Molecular weight metallocene polyethylene

Table 5.8 lists propylene polymerization data for some of the complexes shown in Figure 5.12. Varieties of other polymer products have been prepared using the D/A-bridged complexes. For example the (Flu)(Cp)ZrCl2 complexes (22a-b) produce ultra-high molecular weight (UHMW) polyethylene with intrinsic viscosities as high as 13.5 d/g. The D/A metallocenes are also effective catalysts for the copolymerization of a-olefins with ethylene to produce medium to ultta-low density polyethylene products. ... 

Narrow molecular weight distribution, which is characteristic of metallocene-based polyethylene (Fig. 7), causes processing difficulty in certain applications due to increased melt pressure, reduced melt strength, and melt fracture [14,15]. This problem can be overcome by blending the metallocene polymer with other prod-... 

In practice, product developers often blend two or more resins together in order to obtain a product that has the required melt flow and solid-state characteristics. Thus, we frequently combine metallocene catalyzed linear low density polyethylene, having a most probable molecular weight distribution, with low density polyethylene, having a broad molecular weight distribution. The linear low density polyethylene provides good impact resistance, while the low density polyethylene improves melt flow characteristics. 

Eaves (92) distinguished between polyolefin plastomers (POP) with density >910 kg m and polyolefin (POE) elastomers with densities <910 kg m-3. The density of a polyethylene at 20 °C is a linear function of the crystallinity, with limiting values of 854 kg m 2 for zero crystallinity and 1000 kg m for 100% crystallinity. The polyolefin elastomer foams compete with EVA copolymer foams. Metallocene chemistry also allows the production of copolymers with a larger comonomer content in the high molecular weight part than in the low molecular weight part this... 

Fig. 9.5-1, right, compares the co-monomer incorporated into the polymer, on a molecular basis. With the Ziegler-Natta, multi-site catalysts less monomer is incorporated in the high-molecular-weight fraction. The low-molecular-weight fraction is rich in co-monomer content as shown by the negative slope of curve a. In a metallocene-based polyethylene the comonomer is uniformly distributed (curve b). 

Metallocene supported catalysts are not limited only to those involving inorganic catalyst carriers. Starch [212] and a-cyclodextrin [213] have also been successfully used as supports for zirconocene-AlMe3 [212] and zircono-cene-fA Me b [213] catalysts capable of producing high molecular weight polyethylene. 

The Mw/Mn ratio is usually equal to 5-10 for polyethylene [49,64,66,67, 123,244-247], A much lower polydispersity is displayed by polymers obtained in polymerisation with homogeneous metallocene catalysts the Mw/Mn ratio usually does not significantly exceed a value of 2 [22,95,101,112,138,140], By polymerising propylene with soluble vanadium-based Ziegler-Natta catalysts at low temperature, a very narrow molecular weight distribution of the polypropylene has been found (the Mw/Mn ratio usually reaches values of 1.15-1.25) and a linear increase in its Mn with time has been observed, indicating a noticeable living character of the polymerisation [75,76,241],... 

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