The role of diffusion in propylene polymerization

The role of monomer diffusion in the polymerization of propylene by organometallic catalysis was examined by use of mathematical models which couple the rate of diffusion through the polymer film surrounding the catalyst with the rate of surface reaction. An approximate form of a second-order, integrated rate equation was used to describe the disappearance of active sites on the surface. For the most conservative model conceivable, it was estimated that the particle size would have to be 10–100 times the size for the catalysts presently in use before diffusion time would be significant. The size of the catalysts was determined by photomicrographs and nitrogen adsorption surface areas. The surface areas for three different catalysts were 7, 20–21 and 35 m.²/g., respectively. The kinetic model without the diffusion term was used satisfactorily to correlate productivity data. The characteristic decline in reaction rate was examined in terms of the decay of active sites on the surface of the catalyst. The rate of decay was determined to be second order with respect to the site concentration. The kinetic model indicates that the total polymerization time for a specified productivity is the sum of the monomer diffusion time and the surface reaction time. The model derived by use of an approximate second-order decay function is unique because of the additivity of diffusion and reaction times, which is not the case when the second-order function is used rigorously.