Types of Single-Site Catalysts

There are primarily three types of single-site catalysts used to manufacture polyethylene, each of which will be discussed in detail in this chapter. 

The second type of single-site catalyst, designated as the constrained geometry catalyst (CGC), was developed by James C. Stevens and coworkers in the late 1980s at Dow Chemical Company in Freeport, Texas. This catalyst type contains one Cp ligand as one component in forming a metallocycle structure. The active site is based on titanium [8]. 

The third type of single-site catalyst is best described as a nonmetallocene type of catalyst system, where a ligand system is used that is not based on a cyclopentadiene derivative. This third type of single-site catalyst may utilize a very wide variety of ligands and an early or late transition metal (i.e., iron, cobalt or nickel) maybe utilized as the active center [9-13]. 

Many different variations of these types of catalysts have become commercial since the early 1990s. 

For example, prior to the discovery of this new single-site catalyst type, commercial grades of polyethylene were primarily manufactured over the compositional range of 0-4 mol% of comonomer (1-butene, 1-hexene or 1 -octene) that provided ethylene copolymers over the density range of 0.915-0.970 g/cc. Commercial catalysts were primarily the Cr-based Phillips-type of catalyst or a Ti-based Ziegler catalyst with the xmderstand-ing that both types of catalyst consisted of many different types of active sites. Each type of active site produced a different composition of polyethylene (different molecular weight and branching content) which resulted in a final polyethylene material with a complex molecular structure. These multi-site catalysts limited the composition of the polyethylene that was commercially available due to both process and product constraints imposed by such catalysts. 

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Table 1.2 provides a summary of commonly used classifications in the polyethylene industry. A brief note is warranted here to conclude the survey of polyethylene classifications and nomenclature. In the early 1990s, several types of polyethylene manufactured with metallcxiene catalysts (a type of single site catalyst, see Chapter 6) were introduced to the market. To differentiate polyethylene produced with metallocenes from polyethylene manufactured using older conventional catalysts, metallocene grades are sometimes abbreviated mVLDPE, mLLDPE, etc. 

There are two types of single site catalysts. The most well-known are based on metallocenes. Non-metallocene types are a relatively recent development and most are based upon chelated compounds of late transition metals, primarily Pd, Ni and Fe. Each single site catalyst type is addressed below. 

A new strategy has been used by Tilley et al. to prepare a series of single-site catalysts that consist of iron [13] and cobalt [14] centers supported on mesoporous SBA-15 silica. The iron centers were introduced via grafting reactions of the tris(tert-butoxy)siloxy-iron(III) complex [Fe(OSi Bu3)3(THF)] with SBA-15 in dry hexane to form, finally, an immobilized iron(III) complex of the type [(=SiO) Fe 0Si(0 Bu)3 2(THF)j and to eliminate H0Si(0 Bu)3. Calcination of these species... 

Grubbs group [31, 32] developed another type of Ni-based catalyst. This neutral Ni-catalyst, based on salicylaldimine ligands, is active in ethene polymerisation without any co-activator and originated from the Shell higher olefin process (SHOP). Shortly thereafter another active neutral P,0-chelated nickel catalysts for polymerisation of ethene in emulsion was developed by Soula et al. [33, 34, 35]. The historical development of single site catalysts is represented in Fig. 1. 

The introduction of single-site catalysts for the manufacture of polyethylene is the most recent catalyst innovation that has taken place in the polyethylene industry, with this type of catalyst introduced on a commercial scale in the late 1990s. Single-site catalysts have created new types of polyethylene for the polyethylene industry in which new grades of polyethylene have been introduced into new markets and applications. For the most part, singlesite catalysts have not eliminated any other catalyst system from commercial production. These catalysts have primarily expanded the polyethylene industry into new markets. 

Table 4.10 summarizes some important dates in the commercialization of single-site catalysts during the 1990s and the use of specific trade names by the manufacturers of these new types of polyethylene in order to distinguish these new polyethylene compositions from previous materials. 

Prior to the discovery of single-site catalysts the polyethylene industry was limited to conventional 1-olefins, primarily 1-butene, 1-hexene and 1 -octene, as comonomers that were used to control polyethylene properties. In this regard, the amoxmt and type of 1-olefin in the finished polymer were the important variables. 

Based on the requirements outlined above, industrial scientists recognized soon after the discovery of single-site catalysts based on zircono-cene/MAO solutions that this type of catalyst needed to be supported on a solid support in order to manufacture polyethylene in commercial reactors based on the slurry process and the gas-phase process. Therefore, suitable supports for the single-site Zr/MAO catalyst systems must meet similar requirements as other catalysts in order to perform satisfactorily as particle-form catalysts. 

Based on this model, microreactors might be fabricated by immobilizing different types of single-site metallocene catalysts — or even catalyst cascades — on suitable supports for in situ production of novel polyolefin blends and other environmentally friendly polyolefin materials. 

As discussed in this chapter, metallocene catalysts have unique advantages over conventional Ziegler-Natta catalysts. This new type of catalyst brought us tailor-made polymers which have not been produced by conventional catalysts. The innovation of single-site catalyst by metallocene catalyst and other single-site catalyst technologies (new organometallic catalysts) from multi-site catalysts was a remarkable event in polyolefin industry, as shown in Fig. 21. 

The characterization methods described in Chapter 2 are limited in what they can tell us about structure in the absence of any information about how a sample was made. Chapter 3 surveys the various types of reaction systems used in polymerization and describes the molecular structures that can be produced by each. Anionic and living free-radical polymerizations are used in the laboratory to prepare samples having ideal structures, while processes used in industry produce materials that more complex in structure. The commercial polymer with the most complex structure is low-density (highly branched) polyethylene. The development of single-site catalysts has led to the commercial production of polymers that, while they do not have the homogeneity of ideal samples, do have structures that are reproducible and simply described. 

The two most commonly used single-site catalysts for ADMET today are (1) Schrock s alkylidene catalysts of the type M(CHR )(NAr )(OR)2 where M = W or Mo, AC = 2, 6-C6H3-/-Pr2, R = CMe2Ph, and R = CMe(CF3)2 (14)7 and (2) Grubbs ruthenium-based catalyst, RuCl2(=CHPh)(PCy3)2 (12) where Cy = cyclohexyl.9 While both catalysts meet the requirements to be successful in ADMET, they are markedly different in their reactivity and in die results each can produce. 

We can employ coordination polymerization to produce stereoregular polystyrene. By performing this type of reaction at low temperatures, using Ziegler-Natta or single-site catalysts, we can prepare isotactic and syndiotactic versions of polystyrene. 

Lanthanide-based catalysts, despite finding a lot of application in homogeneous catalysis, can be rather problematic due to the lability of some ligand types and the versatility of their coordination chemistry in the -1-3 oxidation state this makes the controlled synthesis of single-site Ln complexes a quite ambitious goal [92]. McLain and coworkers first demonstrated the high potential of a homoleptic yttrium complex Y(OCH2CH2NMe2)3 as ROP catalyst for the preparation of PLA from rac-lactide and that it promotes a rapid and controlled polymerization... 

The mixture of metallocene and co-catalyst is soluble. Its active center, which is chiral, induces with a very low rate of defects only one type of monomer linkage ( single site catalysts )- That is why high activities (some 1,000 kg polymer/g... 

Various types of catalysts have been described, including Ziegler-Natta catalysts, mixed catalysts and single-site catalysts. These types are explained in detail subsequently. 

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