Polymerization catalysts Phillips

Keywords Chromium Ethylene polymerization Phillips catalyst... 

Different from classical coordination polymerization, Phillips catalysts do not require activation with a cocatalyst however, alkylaluminum complexes are usually used as scavengers in the polymerization medium [33]. 

Second, in the early 1950s, Hogan and Bank at Phillips Petroleum Company, discovered (3,4) that ethylene could be catalyticaHy polymerized into a sohd plastic under more moderate conditions at a pressure of 3—4 MPa (435—580 psi) and temperature of 70—100°C, with a catalyst containing chromium oxide supported on siUca (Phillips catalysts). PE resins prepared with these catalysts are linear, highly crystalline polymers of a much higher density of 0.960—0.970 g/cnr (as opposed to 0.920—0.930 g/cnf for LDPE). These resins, or HDPE, are currentiy produced on a large scale, (see Olefin polymers, HIGH DENSITY POLYETHYLENE). 

Molecular Weight. PE mol wt (melt index) is usually controlled by reaction temperature or chain-transfer agents. Reaction temperature is the principal control method in polymerization processes with Phillips catalysts. On the other hand, special chemical agents for chain transfer are requited for... 

Molecular Weight Distribution. In industry, the MWD of PE resins is often represented by the value of the melt flow ratio (MER) as defined in Table 2. The MER value of PE is primarilly a function of catalyst type. Phillips catalysts produce PE resins with a broad MWD and their MER usually exceeds 100 Ziegler catalysts provide resins with a MWD of a medium width (MFR = 25-50) and metallocene catalysts produce PE resins with a narrow MWD (MFR = 15-25). IfPE resins with especially broad molecular weight distributions are needed, they can be produced either by using special mixed catalysts or in a series of coimected polymerization reactors operating under different reaction conditions. 

Most chromium-based catalysts are activated in the beginning of a polymerization reaction through exposure to ethylene at high temperature. The activation step can be accelerated with carbon monoxide. Phillips catalysts operate at 85—110°C (38,40), and exhibit very high activity, from 3 to 10 kg HDPE per g of catalyst (300—1000 kg HDPE/g Cr). Molecular weights and MWDs of the resins are controlled primarily by two factors, the reaction temperature and the composition and preparation procedure of the catalyst (38,39). Phillips catalysts produce HDPE with a MJM ratio of about 6—12 and MFR values of 90—120. 

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. 

The Phillips Cr/silica catalyst is prepared by impregnating a chromium compound (commonly chromic acid) onto a support material, most commonly a wide-pore silica, and then calcining in oxygen at 923 K. In the industrial process, the formation of the propagation centers takes place by reductive interaction of Cr(VI) with the monomer (ethylene) at about 423 K [4]. This feature makes the Phillips catalyst unique among all the olefin polymerization catalysts, but also the most controversial one [17]. 

The initiation of polymerizations by metal-containing catalysts broadens the synthetic possibilities significantly. In many cases it is the only useful method to polymerize certain kinds of monomers or to polymerize them in a stereospecific way. Examples for metal-containing catalysts are chromium oxide-containing catalysts (Phillips-Catalysts) for ethylene polymerization, metal organic coordination catalysts (Ziegler-Natta catalysts) for the polymerization of ethylene, a-olefins and dienes (see Sect. 3.3.1), palladium catalysts and the metallocene catalysts (see Sect. 3.3.2) that initiate not only the polymerization of (cyclo)olefins and dienes but also of some polar monomers. 

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... 

Since the significance of this transformation increases with increasing temperature, it provides an easy way of controlling molecular weight in polymerization with the Phillips catalyst. 

The Phillips catalyst contains hexavalent chromium after calcining, but the early discoverers quickly realized that reduction takes place in the reactor on contact with ethylene, leaving chromium in a lower oxidation state as the active species. A worldwide debate has continued to this day about the valence of this reduced species. Chromium in every valence state from Cr(II) to Cr(VI) has been proposed as the active site, either alone or in combination with another valence. The question has received far more attention than it probably deserves, undoubtedly at the expense of more fundamental issues, like the polymerization mechanism. 

The traditional catalysts for olefin polymerization were invented by Hogan et al. (their catalyst is the Phillips catalyst) (13-15) in 1952 and Ziegler et al. (16, 17) and Natta and Corradini (18, 19) in 1953. Commercialization of olefin polymerization with these catalysts provided the first linear polyethylene and the first isotactic polypropylene. Before these innovations,... 

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. 

As already mentioned, until about the beginning of 1990 only heterogeneous catalysts have been used for the polymerization of ethylene and propylene. For ethylene polymerization the catalysts used are essentially of three types. These are the Phillips catalyst, the Union Carbide catalyst, and the Ziegler catalyst. 

From this brief discussion it is clear that, even if the CO-reduced catalyst is simpler than the industrial catalyst, the high degree of heterogeneity of the Cr(II) species present on the surface of the amorphous silica makes the comprehension of the polymerization mechanism on the Phillips catalyst a complex and difficult task that still requires work (5). The heterogeneity of the Cr(II) structure is reflected in a... 

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