Magnesium Alkyls in Ziegler-Natta Catalysts

In the late 1960s, it was discovered that inorganic magnesium compounds, espe-cially MgCl2(see section 3.5) are excellent supports for Ziegler-Natta polyolefin

METAL ALKYLS IN POLYETHYLENE CATALYST SYSTEMS 51

catalysts. No doubt sparked by this discovery, chemists in the polyolefins industry began exploring use of magnesium alkyls in catalyst synthesis. The most well-known magnesium alkyls are alkylmagnesium halides. These reagents were discovered by Victor Grignard in 1905, and have become known as Grignard reagents, written "RMgX," where R is usually a simple alkyl and X a chloride ligand. The closely related dialkylmagnesium compounds (R²Mg) received little attention relative to Grignard reagents which are ubiquitous in organic synthesis. In large part, this was due to the excellence of Grignard reagents for formation of new C-C bonds.

Though Grignard reagents were discovered more than a century ago, they remain key synthetic tools in the repertoire of the modern chemist. Indeed, Morrison and Boyd in their classic text describe the Grignard reagent as "one of the most useful and versatile reagents known to the organic chemist" (18). However, for many of the polyethylene catalyst preparations using magnesium alkyls emerging in the 1970s, Grignard reagents were not the material of choice. This was due pri-marily to the catalyst deactivating influence of the Lewis basic solvents used in their preparation, usually diethyl ether or tetrahydrofuran. Catalyst development chemists needed hydrocarbon-soluble magnesium alkyls. Unfortunately, there were few practical options available at the time, since the hydrocarbon solubility was poor for most Grignard reagents and RjMg known in the art of the 1970s.

Using a variety of approaches (3), hydrocarbon-soluble R2Mg were discov-ered, including the so-called "unsymmetrical" dialkylmagnesium compounds (RMgR', where R and R' are C2 to n-Cs alkyl groups). In fact, RMgR' compounds are today the most important of the commercially available dialkylmagnesiums used in manufacture of polyethylene:

• iî-butylethylmagnesium (BEM),

• n-butyl-tt-octylmagnesium (BOM or BOMAG®), and

• "dibutylmagnesium" (DBM)

DBM contains both n-butyl and sec-butyl groups (ratio -1.5) and may be regarded as a "mixture" of di-n-butylmagnesium (DNBM) and di-sec-butylmagnesium (see section 4.4). Likewise, BEM and BOM may be viewed as "mixtures" of DNBM with diethylmagnesium and di-«-octylmagnesium, respectively. However, because dynamic alkyl ligand exchange occurs, these compositions are not true mixtures. Neat (solvent-free) RMgR' are either viscous liquids or amorphous white solids and are commercially available only in hydrocarbon solution (usually hep-tane). RMgR' compounds have been commercially available since the mid-1970s.

Though BEM was discovered in 1978 (19), it remains today the most important dialkylmagnesium compound for production of polyethylene catalysts.

In a 1985 review article, it was declared that dialkylmagnesium compounds are of "limited practical importance" (2). Though this statement may have been accu-rate at that time, it is certainly not true today. In the early 21^st century, the value of polyethylene produced from R2Mg-derived Ziegler-Natta catalysts contributes substantially to the "practical importance" of these materials. Globally, millions of tons of polyethylene are manufactured each year using such catalysts. Dial-kylmagnesium compounds are used in Ziegler-Natta catalyst synthesis in two fundamentally different ways discussed in sections 4.3.2 and 4.3.3 below.

BEM, BOM and DBM are offered in slightly different formulations by several producers in the US and Europe. These companies supply R2Mg formulations to polyolefin producers worldwide. However, no manufacturer offers all prod-ucts. Major suppliers are:

• Akzo Nobel (formerly Texas Alkyls, Inc.)

• Albemarle (formerly Ethyl Corp.)

• Chemtura (formerly Crompton, Witco and Schering)

• FMC (formerly Lithium Corp. of America) 4.3.1 R2Mg for Production of Supports

Reaction of dialkylmagnesium compounds with selected chlorinated compounds produces finely divided MgCl2 that can be used as a support for polyethylene catalysts. Other reagents may be used to produce different inorganic magnesium compounds, also suitable as supports. Examples are shown in Figure 4.1. Treat-ment of these products with transition metal compounds results in a supported

"precatalyst." Typically, the transition metal is subsequently reduced by reaction with an aluminum alkyl and the solid catalyst isolated. The solid catalyst and cocatalyst (usually TEAL) may then be introduced to the polymerization reactor.

As discussed in section on p. 37, fine particles are usually not desirable for Ziegler-Natta catalysts. However, most polyethylene catalysts produced by the method described in this section are used in solution processes where psd and morphol-ogy are less important than in gas phase or slurry processes (see Chapter 7).

4.3.2 R2Mg as Reducing Agent

Reaction of R2Mg with a transition metal compound produces a reduced tran-sition metal compotran-sition co-precipitated with an inorganic magnesium com-pound. In this respect, dialkylmagnesium compounds are functioning in much the same way as aluminum alkyls described in section 4.2.2. As before, addi-tional aluminum alkyl cocatalyst must be introduced in the polymerization reactor to alkylate the transition metal and create active centers.

METAL ALKYLS IN POLYETHYLENE CATALYST SYSTEMS 53

SiOMgR2 0-2 2

Mg (OR')230

Mg(OSiR'3)2 2 8'2 9·*

MgCI223

MgCl·2 7 MgCI225

MgCI²²⁶

Figure 4.1 Reactions of dialkylmagnesium compounds to produce inorganic magnesium compounds useful as supports for Ziegler-Natta catalysts.

An additional point should be made before concluding discussion of the role of dialkylmagnesium compounds in Ziegler-Natta catalyst systems. There are occa-sional citations in the literature suggesting that dialkylmagnesium compounds are used as "cocatalysts." These references are probably using "cocatalysts" in the broadest context. That is, they are merely suggesting that dialkylmagnesium compounds are amongst components employed in the overall catalyst synthesis.

As reducing agents and agents for production of supports, dialkylmagnesium compounds are highly effective. But as cocatalysts are defined in section 4.2.3, dialkylmagnesium compounds are very poor and may even completely deac-tivate some Ziegler-Natta catalysts. This may occur because of deactivation of active centers caused by strong coordination of the magnesium alkyl or perhaps overreduction of the transition metal.