ABSOLUTE RATE CONSTANTS FOR THE FREE RADICAL POLYMERIZATION OF ETHYLENE IN THE SUPERCRITICAL PHAS

An apparatus was designed and constructed which allowed the determination of the absolute rate constants for chain propagation, k(,p), and for chain termination, k(,t), in the free radical polymerization of ethylene at about 1900 kg/cm('2) and at 130(DEGREES)C. This is the first reported measurement of these kinetic constants under supercritical conditions, similar to those existing in the commercial high pressure polymerization process for Low-Density Polyethylene.;The experiment was based on the rotating sector method to determine lifetimes of chain carriers combined with the inhibition period method for initiation rates. Essential to the success of the project was the construction of a quasi-isothermalleak-free batch reactor, in which polymerization rates could be followed by monitoring the pressure change and one in which the entire reactor contents could be illuminated through a window with a large unsupported area (1 inch diameter), that could tolerate the reaction pressure. A photo-initiator with a negligible dark decomposition rate at 130(DEGREES)C was also required and found in the form of 1-tert-butylazo-1-phenoxycyclohexane, was an intense light source. These key experimental problems were solved and satisfactory results were obtained for the lifetime of chain carriers, as well as for initiation rates and polymerization rates, from which the absolute rate constants k(,p) and k(,t) could be calculated.;By varying the initiator concentrations and light intensities, lifetimes of the chain carriers in the range of 0.2 to 1.0 sec were obtained. Satisfactory measurement of the initiation rates could be made through use of the inhibitor hydroquinone. The values of 1.2 x 10('4) 1 mole('-1)sec('-1) and of 7.8 x 10('8) 1 mole('-1)sec('-1) were obtained for k(,p) and k(,t), respectively, at the reaction temperature of 130(DEGREES)C and the reaction pressure of 1900 kg/cm('2). This value for k(,t) is much larger than that for any other free radical vinyl polymerization, but much smaller than what has been estimated for bi-molecular radical-radical reactions that are not diffusion-controlled. The latter result suggests diffusion-control, and the former result is presumably attributable to the relatively large chain flexibility of polyethylene radicals, their relatively large collision diameter and the low solvent viscosity.