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Dramatic changes occur during the initial stages of olefin polymerization over heterogenous catalysts. As polymer accumulates, the catalyst fragments and the void space within the growing particle becomes filled with polymer. The changing monomer transport rate to the active sites, dissipation of heat and stress with the particle, and eventually, dispersion of catalyst fragments within the growing particle can control the polymerization. We focus on the changes in, and influence of, polymer, catalyst and void morphology during the nascent polymerization for silica supported chromium oxide catalysts. We find that the course of the polymerization depends on the initial stages yet is seldom considered in polymerization models. We review several years of morphological characterization that identify the important physical phenomena which occur during nascent heterogeneous olefin polymerization. ~~ ~~ ~ ~~~Des changements importants surviennent durant les stades initiaux de la polymerisation de I'olkfine sur des catalyseurs hkttrogknes. Lorsque le polymkre s'accumule, les fragments de catalyseurs et I'espace intersticiel B I'intCrieur de la particule qui grossit se remplit de polymkre. La variation de vitesse de transport de monomkre vers les sites actifs, la dissipation de la chaleur et des contraintes avec la particule, et finalement, la dispersion des fragments de catalyseurs B I'interieur des particules qui croissent contrblent la polymtrisation. Nous nous interessons en particulier aux changements dans le polymkre, le catalyseur et la morphologie du vide et B leur influence durant la polymkrisation naissante pour de la d i c e supportke sur des catalyseurs d'oxyde de chromium. Nous avons trouvt que le cours de la polymerisation dkpendait des stades initiaux bien que cela soit rarement considCrC dans les modkles de polymerisation. Nous avons examine sur plusieurs annkes la caractirisation morphologique qui difinit les phknomknes physiques qui surviennent durant la polymkrisation naissante de l'olkfine hiterogkne.

The changes in morphology due to the formation of polyethylene in the pores of three silica-supported, chromium oxide catalysts were followed by using mercury porosimetry and electron microscopy.Ethylene polymerization from 0.1 to 20 g of polymer/g of catalyst was carried out from the gas phase in a fluid bed reactor at 1-atm total pressure with a nitrogen diluent. A catalyst with 1.7 cm3/g pore volume fragmented due to the formation of polymer in the pores and thereby maintained an open structure. Catalysts containing 1.1 and 2.3 cm3/g pore volume did not fragment extensively, and the product polymer congested the pores and impeded the continued polymerization. Total pore volume and pore size are not the only controlling factors in the fracturing process. Mercury porosimetry showed that fracture of the 1.7 cm3/g catalyst started after a polymer yield of just 0.4 gPE/g^, maintaining monomer access to the active sites. The 0.1-1-^m catalyst fragments contained a pore microstructure much like that of the starting material, thus demonstrating how the pore structure of the original catalyst particles may influence the polymerization process after fragmentation is complete.

Pore-size distributions measured by both mercury porosimetry and nitrogen adsorption are compared for a series of compressed aerosols. We conclude that both mercury intrusion and nitrogen desorption are characteristic of the constrictions within the void network. Mercury extrusion and nitrogen adsorption are related to the openings within the void network. By consideration of the void structure as a threedimensional network of interconnected pores of varying size, each technique is analyzed independently and in concert. These studies show the correspondence between the two techniques and the differences that are due to the sequential nature of the processes.

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