Abstract
Olefin polymerization starts at the outer surface and at the macropores walls. Here fragmentation occurs by peeling off the catalyst in a layer-by-layer mode with, in ideal conditions, the simultaneous formation of bisection-type fractures across the catalyst inner domains.
Highlights
Polyolefins need to be synthesized as discrete particles with homogeneous bulk density, narrow size distribution, and, ideally, spherical morphology to favor their processability and avoid reactor fouling.[17,18,19] This is achieved by immobilizing metallocenes on inorganic supports, most commonly silica, which provide a template for the growing polymer
The macropores are connected to the outer surface, as evident from the pristine particle scanning electron microscopy (SEM) shown in Fig. S6.† The two different silica phases can be seen as an agglomeration of micrograins, forming interstitial mesopores of ∼15–20 nm, coalescing more in the dense silica shell
The crystallinity of the nascent polymer might be different from the bulk crystallinity of the final polymer, and it will be dependent on the local environment of the growing chain, which is highly affected by the reaction conditions.[19,58]
Summary
Polyolefins need to be synthesized as discrete particles with homogeneous bulk density, narrow size distribution, and, ideally, spherical morphology to favor their processability and avoid reactor fouling.[17,18,19] This is achieved by immobilizing metallocenes on inorganic supports, most commonly silica, which provide a template for the growing polymer. The current most effective lab-scale method to observe fragmentation is cutting particle slices (with a razor blade or diamond knife after embedding them in epoxy, i.e., micro-toming) and observing the obtained crosssections with SEM or TEM.[29,30,48,49,50,51,52,53] This method has its limitations, mainly a possible deformation of the catalyst particle and the difficulty in distinguishing epoxy, polymer, and catalyst phases within the cross-section made It has not been possible so far to visualize the very early onset of catalyst particle fragmentation and polymerization fronts with sufficient spatial resolution as well as accuracy within real-life industrial metallocene-based olefin polymerization catalysts. Polymer formation in ideal reaction conditions induced fragmentation both in layerby-layer and bi-sectioning mode, while mass transfer limitation phenomena and inhomogeneous active site distribution lead to sub-optimal catalyst particle fragmentation
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