Phillips Catalyst Systems

The Phillips catalyst promotes ethylene polymerization only after an induction period. Obviously, the first step in the activation of a freshly prepared Phillips catalyst is ethylene coordination to chromium. After activation, Cr-H is assumed to act as active catalyst species. However, the presence and relevance of Cr-metallacycle species in the active polymerization systems cannot be ruled out. Productivities of the Phillips catalyst are in the range of 5 kg PE per g of catalyst, resulting in a Cr-content of about 2 ppm in the polymer. 

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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. 

Homogeneous model compounds containing chromixun have been prepared. In particular alkylchromium complexes ligated by Cp and NacNac ligands have been shown by Theopold to polymerize ethylene (Equation 22.10). - Moroever, the NacNac compoimd reproduces the copoljmerization of ethylene and a-olefins achieved with the Phillips catalyst. These compoimds would then be models for an active, supported Cr(III) alkyl species in the Phillips catalyst system. 

The discussion of the Phillips catalyst system outlined above can only provide a very limited summary of one of the most important industrial... 

Chromium Oxide-Based Catalysts. Chromium oxide-based catalysts were originally developed by Phillips Petroleum Company for the manufacture of HDPE resins subsequendy, they have been modified for ethylene—a-olefin copolymerisation reactions (10). These catalysts use a mixed sihca—titania support containing from 2 to 20 wt % of Ti. After the deposition of chromium species onto the support, the catalyst is first oxidised by an oxygen—air mixture and then reduced at increased temperatures with carbon monoxide. The catalyst systems used for ethylene copolymerisation consist of sohd catalysts and co-catalysts, ie, triaLkylboron or trialkyl aluminum compounds. Ethylene—a-olefin copolymers produced with these catalysts have very broad molecular weight distributions, characterised by M.Jin the 12—35 and MER in the 80—200 range. 

The Phillips catalyst has attracted a great deal of academic and industrial research over the last 50 years. Despite continuous efforts, however, the structure of active sites on the Phillips-type polymerization systems remains controversial and the same questions have been asked since their discovery. In the 1950s, Hogan and Banks [2] claimed that the Phillips catalyst is one of the most studied and yet controversial systems . In 1985 McDaniel, in a review entitled Chromium catalysts for ethylene polymerization [4], stated we seem to be debating the same questions posed over 30 years ago, being no nearer to a common view . Nowadays, it is interesting to underline that, despite the efforts of two decades of continuous research, no unifying picture has yet been achieved. 

In the Phillips neohexene process147 2,4,4-trimethyl-2-pentene (8) is converted by cleavage with ethylene to neohexene (9) used in the production of a perfume musk. The starting material is commercial diisobutylene. Since it is a mixture of positional isomers (2,4,4-trimethyl-2-pentene and 2,4,4-trimethyl-l-pentene) and the latter (7) participates in degenerative metathesis, effective utilization of the process requires the isomerization of 7 into 8. A bifunctional catalyst system consisting of an isomerization catalyst (MgO) and a heterogeneous metathesis catalyst is employed 131... 

The strongest evidence in favor of propagation at the transition metal-alkyl bond is the existence of one-component, that is, metal-alkyl-free polymerization catalysts. Of these systems the Phillips catalyst was studied most thoroughly because of its commercial importance. Originally it was believed that Cr(VI) ions stabilized in the form of surface chromate and perhaps dichromate resulting from the interaction of Cr03 with surface hydroxyl groups above 400°C are the active species in polymerization 286,294... 

Supported organochromium complexes exhibit catalytic behavior very similar to that of the traditional Phillips catalyst, and both systems are very sensitive to minute experimental variables.322 Modification of the catalyst allows for control of... 

It may be interesting that chain termination with hydrogen, which is utilised in Ziegler-Natta polymerisations, does not operate in polymerisation systems with Phillips catalysts no influence of hydrogen to control the molecular weight of polyethylene in the latter case was achieved [37],... 

In common with the polymerisation of acyclic olefins (oc-olefins) by Ziegler Natta catalysts, the ring-opening metathesis polymerisation of monocyclic and bicyclic olefins is promoted by alkylmetal-activated transition metal halides, and only a relatively small proportion of the transition metal atoms introduced into the system is converted into the active sites for the polymerisation. Also, as in the polymerisation of ethylene by Phillips catalysts, the metathesis polymer-... 

The use of Nd-hydride-based catalyst systems is described in a patent filed by Phillips Petroleum Co. in 1987 [351,352], The hydride-containing catalyst system NdH3/AlBr3/Et2AlH yields BR with a high cis- 1,4-content of 97%. 

HDPE is produced mainly by a suspension (slurry) process in various types of reactors and with various polymerization procedures. In these processes, a supported Ziegler-Natta catalyst system or a Phillips catalyst in a solvent is used. Because the temperature (80-100°C) is lower than the melting point of the polyethylene (140°C), the polymer produced is separated as a solid. This process is highly versatile and can be used to produce many kinds of polyethylenes. 

High-Density Polyethylene (HDPE). Polymerization of ethylene to polyethylenes is most often carried out at low temperature and pressure, using either the Ziegler aluminum triethyl plus titanium tetrachloride catalyst system, the Phillips chromic oxide plus silica plus alumina system, or more recently the newer metallocene single-site catalyst systems. 

While molybdenum and tungsten catalysts are associated with metathesis polymerization (see Organic Synthesis Using Metal-mediated Metathesis Reactions), chromium catalysts are widely used in the production of polymers. The best-known system is the so-called Phillips catalyst, Cp2Cr (see... 

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