Metallocene catalysts advantages

Improved co-monomer incorporation. Metallocene catalysts are very efficient at co-monomer incorporation, which means that co-monomer use can be reduced by a factor of ten or more. This has some cost advantages but, more importantly, there is less unincorporated comonomer in the final product, improving efficiency and mass balance and reducing VOCs. 

These are some key advantages that the metallocene catalysts have over conventional Ziegler-Natta catalysts and hence it is highly probable that inter-and intra-chain heterogeneity expected in ethylene-a-olefins copolymers can be controlled through the use of the metallocene system. 

In recent years metallocene catalysts have been introduced into low-pressure gas-phase-, solution-, and slurry-processes to manufacture polyethylene and polypropylene. The new technology extends not only the range of conventional materials but generates new speciality polymers. Some companies have also retro-fitted high-pressure reactors to make use of the advantages of metallocene catalysts. 

The advantages known from the production of low-density polyethylene (LDPE) become obvious also when metallocene catalysts are used under high-pressure conditions. The compressed monomer can dissolve the polymer which is formed during polymerization, which means that no additional solvent is required for the polymer. The high-pressure polymerization proceeds with a high rate, which requires a short residence time and small reactor volume. Established technology, with stirred autoclaves as well as tubular reactors, can be applied. 

The main benefits of metallocene catalysts in comparison to the conventional Ziegler-Natta catalysts are well-defined microstructures, high activity, narrow molar mass distribution and the possibility of tailor-made polyolefins. The advantages in metallocene catalyst chemistry offer a promising novel... 

Metallocene catalysts supported on SiC>2 are practically free of chlorine. This could be an advantage from an ecological standpoint when the chemical or thermal recycling of polymeric products reaches a large scale [211]. 

Occasional regioerrors appear significantly to inhibit the polymerisation of a-olefins by methylaluminoxane-activated metallocene catalysts [114, 138, 253— 261], In order to reduce the number of secondary Zr-CH(R)-CH2 species, and therefore to accelerate the polymerisation, advantage has been taken of the chain transfer reaction with hydrogen ... 

The discovery of homogeneous metallocene catalysts in the 1980s was a very important milestone in polymer technology. With these catalysts the plastic industry is poised to move into an era of an entirely new range of polymeric materials with several specific advantages. From the initial discovery in the 1980s, close to five billion dollars is estimated to have been invested by several large chemical companies in research and development. In a relatively short time, this has resulted in approximately fifteen hundred patent applications Close to 0.5 million tons of metallocene-catalyzed polypropylene is expected to be manufactured by the year 2003. 

The mechanical, thermal, optical, and other properties of a polymer depend on the structure of the monomer units. Where a copolymer is used, it depends additionally on the relative amounts and distribution of the two monomeric building blocks. Metallocene catalysts have four main advantages over the conventional polymerization catalysts. They can polymerize a very wide variety of vinyl monomers irrespective of their molecular weights and steric features. They also can polymerize mixtures of monomers to give polymers of unique properties. 

The third advantage associated with metallocene catalysts is that the predominant mechanism for chain termination is by /3-hydride elimination. This produces a vinyl double bond at the end of each polymer chain. Further functionalization of the vinyl group by graft polymerization with maleic anhydride and other functional monomers is far more effective than is typical for polyolefins obtained by conventional catalysts. 

The advantages of high pressure known from the radical polymerization process for low density polyethylene are listed in Table 1. They can also be observed in the polymerization with metallocene catalysts. 

Nowadays homogeneous metallocene catalysts activated with oligomeric methylalumoxanes or other co-catalysts [16, 20, 46-54] open new prospects. These systems have an excellent activity, they have the ability to form random copolymers in combination with a narrow molecular mass and comonomer distribution. Further important advantages are that a broad variety of structures can be synthesized to obtain tailor-made catalysts [49, 53], and that zirconium compounds are scarcely reduced with the co-catalyst [54]. It is further reported that metallocenes have been used in combination with methylalumoxanes for EPDM production at temperatures below 100 °C in liquid propylene [55]. 

The tunable metallocene catalyst with a well-defined polymerization mechanism provides distinctive advantages in the preparation of new polymers with well-controlled molecular structures, especially functional polyolefins that are very difficult to prepare by other methods. Since the discovery of HDPE and i-PP about half a century ago, functionalization of polyolefin has been a scientifically challenging and industrially important area. The constant interest, despite lack of effective functionalization chemistry, is due to the strong desire to improve polyolefin s poor interactive properties. The hydrophobicity and low surface energy of polyolefin has limited its applications, especially in the areas of coating, blends, and composites, in which adhesion, comparability, dispersion, and paintability are paramount. 

In the past decade, our group at Penn State has been focusing on a functionalization approach by the combination of metallocene catalysts and reactive comonomers. The chemistry takes the advantage of metallocene catalyst with a tunable single active site to prepare polyolefin copolymers with narrow molecular weight and composition distributions, high catalyst activities, and predictable tacticities and copolymer compositions. 

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