Cocatalysts for Single Site Catalysts

As noted above, conventional aluminum alkyls are not effective cocatalysts for single site catalysts, probably because they are incapable of abstracting a ligand to generate the cationic active center (see section 6.4 on mechanism). Two main 

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Organoboron compounds constitute a broad and rich area of organometallic chemistry and a detailed discussion is inappropriate for an introductory text on polyethylene. However, several organoboron compounds are crucial for selected polyethylene catalyst technologies. For example, arylboranes are used as cocatalysts for single site catalyst systems and will be discussed in Chapter 6 (see section 6.3.2). The purpose of this section is to introduce the trialkylborane that is a component of 3 generation Phillips catalyst systems (Chapter 5) employed in industrial processes in for linear polyethylene. 

However, these alternative aluminoxanes perform poorly as cocatalysts for single site catalysts. Preparation and properties of aluminoxanes have been extensively reviewed (20-22). 

Modified methylaluminoxanes exhibit much improved storage stability and several are highly soluble in aliphatic hydrocarbons. (Manufacturers of polyethylene prefer to avoid toluene because of toxicity concerns, especially if resins are destined for food contact.) Most importantly, because yields are higher, modified methylaluminoxane formulations are less costly than MAO. However, since modified methylaluminoxanes contain other types of alkylaluminoxanes, they do not match the performance of conventional methylaluminoxane in some single site catalyst systems. Consequently, modified methylaluminoxanes should be considered niche cocatalysts for single site catalysts. 

Tris(pentafluorophenyl)borane, known as "FAB" (structure below), is the most common arylborane used as cocatalyst for single site catalysts. FAB is a strongly Lewis acidic, air-sensitive solid (T 126-131 °C) that is only slightly soluble in hydrocarbon solvents. The structure of FAB is given below. 

Uses Cocatalyst for single-site catalyst systems for polymer prod. 

Toxicology Toxic by inh., ing., skin contact target organs liver, kidneys, bladder Precaution Pyrophoric spontaneously flamm. in air reacts violently with water Storage Handle and store under nitrogen Uses Cocatalyst for single-site catalyst systems for polymer prod. 

Keys to the high polymerization activities of single-site catalysts are the cocatalysts. MAO is most commonly used and is synthesized by controlled hydrolysis of trimethyl aluminum. Other bulky anionic complexes which show a weak coordination, such as borates, also play an increasingly important role. One function of the cocatalysts is to form a cationic metallocene and an anionic cocatalyst species. Another function of MAO is the alkylation of halogenated metallocene complexes. In the first step, the monomethyl compound is formed within seconds, even at -60°C (69). Excess MAO leads to the dialkylated species, as shown by NMR measurements. For the active site to form, it is necessary that at least one alkyl group be bonded to the metallocene (70). 

After activation, the catalyst is intrcxiuced into the polymerization reactor as slurry in a saturated hydrocarbon such as isobutane. The precise mechanism of initiation is not known, but is believed to involve oxidation-reduction reactions between ethylene and chromium, resulting in formation of chromium (II) which is the precursor for the active center. Polymerization is initially slow, possibly because oxidation products coordinate with (and block) active centers. Consequently, standard Phillips catalysts typically exhibit an induction period. The typical kinetic profile for a Phillips catalyst is shown in curve C of Figure 3.1. If the catalyst is pre-reduced by carbon monoxide, the induction period is not observed. Unlike Ziegler-Natta and most single site catalysts, no cocatalyst is required for standard Phillips catalysts. Molecular weight distribution of the polymer is broad because of the variety of active centers. 

As isolated from toluene solution, neat MAO is an amorphous, friable white solid containing 43-44% Al (theory 46.5%). Like most commercially available aluminum alkyls, it is pyrophoric and explosively reactive with water. Freshly prepared MAO solutions form gels within a few days when stored at ambient temperatures (>20 °C). However, lower storage temperatures (0-5 °C) delay gel formation. Consequently, manufacturers store and transport MAO solutions in refrigerated containers. Commercially available MAO contains residual TMAL (15-30%), called "free TMAL" or "active aluminum." The literature is contradictory on the influence of free TMAL on activity of single site catalysts both reductions and increases have been reported (18-20). Perhaps the most important drawback of methylaluminoxane is its cost, which is substantially higher than conventional aluminum alkyls. Despite these untoward aspects, methylaluminoxane remains the most widely used cocatalyst for industrial single site catalysts. 

The Borstar process employs a small loop prepolymerization reactor (see section 3.6 for a discussion of the advantages of prepolymerization). Ziegler-Natta catalysts and triethylaluminum cocatalyst are commonly used but the process is capable of using single site catalysts (15). 

Single site catalysts, such as metallocene compounds, CGCs, and nickel or palladium diimine complexes, used in combination with MAO or borate cocatalysts, are highly active for the homopolymerization of norbornene and its copolymerization with ethylene. The structure of the norbornene homo- and copolymers can be widely influenced by the symmetry and structure of the ligands on the transition metal complexes. 

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