Metallocene catalysts molecular weight

Molecular Weight Distribution. In industry, the MWD of PE resins is often represented by the value of the melt flow ratio (MER) as defined in Table 2. The MER value of PE is primarilly a function of catalyst type. Phillips catalysts produce PE resins with a broad MWD and their MER usually exceeds 100 Ziegler catalysts provide resins with a MWD of a medium width (MFR = 25-50) and metallocene catalysts produce PE resins with a narrow MWD (MFR = 15-25). IfPE resins with especially broad molecular weight distributions are needed, they can be produced either by using special mixed catalysts or in a series of coimected polymerization reactors operating under different reaction conditions. 

More recent inventions are the metallocene catalysts based on zirconium. They offer more uniform catalyst activity and can give a relatively narrow molecular weight distribution. More importantly, they offer better control over structure and copolymer composition distributions. 

In order to enhance the activity of coordination catalysts we typically add a cocatalyst. The cocatalyst works synergistically with the catalyst to allow us to tailor the tacticity and molecular weight of the product while also enhancing the rate of the reaction. An example of a commercially used cocatalyst is methylaluminoxane used in conjunction with metallocene catalysts. 

The molecular weight distribution in Fig. 5.3 a) exhibits a most probable molecular weight distribution , which is characteristic of polymers produced by metallocene catalysts. This distribution contains relatively few molecules with either extremely high or low molecular weights. Products made with this type of distribution are relatively difficult to process in the molten state, exhibit modest orientation, and have good impact resistance. 

How do metallocene catalysts define the molecular weight distribution and tacticity of polypropylene ... 

At present, the difficulties described in 7-9 can be solved. The use of MAO can be avoided by the use of non-MAO catalysts such as cationic Group 4 metallocenes or neutral Group 3 metallocenes. It has been found that the molecular weight of the resulting polymer can be increased by introducing substituents at the 2,5-position in the cyclopentadienyl group of the... 

An important step in the field of investigation of metallocenes was the design of zirconocene catalysts. These made it possible to produce polypropylenes with a great variation in tacticity [5], Although the ligands of the catalysts were varied, the molecular weights mostly did not exceed 300,000 g mob1. 

The exchange of zirconium in isostructural complexes leads to a new family of asymmetric metallocenes (Fig. 1) bearing a 2-methyl substituent and varied substituents in positions 5, 6, and 7 of the indenyl moiety. After borate activation all catalysts show an unexpected high and constant activity toward the polymerization of propylene and lead to significantly increased molecular weight products compared to the zirconocene species [9-11],... 

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