Generations of Phillips catalysts

Since discovery in the early 1950s, a variety of modifications have been made to improve the Phillips catalyst. Beaulieu has identified four generations of Phillips catalysts (9), summarized in Table 5.1. These modifications are primarily intended to affect molecular weight and molecular weight distribution of the polymer, rather than catalyst activity. 

Third generation Phillips catalysts use triethylborane (TEB) as an adjuvant. In these systems, triethylborane is sometimes called a cocatalyst. However, it 

P 1955 Chromium on silica after induction period produces high MW polymer with relatively broad MWD. 

2nd 1975 TIPT used to modify support surface results in lower polymer MW. 

3rd 1983 TEB used as cocatalyst broadens MWD of polymer by increasing low MW fraction 

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Table 5.1 Principal features of generations of Phillips catalysts (B. Beaulieu, M. McDaniel and P. DesLauriers, Society of Plastics Engineers, Houston, TX, February 27,2005).

Figure 3.59 Life cycles of catalysts for olefin coordination polymerisation (a) early-generation Ziegler-Natta catalysts for ethylene and propylene polymerisation (b) Phillips catalysts for ethylene polymerisation (c) fourth-generation Ziegler-Natta catalysts for ethylene polymerisation (d) fourth-generation Ziegler-Natta catalysts for propylene polymerisation (e) metallocene-based catalysts for olefin polymerisation leading to polymers of various stereoregularity...

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. 

Second generation Phillips catalysts involve use of titanium compounds that modify the surface chemistry of the support and enables production of polyethylene with higher MI (lower MW) (12). Titanium tetraisopropoxide, also known as tetraisopropyl titanate (TIPT), is the most commonly used modifier for these catalysts. Hexavalent chromium titanate species are probably formed on the surface as shown in Figure 5.3 (13). Catalyst surfaces contain a diversity of active sites and molecular weight distribution of the polymer is broader than that from generation catalysts. 

Figure 5.3 Structure of hexavalent chromium titanate species formed in 2" generation Phillips catalyst. Catalyst produces polyethylene with higher MI (lower molecular weight) and broader MWD than chromium on silica alone.

Figure 5.4 Structures proposed for 4" generation Phillips catalyst. Because of diversity of active centers, catalyst produces polyethylene with broadest MWD (M /M > 50) relative to any other single catalyst in commercial use (7).

The initial step in the mechanism of ethylene polymerization using Phillips catalysts is believed to occur by way of an oxidation-reduction reaction between Cr (VI) and ethylene as depicted in eq 5.1. This generates Cr (II) and vacant coordination sites. As mentioned above, polymerization may be initially slow because of sluggish reduction or desorption of the oxidation by-products which can coordinate with (and block) active centers. 

The Phillips catalyst is not alkylated when it goes into the reactor, and metal alkyl cocatalysts are not normally used. Thus, in contrast to Ziegler, Ballard, or metallocene catalysts, the Phillips catalyst has no Cr-alkyl bond into which ethylene may be inserted. Instead, the chromium somehow reacts with ethylene to generate such a bond. This characteristic is not unique, as many catalyst types also display this ability.8 This issue has been the source of much interest and speculation for half a century. On some catalysts, CO reduction is known to cleanly produce Cr(II). Reaction with ethylene could involve a formal oxidation [52,94,141,250-252,269,322-325,339-345] and many pathways involving Cr(IV) have been proposed, sometimes based on organochromium analogs, such as shown in Scheme 8 [94,250-252,315-319,321-325,342,346-349]. 

In contrast, the typical polydispersity of polymers produced with Phillips catalysts varies from 6 to 20, and specialized catalyst treatments can provide polymers of PDI as low as 4.0 or as high as 100. Thus, 2 to 12 unique site types are required to reproduce the MW distribution from Phillips catalysts, because the catalyst contains a heterogeneous population of sites, differing widely in propagation and termination rate constants. Each site type generates polymer with its own characteristic MW, and consequently the polymer MW breadth reflects the heterogeneity of the site population. Differences in site reactivity no doubt derive from the... 

Like metallocenes, Phillips catalysts are usually thought to generate LCB through macromer incorporation [531,532]. The liberated vinyl end-group of one terminated chain is thought to become copolymerized into another... 

There were three kinds of Cr(II) sites generated after the reduction of hexavalent chromate species by ethylene monomers. 4g represented the naked cluster model for the Cr(II) site of the Phillips catalyst, providing more room for ethylene coordination to the Cr center. The calculations showed that the initiation reactions between the Cr(ll)0 c surf species and ethylene molecules may occur after the desorption of one or two formaldehyde molecules (on 4g-l or 4g-2). For 4g-2, two formaldehyde molecules were adsorbed on the Cr(ll) center from the opposite side above the chromasiloxane ring, with formation of two Cr-O bonds of 2.131 A. 

The FT-IR analysis showed that SiH4-modified catalysts generated PE at a rate about seven times faster than that of the standard Phillips catalyst. Both catalysts were exposed to ethene (at low pressure) for eight minutes at room temperature, during which time operando FT-IR was collected. It was... 

These catalysts were developed from the generation. At low temperatures (below 100 °C) the active violet y or 5 form of the brown P-TiCls is formed. Through the smaller size of the primary crystallites, the surface area and activity of the catalyst was increased. The and generation catalysts (unsupported catalysts) were used in suspension processes with hexane as a solvent, in mass polymerisation processes (Rexene, Phillips), in the BASF gas phase process (vertical agitation) and in the solution process (Eastman). 

Soon after the discovery of the first generation Ziegler catalysts and the chromium-based Phillips catalysts in the mid-1950s, the need developed... 

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