Chromium oxidants, polymer

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). 

Processes for HDPE with Broad MWD. Synthesis of HDPE with a relatively high molecular weight and a very broad MWD (broader than that of HDPE prepared with chromium oxide catalysts) can be achieved by two separate approaches. The first is to use mixed catalysts containing two types of active centers with widely different properties (50—55) the second is to employ two or more polymerization reactors in a series. In the second approach, polymerization conditions in each reactor are set drastically differendy in order to produce, within each polymer particle, an essential mixture of macromolecules with vasdy different molecular weights. Special plants, both slurry and gas-phase, can produce such resins (74,91—94). 

The preferred catalyst is one which contains 5% of chromium oxides, mainly Cr03, on a finely divided silica-alumina catalyst (75-90% silica) which has been activated by heating to about 250°C. After reaction the mixture is passed to a gas-liquid separator where the ethylene is flashed off, catalyst is then removed from the liquid product of the separator and the polymer separated from the solvent by either flashing off the solvent or precipitating the polymer by cooling. 

Since the publication by the discoverers (3) of chromium oxide catalysts a considerable number of papers devoted to this subject have appeared. Most of them (20-72) deal either with the study of the chromium species on the catalyst surface or with the problem of which of this species is responsible for polymerization. Fewer results have been published on the study of processes determining the polymer molecular weight (78-77) and kinetics of polymerization (78-99). A few papers describe nascent morphology of the polymer formed (100-103). 

In the propagation centers of chromium oxide catalysts as well as in other catalysts of olefin polymerization the growth of a polymer chain proceeds as olefin insertion into the transition metal-carbon tr-bond. Krauss (70) stated that he succeeded in isolating, in methanol solution from the... 

Table IV presents the results of the determination of polyethylene radioactivity after the decomposition of the active bonds in one-component catalysts by methanol, labeled in different positions. In the case of TiCU (169) and the catalyst Cr -CjHsU/SiCU (8, 140) in the initial state the insertion of tritium of the alcohol hydroxyl group into the polymer corresponds to the expected polarization of the metal-carbon bond determined by the difference in electronegativity of these elements. The decomposition of active bonds in this case seems to follow the scheme (25) (see Section V). But in the case of the chromium oxide catalyst and the catalyst obtained by hydrogen reduction of the supported chromium ir-allyl complexes (ir-allyl ligands being removed from the active center) (140) C14 of the...

Witt, D. R., Reactivity and Mechanism with Chromium Oxide Polymerization Catalysts, Chap. 13 in Reactivity, Mechanism, and Structure in Polymer Chemistry, A. D. Jenkins and A. Ledwith, eds., Wiley, New York, 1974. 

Unlike Ziegler catalysts, chromium oxide based catalysts are extremely sensitive to minor changes in the preparation or calcining history. The active sites no doubt respond to the local electronic environment, which determines the molecular weight distribution of the polymer. Therefore, replacing the... 

In all the low pressure PE processes the polymer is formed through coordination polymerisation. Three basic catalyst types are used chromium oxide, Ziegler-Natta and single-site catalysts. The catalyst type together with the process defines the basic structure and properties of the polyethylene produced. Apart from the MWD and comonomer distribution that a certain catalyst produces in polymerisation in one reactor, two or more cascaded reactors with different polymerisation conditions increase the freedom to tailor... 

We begin with the structure of a noble metal catalyst, where the emphasis is placed on the preparation of rhodium on aluminum oxide and the nature of the metal support interaction. Next, we focus on a promoted surface in a review of potassium on noble metals. This section illustrates how single crystal techniques have been applied to investigate to what extent promoters perturb the surface of a catalyst. The third study deals with the sulfidic cobalt-molybdenum catalysts used in hydrotreating reactions. Here, we are concerned with the composition and structure of the catalytically active surface, and how it evolves as a result of the preparation. In the final study we discuss the structure of chromium oxide catalysts in the polymerization of ethylene, along with the polymer product that builds up on the surface of the catalyst. 

