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Single-site catalysts molecular weight distribution

Unlike the conventional UHMWPE made with the Ziegler catalysts, the single-site UHMWPE has narrow molecular weight distribution. A suitable catalyst is supported 8-quinolinoxytitanium trichloride and triethylaluminum. The supported catalyst is prepared by (9) ... [Pg.79]

Polymers manufactured via single site catalyst technologies, because of the unique chemical catalytic environment, exhibit a more controlled molecular weight distribution and tacticity than seen with Ziegler-Natta catalyst systems. [Pg.49]

We can vary the density of very low density polyethylene from 0.90 down to 0.86 g/cm3 by varying the comonomer level from approximately 8 to 14 mole %. At the highest comonomer levels, crystallization is impeded by the branches to such an extent that only about 5% of the material crystallizes. The crystallites of very low density polyethylene are small and poorly organized. We polymerize these resins using single-site catalysts, which give us relatively narrow molecular weight and composition distributions... [Pg.298]

The theoretical lower limit of the molecular weight distribution for the diblock OBC is 1.58. The observed MJMn of 1.67 indicates that the sample contains a very large fraction of polymer chains with the anticipated diblock architecture. The estimated number of chains per zinc and hafnium are also indicative of a high level of CCTP. The Mn of the diblock product corresponds to just over two chains per zinc but 380 chains per hafnium. This copolymer also provides a highly unusual example of a polyolefin produced in a continuous process with a molecular weight distribution less than that expected for a polymer prepared with a single-site catalyst (in absence of chain shuttling). [Pg.99]

With a constant temperature and constant concentration of reacting components near the active site the molecular weight distribution of the polymer formed by a single-site catalyst can be described by a Flory-Schulz equation, that can be derived easily from the polymerization mechanism ... [Pg.345]

This diversity of sites explains why the molecular weight distribution (MWD) of polymers produced by Cr/silica is broad (71). Model calculations which assume a single type of active site usually predict Mw/Mn 2,4 but in reality Mw/Afn = 6-15 is common, and 20-30 can be achieved with catalyst modifications. The distribution is also broader than that generally obtained from Ziegler catalysts, for which Mw/Afn = 3-6 under similar conditions. Experience with organometallic compounds suggests that a broad MWD may be a general feature of catalysts which terminate by -elimination. [Pg.68]

The first single-site metal catalyst which was shown to homogeneously catalyze the coupling of epoxides and C02 was (tpp)AlCl (tpp = tetraphenylporphyrin) in the presence of a quaternary organic salt or triphenylphosphine [20]. Although this catalytic system was extremely slow at ambient temperature, copolymers from ethylene oxide, PO, and CHO and C02 were obtained that possessed very narrow molecular weight distributions (polydispersity = 1.06-1.14). The low reactivity of... [Pg.217]

The soluble catalysts can prepare polymers with very good stereospecificity and narrow molecular weight distributions, as a result of the uniformity of the active sites. In fact, these catalysts are often referred to as single site catalysts .26 The polydispersity (MJMn),... [Pg.634]

Application The SCLAIRTECH1 technology (PE) process can produce linear-low-density, medium-density and high-density polyethylene (PE) with narrow to broad molecular weight distribution using either Ziegler-Natta (ZN) or proprietary single-site catalyst (SSC). [Pg.157]

For polyethylene produced with transition metal catalysts, molecular weight distribution is dictated largely by the catalyst employed. Polydispersities typically range from 2-3 for polyethylene made with single site catalysts, 4-6 for polymer produced with Ziegler-Natta catalysts and 8-20 for polyethylene made with supported chromium catalysts. These differences are illustrated... [Pg.17]

Bimodal molecular weight distribution may be achieved by several techniques. The simplest method is post-reactor blending of polyethylene with different melt indices. Two other methods involve in-reactor production of polyethylene. One approach involves use of mixed catalyst systems that polymerize ethylene in different ways to produce polyethylene with different molecular weights. The latter requires that the catalysts are compatible. Another technique employs use of reactors in series operated under different conditions (see section 7.6 in Chapter 7). Figure 1.9 illustrates polyethylene with a bimodal molecular weight distribution produced with a single site catalyst system in a Unipol gas-phase process. [Pg.18]

After activation, the catalyst is intrcxiuced into the polymerization reactor as slurry in a saturated hydrocarbon such as isobutane. The precise mechanism of initiation is not known, but is believed to involve oxidation-reduction reactions between ethylene and chromium, resulting in formation of chromium (II) which is the precursor for the active center. Polymerization is initially slow, possibly because oxidation products coordinate with (and block) active centers. Consequently, standard Phillips catalysts typically exhibit an induction period. The typical kinetic profile for a Phillips catalyst is shown in curve C of Figure 3.1. If the catalyst is pre-reduced by carbon monoxide, the induction period is not observed. Unlike Ziegler-Natta and most single site catalysts, no cocatalyst is required for standard Phillips catalysts. Molecular weight distribution of the polymer is broad because of the variety of active centers. [Pg.64]


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Molecular weight distribution

Single-molecular

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