Polymer particle growth, Ziegler-Natta

Polyethylene particle growth, over Ziegler-Natta catalysts, 26 530-531 Polyethylene polymer properties, control of, 17 703... 

Assume as a model for a Ziegler-Natta system the diffusion of monomer to a site of catalytic activity—presumably one of a number of sites on a solid particle—where it is inserted into a growing polymer chain. For the bulk polymerization of a monomer such as 4-methylpentene-l where polymer is insoluble in monomer, the solid catalyst particle becomes the center of an expanding sphere of precipitated polymer chain (s) growing from the inside. On this molecular level, the rate of chain growth will be directly proportional to the monomer activity at the individual sites. At equilibrium the monomer activity at each site encapsulated in precipitated polymer will equal that of the surrounding bulk monomer, [Mo]. Under nonequilibrium conditions, where the rate of diffusion of monomer from the bulk monomer thru the precipitated polymer to the polymerization site becomes comparable to the rate of polymerization at that site, the localized activity will be lower, and the rate of polymerization will be correspondingly lower. 

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