Catalysts polymeric, properties

The CO-reduced catalyst polymerizes ethylene much like its ethylene-reduced hexavalent parent and produces almost identical polymer [4]. Since the polymer properties are extremely sensitive to the catalyst pretreatment. 

Metallocenes have a transition metal sandwiched between two cyclopentadienyl rings. The cyclopentadienyl rings may not necessarily be parallel because of bridging. They may be further substituted to restrict access to the metal. The catalyst structure can be changed to make different polymeric properties. A typical example would be the zirconium metallocene shown below. 

Krauss and Stach (31) demonstrated that the hexavalent catalyst can be quantitatively reduced by CO at 350°C to divalent chromium. This material has no y-phase resonance but is active for ethylene polymerization, indicating that Cr(II) is definitely an active valence.1 These results have since been confirmed by several other laboratories, including this one (30). In fact, Hogan concluded, as early as 1959 from similar reduction studies, that the active species must be divalent. The CO-reduced catalyst polymerizes ethylene in a high-pressure autoclave much like its hexavalent parent, and produces almost identical polymer. Since the polymer properties are extremely sensitive to the catalyst pretreatment, this is a strong endorsement for the conclusion that Cr(II) is probably also the active species on the commercial catalyst after reduction by ethylene. 

Metallocene Catalysts. Higher a-olefins can be polymerized with catalyst systems containing metallocene complexes. The first catalysts of this type (Kaminsky catalysts) include metallocene complexes of zirconium such as biscyclopentadienylzirconium dichloride, activated by methylaluminoxane. These catalysts polymerize a-olefins with the formation of amorphous atactic polymers. Polymers with high molecular weights are produced at decreased temperatures and have rubber-like properties. 

C NMR spectroscopy is a powerful method for analysing the structure of copolymers providing detailed information about their constitution, sequences, stereo- and regio-errors and chain-end structures. The presence of various microstructures influences the polymeric properties and provides an insight into the mechanism by which the polymerisation catalyst operates. However, because some structures occur in very low concentrations, early application of 2D INADEQUATE to polymers77 found very few follow-up studies. 

Copolymers of ethylene and propylene have come to stay as important materials with diverse practical applications. They span the full range of polymeric properties, from soft elastomers to hard thermoplastics depending on the relative composition of the two monomers and the manner of their enchainment. Ethylene-propylene copolymers are manufactured commercially using Ziegler-Natta catalysts [1]. For the purposes of this discussion, we will treat these copolymers in terms of three distinct classes of materials ... 

Elevated temperatures, 70 °C, and enhanced pressures, 5—10 atm, are conventionally used for olefin polymerization. The catalytically active species is formed in situ from a catalyst precursor. The solvent medium changes during the course of reaction. Initially a solvent such as propylene or toluene surrounds the active site, but shortly the active site is encased in a polymeric solution. Seemingly insignificant changes in reaction medium perturb the observed polymeric properties. For these reasons, for modeling studies one must proceed with caution... 

The property envelope of polyolefins has been expanded by the direct synthesis, in the reactor, of novel alloys and blends with a balance of properties not possible ftom conventional, third-generation catalyst polymerization. Such advances in Ziegler-Natta polymerization technology show that polyolefin-based resins can continue to provide the most cost-effective solutions for a wide and ever-increasing range of plications in the global plastics market. 

A decisive improvement in the polymeric properties, and much-improved applicability, were brought about by the metallocene catalysts that have been known since the 1980s. 

For many processes performed in supercritical fluids, the transport properties of the medium will play an important role. For polymerizations, this includes mass transfer for mixing reactants and to allow proper contact between monomer and catalyst. Polymerization reactions are usually highly exothermic, so that the heat of reaction needs to be absorbed and transported through the supercritical fluid. Virtually aU studies described in the literature have been performed on a relatively small scale, and scale-up aspects, for which mass and heat transfer are major issues, have generally been disregarded. This chapter will describe an experimental study of some aspects of mass and heat transfer in supercritical CO2 (SCCO2), and a comparison will be made with the behavior of standard liquid systems. 

Frontier with the subject of this paragraph, the use of early and late metal complexes combinations for reactor blending during ethylene polymerization can also be mentioned. In such a process, known since the 1980s, the early and late catalysts polymerize ethylene independently to afford an intimate mixture of polyethylene PE) chains of different structure. Although in this case there is no cooperative effect of the two metals from a molecular or mechanistic point of view, the beneficial simultaneous use of the two catalysts, such as a dichlorozirconocene and a diimine nickel complex (Scheme 58) ([147] and references therein), is found in the bulk physicochemical properties of the obtained PE. 

EPM [poly(ethylene-co-propylene)] and EPDM [poly(ethylene-co-propylene-co-5-ethylidene-2-norbomene)P can be metallocene catalyst polymerized. Metallocene catalyst technologies include (1) Insite, a constrained geometry group of catalysts used to produce AfiGnity polyolefin plastomers (POP), Elite PE, Nordel EPDM, and Engage polyolefin elastomers (POP) and (2) Exxpol ionic metallocene catalyst compositions used to produce Exact plastomer octene copolymers.2 Insite technology produces EPDM-based Nordel IP with property consistency and predictability (see Sec. 3.2.2). 

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