Polymerization in expanding catalyst particles

Abstract

Four models are presented to describe polymerization i n expanding catalyst particles. The globules are presumed t o be expanding with accumulating polymer, and catalytic reaction sites are dispersed throughout the polymer matrix which i s forming about them. Monomer must diffuse through the polymer t o polymerize a t the catalyst sites.i s a ratio of characteristic diffusion time t o reaction time which i s a measure of the importance of diffusion relative to reaction.Polymerization rates are predicted by the models which are generally dependent on the controlling mechanism. Broad molecular weight distributions are predicted for cases of diffusion control (large IY.) for those models i n which catalyst sites are not equally accessible to monomer.Polymerization rates decline toward an asymptotic final value as the particles expand i n diffusion-controlled cases. Most of the decline which would be readily observable i n a laboratory experiment would have occurred by the time the particle radii had increased to about three times their original value. The Thiele modulus (a), R d m ,Solid polymerization catalysts fluidized in a material that does not dissolve the polymer expand with accumulating polymer during reaction. These particles may grow more than 10,000-fold in volume during polymerization. In fact, since catalysts are generally much more expensive per unit weight than the polymers they produce, a considerable increase in size during polymerization i s usually a prerequisite for the associated process to have commercial value. Otherwise, the catalyst must be recovered. aPresent address,

On the Mechanism of Polypropene Growth over MgCl2/TiCl4 Catalyst Systems

“…Polymer subglobules thus tend to behave like an expanding "microcore" that can be assimilated to the polymeric flow model described by Schmeal and Street. [8] As a result, the whole system can be described as a double-grain model with expanding "microcore".…”

“…Four models were reported by Schmeal and Street [8] to describe polymerization in expanding catalyst particles; among these, it is worth considering the following two limiting case models:…”

Modeling of morphogenesis of growing polyolefin particles

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Uma revisão sobre polimerização de olefinas usando catalisadores Ziegler-Natta heterogêneos

“…Os modelos propostos por Schmeal e Street [112] (Centro Sólido, Centro Polimérico, Fluxo Polimérico), Singh e Merrill [113] (Sítio Físico, Conservação e Expansão de Sítio)…”