Cover Picture: Macromol. Rapid Commun. 1/2006

Abstract

Cover: The cover shows a scanning electron microscopy image of a cross-section through a polymer particle with catalyst fragments in the early stage of polymerization. Further details can be found in the Communication by X. Zheng, M. S. Pimplapure, G. Weickert and J. Loos* on page 15.

Polymerization on Molecular Catalysts

“…Since many industrial catalysts are in fact prepolymerized under milder conditions in a separate reactor before being fed to the polymerization reactor, this should not be seen as limitation of the [213,214] covers these questions and some alternative models. According to Loos and coworkers [30,[217][218][219], these investigations show that there are basically two different ways by which the catalyst particles can break up in the case of MgCl 2 -supported Ziegler-Natta catalysts as in the case of silica-supported metallocene/MAO catalysts. 0.8-1.4 g polymer g −1 cat ), respectively [30,200,215,216].…”

“…0.8-1.4 g polymer g −1 cat ), respectively [30,200,215,216]. 42 shows the cross-sectional morphology of a propylene polymer particle generated with an MgCl 2 -supported TiCl 4 /AlEt 3 Ziegler-Natta catalyst at a yield of 0.8 g pp g −1 cat [217]. One way is fragmentation ''shell by shell'' or ''layer by layer'' from the outer surface gradually to the center of the particle (as described above); the other way is instantaneous break up at the beginning of the polymerization into a large number of small sub-particles [218].…”

Alternation of Pore Architecture of Ziegler–Natta Catalysts through Modification of Magnesium Ethoxide

“…Competitive catalysis and diffusion are believed to play an important role in heterogeneous olefin polymerization: higher a-olefins like 1-hexene are susceptible to diffusion limitation, and narrower pores may restrict its incorporation in copolymerization with ethylene. [11][12][13][14][15] The preparation of recent ZN catalysts mainly employs support precursors having spherical morphologies such as MgCl 2 Á alcohol adducts and magnesium alkoxides (Mg-(OR) 2 ). [16,17] A variety of methods have been proposed to modify MgCl 2 Á alcohol adducts, but only a few studies actually reported the alternation of porosity as a result of the modification.…”

Initial Particle Morphology Development in Ziegler‐Natta Propylene Polymerization Tracked with Stopped‐Flow Technique

“…catalyst particles. [5][6][7][8][9][10] For example, Noristi et al [5] reported that fragmentation occurs throughout the MgCl 2 -supported catalyst particles at the polymer yield of 1 g-pol/ g-cat. On the other hand, SiO 2 -supported catalyst particles are fragmented at higher polymer yield, and the fragmentation tends to occur in a ''layer-by-layer'' (LbL) manner.…”

“…It was observed that MgCl 2 -based catalysts could fragment either in an LbL manner or in a homogeneous manner, according to the pore size and volume of the particles. [6,11] These effects of porosity on fragmentation can be explained in term of mass transfer limitation: once a catalyst is mixed with a cocatalyst in the presence of monomer, polymerization first happens at active sites located on the particle's outermost surface, and on the surfaces of pores which are most easily accessed by the cocatalyst. The formed polymer restricts the monomer and cocatalyst diffusion into inner parts.…”