Polymerisation with Phillips Catalysts

The typical Phillips catalyst comprises chemically anchored chromium species on a silica support. The formation of a surface silyl chromate, and eventually silyl dichromate [scheme (29)], is significant during the catalyst preparation, because at the calcination temperature chromium trioxide would decompose to lower-valent oxides. Chromium trioxide probably binds to the silica as the chromate initially, at least for the ordinary 1% loading. However, some rearrangement to the dichromate at high temperature may occur. It is incorrect to regard only one particular valence state of chromium as the only one capable of catalysing ethylene polymerisation. On the commercial CrOs/silica catalyst the predominant active species after reduction by ethylene or carbon monoxide [scheme (59)] is probably Cr(II), but other species, particularly Cr(III), may also polymerise ethylene under certain conditions  

The catalyst reduced by carbon monoxide or ethylene becomes air and moisture sensitive it undergoes an easy oxidation to Cr(VI) species when exposed to air. 

The reduction brought about by treatment with carbon monoxide leads to the catalyst being capable of polymerising ethylene immediately without an induction period ethylene polymerisation in the presence of a catalyst prepared by reduction with ethylene is characterised by an induction period, which is connected with the reduction [28,37,43,224,240]. 

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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],... 

High density polyethylene (HDPE) is defined by ASTM D1248-84 as a product of ethylene polymerisation with a density of 0.940 g/cm or higher. This range includes both homopolymers of ethylene and its copolymers with small amounts of a-olefins. The first commercial processes for HDPE manufacture were developed in the early 1950s and utilised a variety of transition-metal polymerisation catalysts based on molybdenum (1), chromium (2,3), and titanium (4). Commercial production of HDPE was started in 1956 in the United States by Phillips Petroleum Company and in Europe by Hoechst (5). HDPE is one of the largest volume commodity plastics produced in the world, with a worldwide capacity in 1994 of over 14 x 10 t/yr and a 32% share of the total polyethylene production. 

With the exception of LDPE, polyolefins like other polyethylenes and polypropylene, which represent the largest amount of vinyl-type polymers produced in the world, are neither synthesized by radical nor by classical ionic polymerisation processes. Different types of polymerisation catalysts are in use for these purposes. The Cr-based Phillips catalyst, Ziegler-Natta type catalysts, metallocene or other more recently discovered catalysts, including late transition metal catalysts, are all characterized by their propagation step where the olefin monomer inserts into a carbon-transition metal link. ... 

Supported CrC>3 catalysts, referred to as Phillips catalysts, are important industrial catalysts and are employed in high-density polyethylene production. Phillips catalysts polymerise ethylene with an induction period, which has been ascribed to the slow reduction of Cr(VI) by the monomer and to the displacement of oxidation products (mainly formaldehyde) from the catalytic species [226]. The prereduction of the catalyst with the use of H2 or CO enables the induction period to be eliminated. Active sites thus formed involve surface low-valence Cr(II) and Cr(III) centres, which can appear as mononuclear (formed from chromate species) and binuclear (formed from dichromate species) [227-232],... 

The selection and treatment of the support is fundamental to the process, and a plant may use catalysts made from a variety of supports to produce a whole range of products. Catalyst productivities are of the order of 5 kg of polyethylene per gram of catalyst or higher, with a corresponding chromium content of 2 ppm or less. The percentage of Cr atoms that form active polymerisation centres has been estimated as 12% [43]. Typically, commercial Phillips catalysts contain ca 1 % total Cr and have particle sizes of 30-150 pm [224]. 

Phillips catalysts are active in the polymerisation of propylene and higher a-olefins, yielding tacky polymers with irregular structure and small amounts of crystalline polymers in the case of polypropylene, a small amount of crystalline fraction appeared to constitute the isotactic polymer [236]. 

Somewhat similar to the Phillips catalyst is the Standard Oil of Indiana catalyst, which was the first olefin polymerisation coordination catalyst to be discovered [239], It typically consists of M0O3 supported on alumina or silica and calcined in air at high temperature. Unlike the Phillips catalyst, it is necessary to reduce the catalyst precursor thus obtained with hydrogen at elevated temperature before using it for polymerisation. Despite extensive studies, Standard Oil of Indiana catalysts have not been widely commercialised [43],... 

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-... 

Continuing with this study, Delley et investigated CO-reduced Phillips catalyst to study the initial bond formation that occurs during the initiation step of ethene polymerisation. Published literature has included much debate about the initial bond formation, stating that it could result, for example, via C-H or bond activation of ethene or silanol, re-... 

By reaction with the Fischer type carbyne complexes loose the surface chromium(II) atoms of the reduced Phillips their polymerisations activity for 1-alkenes (16). The surface chromium(II) atoms enhance the metathesis activity of the bimetallic catalysts (Table 1). In contrast to the original Fischer type carbyne complexes are the bimetallic catalysts stable at room temperature. They can be stored for a year at 25°C without changing their metathesis activity (14). 

Fig. 2.2. Simplified flow chart of the Phillips process (solution phase polymerisation) (Elias 1992). The solvent (for example isobutane), ethylene, co-monomers and the catalyst are fed into the reactor. The polymer solution is de sed (flashed) from the reactor through a gas separator after a certain reaction time. In a cleaning nnit (centrifuge, washing device, drier) the polymer is separated out and ethylene and solvent residues are processed. The polymer emerges in the drier in the form of snow white flakes. Flakes, carbon black, stabiliser and further additives, for example Ca-stearate, are mixed in a mixer and this mix is fed into an ex-tmder at an adequate mix raho with the polymer flakes. Here the mix is melted, homogenised and finally granulated. The black pellets are then transported to the storage facihties...

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). 

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