Metallocene polyethylene technology

Since the last edition several new materials have been aimounced. Many of these are based on metallocene catalyst technology. Besides the more obvious materials such as metallocene-catalysed polyethylene and polypropylene these also include syndiotactic polystyrenes, ethylene-styrene copolymers and cycloolefin polymers. Developments also continue with condensation polymers with several new polyester-type materials of interest for bottle-blowing and/or degradable plastics. New phenolic-type resins have also been announced. As with previous editions I have tried to explain the properties of these new materials in terms of their structure and morphology involving the principles laid down in the earlier chapters. 

New types of LLDPEs based on the metallocene catalyst technology have been introduced recently in the market place. Such LLDPEs are characterized by narrower molecular weight and homogeneous short-chain branching distribution. Some of the metallocene catalyst based octene-1 LLDPE copolymers made by the Dow Chemical Company are known to have LCB. For the properties of metallocene LLDPE see the entry Polyethylene, metallocene linear low density, in this handbook. 

INTRODUCTION New types of linear low-density polyethylenes (LLDPE) based on the metallocene catalyst technology have been introduced recently in the market place. Metallocene-based Ziegler-Natta catalysts utilize a new synthetic approach for the polymerization of poly(a-olefins).( 5) Metallocene precatalysts are based primarily on group IV transition metals (primarily titcuiium and zirconium straddled by a pair of cyclic alkyl molecules) and require a coactivator, which is typically methylalumoxane but certain acids containing noncoordinating anions as bases also work well. 

While metallocenes have been known to polymerize olefins since the 1950s, only in the last 10 years have they been introduced commercially. Exxon introduced its first generation metallocene in 1989 for the limited production of polyethylene. This was a homogeneous catalyst used in solution. Other companies have followed with their own metallocene catalyst technologies, which are supported and are used in solution, gas phase, and supercondensed phases and processes. 

The commercial processes used to produce CPOF are well estabhshed and based on time-tested concepts and empirical methods. Different technologies result in foams with different cellular structures, and, as a consequence, different properties should be expected. In the last few years, investigations in the area of foaming have focused mainly on the foaming of new resins such as ESI, metallocene polyethylenes, or PP and on the improvement of formulations. In addition, to increase the range of properties and apphcations of PO foams, open-cell materials have also been produced. 

Metallocene Polyethylene 2000 Technology, Markets and Players, Phillip Townsend Associates Inc., Houston, TX, USA, 2000. 

It should also be noted that recently a new high-pressure autoclave process has been developed by Exxon Chemical Company to produce linear low-density polyethylene using metallocene catalyst technology. Because the metallocene catalyst is a single-site catalyst, the molecular-weight distribution of the resulting polyethylene is very low (M /M = 2.0). The polymerization is carried out in a staged autoclave reactor at 1000-2000 atm and 150-250°C with 30-120 sec of reactor residence time [13]. 

Just recently a new polyolefin wax family had been developed by using metallocene catalyst technology. The family of waxes includes polyethylenes and specialty grades of polypropylenes and copolymers. These low viscosity and low softening point resins have already been successfully used as dispersing aids in the production of master-batches, in adhesives and sealants as well as in fibre glass coatings for composites [2],... 

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. 

In only a few polymerization processes are metallocene catalysts used in a soluble form. Supported metallocene catalysts are preferred for the production of polyethylene or isotactic polypropylene on an industrial scale, especially in the slurry and gas-phase processes. To use them in existing technological processes (drop-in technology) as replacements for the conventional Ziegler-Natta catalysts, the metallocenes have to be anchored to an insoluble powder support, including silica, alumina, and magnesium dichloride (208-217). Various methods of anchoring catalysts to supports are possible (Fig. 25) ... 

The sheer size and value of the polyethylene industry ensure that there is continued research, progress, and development in catalysis, for their potential commercial impact. Although this whole subject is not within the scope of this chapter, we mention a couple of aspects of the progress, which offer the potential to impact this industry. In 1995, DuPont introduced work, carried out with them at the University of North Carolina—via the largest patent applicafion ever in the USA. They disclosed what are described as post-metallocene catalysts. These are transition and late transition metal complexes with di-imine ligands, which form part of the DuPont Versipol technology. Such catalysts create highly branched to exceptionally linear ethylene homopolymers and linear alpha-olefins. Late transition metals offer not only the potential for the incorporation of polar comonomers, which until now has only been possible in LDPE reactors, but also their controlled sequence distribution, compared to the random composition of free radical LDPE copolymers. Such copolymers account for over 1 million tons per annum [20]. Versipol has so far only been cross-licensed and used commercially by DuPont Dow Elastomers (a former joint venture, now dissolved) in an EPDM plant. 

We have seen the development of polyethylene, from low molecular weight polymers first mentioned by name in the literature in 1869, to the first reported solid polymers of linear polyethylene by Prof. Marvel in 1930 then the unintentional synthesis and chance observation of 0.4 g of solid polyethylene in March 1933 by ICI (prepared under high pressure, later described as LDPE) the onset of catalyst technology in the industry, from the simultaneous discoveries of transition metal catalysts a few decades later, that created the HOPE industry the development of LLDPE copolymers and the discovery in 1979 of metallocene catalysts for polyolefin polymerization - all of which are now part of the mainstream polyethylene industry. Post-metaUocene catalysts offer the promise of branching without high pressure or comonomers the potential to incorporate polar groups without high pressure, and to control this copolymer microstructure. 

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