Metallocene polyethylene catalyst research

A unique feature of polymer products obtained with homogeneous metallocene-based catalysts is their narrow molecular weight distribution (MWD). Thus the values of My,jMn for polyethylenes and polypropylenes are typically near 2 which would indicate the presence of uniform active species in catalysts. Some researchers have thus suggested that the catalysts could be single site, while others have explained that two or more kinds of species can have very similar values for the kpjktr ratio, giving rise to narrow MWD products. 

Metallocenes, especially zirconocenes but also titanocenes, hafnocenes, and other transition metal complexes treated with MAO are highly active for the polymerization of olefins, diolefins, and styrene. The polymerization activity, which is up to 100 times higher than for classical Ziegler catalysts, as well as the possibility to easily tailor the microstructure of the polymer chain and to obtain polymers with special properties have motivated research groups worldwide to produce thousands of patents and publications in the last 20 years. An overview can be found in selected review articles and books [55-68]. A metallocene/MAO catalyst containing 1 g zirconium produced 40 x 10 g polyethylene in 1 h at 95°C and 8 bar ethene pressure (Table 1). 

The focus of commercial research as of the mid-1990s is on catalysts that give desired and tailored polymer properties for improved processing. Development of metallocene catalyst systems is an example. Exxon, Dow, and Union Carbide are carrying out extensive research on this catalyst system for the production of polyethylene and polypropylene. 

In this chapter we will discuss a few topics in the area of alkene polymerisations catalysed by homogeneous complexes of early and late transition metals (ETM, LTM). One of the main research themes for the ETM catalysts has been the polymerisation of propene, while industries have also paid a lot of attention to metallocenes giving LLDPE (linear low-density polyethylene, for thinner plastic bags). In less than a decade a completely new family of catalysts has been developed which enables one to synthesise regioselective and stereoselective polymers of a wide variety of monomers. These new catalysts are starting to find application on industrial scale, but as yet only reports on commercialisation of (branched) polyethylene and polystyrene have appeared. 

The Phillips catalyst has now been used commercially for more than half a century. It has been adapted and improved and tailored for new markets and new reactors. Research, which is still ongoing, continues to lead to new ways to modify the catalyst and to new grades of polyethylene to make with it. These facts are contrary to the impression often taken from trade magazines, which sometimes describe the Phillips and Ziegler catalysts as "mature," in contrast to the many exciting discoveries of metallocenes that are being made. Indeed, some writers even predicted that metallocenes would replace Phillips and Ziegler catalysts. 

The improved control over the polymer structure offered by these catalysts offers the polymer producer a significantly greater ability to tailor the polymer to the end-user requirements. Polymer research with metallocene catalysts continues, so more advances can be expected for polyethylene, polypropylene, and other polyolefins. 

Brinell hardness See test, Brinell hardness. Brintzinger A chemist from Konstanz University, Hamburg, Germany, who m 1982 was the first to report on rnetalloorganic (metallocene) complex compounds of titanium and zirconium. Science and industry soon used his discovery to develop new polymerized compounds such as polyethylene and polypropylene plastics. Unlike its classical predecessors, the structure of metallocenes can be analyzed in all types of details, enabling it to be adapted to suit the requirements of the plastic researchers with a broad scope of possible variations. See catalyst, metallocene. 

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. 

Because of the commercial significance of polyethylene and polypropylene and to a lesser extent, polystyrene, and the ability to tune polymer properties such as stereochemistry, molecular weight, and comonomer incorporation, research on these organometallic catalyst systems continues to this day. Metallocene catalyst systems are often based upon substituted... 

Here, we review (1) research activities in metallocene catalysts, (2) polymerization performances of metallocene catalysts and other single-site catalyst technologies, with examples for polyethylene (PE), cyclo-olefin copolymer (COC), polypropylene (PP), syndiotactic polystyrene (SPS), and cyclo-olefin polymers, and (3) the computational design of metallocene catalysts. 

---