 30, 1973) S U M M A R Y :The polymerization of styrene, photosensitized by azocyclohexane-l,l'-dinitrile, was studied at 30°C over a pressure range of 1 to ca. 1000 bar. The average lifetimes of the growing polymer chains were determined by the rotating-sector method. By measuring the effect of pressure on the individual reactions of styrene polymerization, the activation volumes for the propagation and the termination were obtained as -17,9 cm3,'mol and 13,l cm3/mol, respectively. The overall activation volume was also measured as --17,1 cm3/mol and was nearly equal to that calculated from the individual activation volumes (-1 7 , s cm3/mol). The propagation rate constant increases exponentially with pressure, whereas the termination rate constant decreases rapidly up to 500 bar and then decreases slowly as the pressure increases further. Z U S A M M E N F A S S U N G :Die Polymerisation von Styrol wurde mit Azocyclohexan-l,l'-dinitril als Photo-Sensibilisator bei 30°C uber einen Druckbereich von 1 bis ca. 1000 bar untersucht. Die mittlere Lebensdauer der wachsenden Polymerketten wurde mit der Methode des rotierenden Sektors bestimmt. Durch Messung des Druckeinflusses auf die einzelnen Reaktionen der Styrol-Polymerisation wurden die Aktivierungsvolumina fur die Wachstums-und die Abbruchsreaktion zu --17,9 cm3/mol bzw. 13,l cm3/mol ermittelt. Das Aktivierungsvolumen der Gesamtreaktion wurde zu -17,l cm3/mol gemessen und war nahezu gleich dem a m den einzelnen Aktivierungsvolumina errechneten Wert (-I 7 , s cm3, mol). Die Geschwindigkeitskonstante der Wachstumsreaktion nimmt mit zunehmendem Druck exponentiell zu, wahrend die Geschwindigkeitskonstante der Abbruchsreaktion bis zu 500 bar schnell, bei hoheren Driicken langsam abnimmt.
In order to calculate the average molecular weight, the molecular weight distribution or the branching of low density polyethylene, the individual values of the rate constants of the different reactions involved in the free radical polymerization are required. The method of rotating sector was applied to determine the rate constants of chain propagation and chain termination in a wide range of temperatures and pressures. This first part of the work reports the influence of the pressure. It was found that the rate constant kp of the chain propagation increases with rising pressure whereas the rate constant k, of chain termination decreases. The effect of the pressure on kp can be explained in terms of the activation volume. The decrease of k, with pressure is caused by a diffusion controlled termination reaction. Einleitung
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