Copolymers may also be produced with a catalyst containing both chromium oxide and nickel oxide supported on silica-alumina. It is well-known that nickel oxide—silica—alumina by itself makes predominantly butenes from ethylene. In the mixed catalyst, butenes that are formed on nickel oxide copolymerize with ethylene on the chromium oxide to form ethylene-butene copolymers. The fact that infrared shows only ethyl branching in the polymer indicates that the initial product... 

An increasing number of colored inorganic pigments are FDA-compliant. Historically, yellow iron oxide, red iron oxide, black iron oxide, zinc ferrite, burnt umber, raw and burnt sienna, channel carbon black, chromium oxide green, ultramarine blue, cobalt blue and copper chrome black have enjoyed FDA—compliant status, under 21 CFR 178.3297, Colorants for Polymers . More recently, the FDA has been successfully petitioned with regard to nickel titanium yellow, chrome titanium yellow, and cobalt green under 21 CFR 170.39, Threshold of Regulation for Substances Used in Food-Contact Articles . 

Supported chromium oxidants fall in to three main categories (i) adsorbed on alumina, silica or celite (Section 2.7.5.1) (ii) adsorbed on a polymer or resin (Section 2.7.5.2) and (iii) adsorbed on carbon (Section 2.7.S.3). 

Direct reaction of soluble polymers that contain phenyl groups, for example, polystyrene, with chromium atoms leads to polymers crosslinked with bis(benzene)chromium. Oxidation leads to aggregates of Cr203 imbedded with the polymer. 

A typical coating contains iron oxides (sometimes doped with cobalt or chromium), a polymer system that acts as a binder, various additives (including up to 1.5% lecithin on a dry solids basis), and solvents such as methyl ethyl ketone and cyclohexanone (454 58). This material is milled into a dispersion and deposited onto the tape and dried. 

If a copolymer such as VLDPE or LLDPE is the target resin, satisfactory comonomer incorporation must be achieved. This is manifested by the amount of comonomer incorporated (evidenced by density) and the distribution of comonomer in the polymer (evidenced by composition distribution). In general, supported chromium oxide catalysts incorporate comonomer more easily than Ziegler-Natta catalysts. 

Butadiene, a substance used industrially to make polymers, is prepared by thermal cracking of butane over a chromium oxide/aluminum oxide catalyst, but this procedure is of little use in the laboratory. 

The difference in polymerization mechanism between one-component metal oxide catalysts and traditional Ziegler-Natta two-component catalysts seems to exist only in the initiation stage, while the mechanism of continued propagation of polymer chain has many common features for all the catalyst systems based on transition metal compounds. Thus most studies of the chromium oxide catalyst system, for example, deal either with the nature... 

Commercial linear polyethylene, the most commonly used type of plastic, was bom more than half a century ago with the accidental discovery at Phillips Petroleum Company that chromium oxide supported on silica can polymerize a-olefins.1 The same catalyst system, modified and evolved, is used even today by dozens of companies throughout the world, and it accounts for a large share of the world s high-density polyethylene (HDPE) supply, as well as some low-density polymers. The catalyst is now more active and has been tailored in numerous ways for many specialized modem applications. This chapter provides a review of our understanding of the complex chemistry associated with this catalyst system, and it also provides examples of how the chemistry has been exploited commercially. It is written from an industrial perspective, drawing especially on the commercial experience and the research of numerous scientists working at Phillips Petroleum... 

Although there are many differences between chromium oxide catalysts and the organochromium catalysts, when they are bonded to the support, organochromium catalysts usually display a similar, but exaggerated, MW response in the polymer produced relative to what is observed with chromium oxide catalysts. For example, the MW of polymer produced with each type of catalyst usually decreased as the support calcination temperature was raised. Similarly, when both chromium oxide and the organochromium compounds were deposited onto aluminophosphate supports, they always yielded lower-MW polymer as the amount of phosphate in the support was raised. 

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