Kinetics of the γ‐radiation‐induced polymerization of ethylene in alkyl chlorides

A study has been made of the kinetics and mechanism of the liquid‐phase, γ‐radiation‐induced, alkyl chloride solution polymerization of ethylene. The γ‐radiation was provided by a 4.07 × 105 r/hr. Co60 source. The kinetics of these polymerizations were studied up to 10% conversion and over the temperature range 0 to −25°C. Three solvents (n‐propyl chloride, iso‐propyl chloride, and tert‐butyl chloride) were used with concentrations of ethylene ranging from 6.76 to 8.89 moles/l. Two different grades of ethylene were used: the first contained an upper limit of 1000 ppm of oxygen, the second 60 ppm. The ethylene containing only 60 ppm of oxygen was found to give a smaller induction period, and faster rate of polymerization than the ethylene containing 1000 ppm. Activation energies of 5.0 kcal./mole for the reaction in n‐propyl chloride and 7.4 kcal./mole for the reaction in tert‐butyl chloride were observed for the temperature range studied. These values were higher than the 4.4 kcal./mole obtained for the bulk polymerization of ethylene in this temperature range. The order of the reaction with respect to ethylene concentration in n‐propyl chloride at 0°C. was found to be 1.41. Although the heterogeneity of the system is a complicating factor, this rate expression agrees well with the general scheme proposed for other heterogeneous polymerizations. Molecular weight information which shows an increase in molecular weight with increasing temperature leads to the conclusion that a termination step involving weakly reactive solvent radicals may be important.