Single-Site Ethylene Polymerization Catalysts

By 1980, Professor Walter Kaminsky and coworkers at the University of Hambm-g reported the development of a new class of ethylene 

PE Type Molecular Weight Distribution Process/Catalyst 

LDPE intermediate high-pressure/free radical 

In order to put the global polyethylene market as of 2010 into perspective, it is helpful to compare the demand of the five most common thermoplastic materials. These are polyethylene, crystalline polypropylene, polyvinyl chloride, polyethylene terephthalate and polystyrene. 

Polyethylene is a thermoplastic material that had a 2010 global demand of about 154 billion pounds. Polyethylene has the largest market of the five most common thermoplastic materials, as summarized in Table 1.4. 

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Esteruelas et al. [40] reported very high activity, single-site, ethylene polymerization catalysts prepared with tridentate (N,N,N) ligands based on Bis(imino)pyridyl chromium (III) complexes that were activated with tri-isobutyl aluminum (TIBA) or alkylaluminoxane (-Al(R)-O), where R is methyl or isobutyl. 

Single-site ethylene polymerization catalysts based on titanium or zirconium as the active center are primarily used in the low-pressure manufacture of polyethylene with a relatively very narrow molecular weight distribution to include high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE) and very low density LLDPE to include amorphous material. Table 4.1 summarizes a few examples of end-use applications for this type of polyethylene in terms of the density, molecular weight distribution and molecular weight for various applications. Note that in Table 4.1 a Melt Flow Ratio (MFR) value of 16 approximately corresponds to an polydispersity value of 2.0, which is indicative of a... 

The role of methylalumoxane (MAO) as a cocatalyst to activate zirconocene compounds such as Cp ZrCl to create a single-site ethylene polymerization catalyst is similar, in some respects, to the role simple aluminum alkyls (AlRj) play in activating Ziegler catalysts. For example, MAO acts as an alkylating agent to form the initial Zr-carbon bond (Zr-CH ) necessary to initiate the polymerization process. However, experimental evidence obtained by a variety of methods clearly has shown that the MAO reacts with the zirconium center to form a zirconium cation of the type [CpjZr-CHS] in which the zirconium is not reduced to a lower oxidation state, but remains as a d° metal and Zr(IV) oxidation state. The MAO, therefore, forms an anion moiety to complete the ion pair necessary to create the active species, as illustrated in Equation 4.1. 

Last but not least, Tl-based MOFs were tried in the Zieglff-Natta polymerization of ethylene and propylene, though these systems were infaior to the well-known single-site metallocene polymerization catalysts with their trCTiendous turnover frequencies and space-time yields [107]. 

McKittrick, M.W. and Jones, C.W. (2004) Toward single-site, immobilized molecular catalysts site-isolated Tr ethylene polymerization catalysts supported on porous silica. J. Am. Chem. Soc., 126, 3052. 

McKittrick, Michael W., and Christopher W. Jones, Toward Single-Site, Immobilized Molecular Catalysts Site-Isolated Ti Ethylene Polymerization Catalysts Supported on Porous Sihca , JACS Communications (2004) 126,3052... 

Additional homogeneous, single-site, chromium-based ethylene polymerization catalysts were reported by Theopold [35], These complexes were [Cp CrCTHFl BzjBPh, Cp CrfTHFKBz), and Li[Cp Cr(Bz)3], which represent a cationic, neutral, and anionic Cr(III) complex, respectively. Table 3.9 summarizes the ethylene polymerization data for each of these three complexes. 

Most of the complexes examined as ethylene polymerization catalysts also produced a large amount of a low molecular weight polymer component (waxes), but if this polymer fraction was removed from the product, the remaining polyethylene exhibited a narrow MWD, indicating a single-site catalyst. Obviously, elimination of the species producing the low molecular weight polymer component would be necessary to develop a commercial catalyst. 

Structurally, plastomers straddle the property range between elastomers and plastics. Plastomers inherently contain some level of crystallinity due to the predominant monomer in a crystalline sequence within the polymer chains. The most common type of this residual crystallinity is ethylene (for ethylene-predominant plastomers or E-plastomers) or isotactic propylene in meso (or m) sequences (for propylene-predominant plastomers or P-plastomers). Uninterrupted sequences of these monomers crystallize into periodic strucmres, which form crystalline lamellae. Plastomers contain in addition at least one monomer, which interrupts this sequencing of crystalline mers. This may be a monomer too large to fit into the crystal lattice. An example is the incorporation of 1-octene into a polyethylene chain. The residual hexyl side chain provides a site for the dislocation of the periodic structure required for crystals to be formed. Another example would be the incorporation of a stereo error in the insertion of propylene. Thus, a propylene insertion with an r dyad leads similarly to a dislocation in the periodic structure required for the formation of an iPP crystal. In uniformly back-mixed polymerization processes, with a single discrete polymerization catalyst, the incorporation of these intermptions is statistical and controlled by the kinetics of the polymerization process. These statistics are known as reactivity ratios. 

Figure 79 The polymer-supported o-carborane (143) utilized further for its decapitation, deprotonation and reaction with ZrCl4 2THF to generate a polymer-supported single-site polymerization catalyst (144) for the polymerization of ethylene and vinyl chloride. (Adapted from ref. 158.)...

Exxpol [Exxon polymerization] A gas-phase process for making polyethylene from ethylene. The process uses single-site catalysis (SSC), based on a zirconium metallocene catalyst. Developed by Exxon Chemical Company in 1990 with plans to be commercialized in 1994. 

After activation with MAO (molar ratios [Al] [Zr] = 1000) the polymerization of ethylene has been successfully carried out using the zirconocene functionalized dendrimer at 40 bar ethylene pressure and 70 °C. We obtained high activity and productivity values for the ethylene polymerization and polymers with very high molecular masses in the range of 2 x 10 g/mol. The polydispersity of the polymer is quite low (3.0) indicating the single site character of the catalytically active species. Optimization of this system and study of the mechanism are stiU under investigation. Nevertheless, these preliminary results reveal the suitability of polyphenylene dendrimers as supports for zirconocene catalysts. 

This is a major achievement, mainly due to Basset and his group, in surface organometallic chemistry because it has been thus possible to prepare single site catalysts for various known or new catalytic reactions [53] such as metathesis of olefins [54], polymerization of olefins [55], alkane metathesis [56], coupHng of methane to ethane and hydrogen [57], cleavage of alkanes by methane [58], hydrogenolysis of polyolefins [59] and alkanes [60], direct transformation of ethylene into propylene [61], etc. These topics are considered in detail in subsequent chapters. 

Michalak A, Ziegler T, Exploring the scope of possible microstructures accessible from polymerization of ethylene by late transition metal single-site catalysts A theoretical study, Macromolecules, 36, 928-933 (2003)... 

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