Aspects of polymerization at high pressures

Abstract

The catalyzed polymerization of styrene at pressures in the range 0 to 5000 kg. /cm. 2 has been studied by an improved technique utilizing a closed system containing a very sensitive electrical resistance pressure gauge. Correlation of the results obtained by this method with the more usual analytical methods has given reliable results for the rates and molecular weights over the observed initial period of the polymerization. The results show that ( a ) the rate varies with the catalyst concentration raised to the power 0·50 for zero pressure, dropping to 0·40 for 3000 kg. /cm. 2 and thereafter remaining almost constant; ( b ) the rate increases almost exponentially with pressure above 2 to 3000 kg. /cm. 2 ; ( c ) the molecular weight rises with pressure up to about 3000 kg. /cm. 2 and thereafter remains constant. These facts have been explained on both a thermodynamic and a kinetic basis. The former shows the increase in rate as the result of the 'slow' bimolecular reaction of propagation being accelerated by pressure. Comparisons are drawn between the effects of pressure on simple chemical reactions (due to Gibson, Fawcett & Perrin and others) and the results given above. The kinetic approach suggests that the pressure effects a change in the relative values of the termination constants, a larger proportion of the termination at higher pressure being due to chain transfer and to a reaction with the catalyst.