Low pressure polymerization

Polyethylene. Low pressure polymerization of ethylene produced in the Phillips process utilizes a catalyst comprised of 0.5—1.0 wt % chromium (VI) on siUca or siUca-alumina with pore diameter in the range 5—20 nanometers. In a typical catalyst preparation, the support in powder form is impregnated with an aqueous solution of a chromium salt and dried, after which it is heated at 500—600°C in fluid-bed-type operation driven with dry air. The activated catalyst is moisture sensitive and usually is stored under dry nitrogen (85). 

Low pressure polymerization via ionic catalysts, using Ziegler catalysts (aluminum alkyls and titanium haUdes). 

Low pressure polymerization with Ziegler catalysts supported on inorganic carriers. 

Efficient contact is produced between the phases in agitated gas-liquid contactors and, therefore, this type of equipment can also be useful for those absorption and stripping operations for which conventional plate or packed towers may not be suited. It may also be useful where the operation involves the contact of three phases—say, gas, liquid, and suspended solids. The latter application could be represented by the low-pressure polymerization of ethylene with solid catalysts (F5). 

The low-pressure polymerization of olefins using Ziegler-Natta catalysts, i.e., mixtures of compounds of transition groups IV to VI of the periodic table of the elements together with organometallic compounds of groups I to III is widely applied. Such catalysts, consist of titanium alkyl compounds and aluminum alkyl compounds or alkylaluminum halides. 

From polymerization tests under the high pressure of 150 MPa, and varying concentrations of ethylene, Rau, Schmitz, and Luft [6] found that chain-propagation is the rate-limiting step and depends linearly on the concentration of ethylene (a = 1). This is different from polymerization at low pressure where an order of 1.2 - 2 was observed [7]. In agreement with results from low-pressure polymerization of ethylene, the overall rate at high pressure also depends linearly on the concentration of metallocene catalyst (b = 1). 

Figure 17.14. Some unusual reactor configurations, (a) Flame reactor for making ethylene and acetylene from liquid hydrocarbons [Patton et al., Pet Refin 37(li) 180, (1958)]. (b) Shallow bed reactor for oxidation of ammonia, using Pt-Rh gauze [Gillespie and Kenson, Chemtech, 625 (Oct. 1971)]. (c) Sdioenherr furnace for fixation of atmospheric nitrogen, (d) Production of acetic acid anhydride from acetic acid and gaseous ketene in a mixing pump, (e) Phillips reactor for low pressure polymerization of ethylene (closed loop tubular reactor), (f) Polymerization of ethylene at high pressure.

Titanocene dichloride (8) is readily reduced to the [(ir-C5H5)2Ti(III)]+ species which in turn can be converted into alkyl and hydrido derivatives (18-20). These titanocene derivatives have been found to be active catalysts in the low-pressure polymerization of ethylene (21, 22) and the... 

The low values of dissociation energy of a bond metal-carbon are characteristic particularly for alkyl transition-metal compounds. Some of them are the key intermediates of Ziegler-Natta low-pressure polymerization of alkenes. Outside the laboratory or chemical plant [43, 44], Nature gives us also important representatives of such compounds namely vitamin B12 or coenzyme B12. 

K. H. Reichert, and K. R. Meyer, Kinetics of Low Pressure Polymerization of Ethylene by Soluble Ziegler Catalysts, Makromol. Chem. 169, 163-176 (1973). 

The revolutionary discoveries by Ziegler and Natta, relating to the low pressure polymerization, respectively, of ethylene and of propylene and other a-olefins onto the previously unknown crystalline polymers, opened a new era in science and technology. Since then, remarkable progress has been made in the fields of coordination catalysis, macromolecular science and stereochemistry. With the discovery and development of the new generation catalytic systems for polyethylene in the late 1960 s, and more recently for polypropylene, enormous progress was made in terms of polymerization process as to economics and product quality Further process simplification and, above all, ever more accurate product quality control by taylor made catalytic systems is the aim of the 1980 s. 

Phillips,G.W., Carrick,W.L. Transition metal catalysts. IX. Random ethylene-propylene copolymers with a low pressure polymerization catalyst. J. Am. Chem. Soc. 84, 920-925 (1962). 

Conventionally, HAS are blended with PO during processing. 2-(Diethy-lamino)-4,6-bis[butyl(l,2,2,6,6-pentamethyl-4-piperidyl) amino]-l,3,5-triazine may be fed with an olefin directly into the low pressure polymerization process catalyzed with a modified MgCl2 supported Ziegler-Natta catalyst [142]. The catalytic activity was not impaired [143], Tetramethylpiperidine was reported to be a useful component in MgC -supported Ziegler-Natta catalysts as well. Very high stereospecificity of the synthesised PO was achieved. A complex of HAS with the alkyl aluminium activator was envisaged without interaction with the catalytically active alkyl titanium compound [144],... 

Alkylaluminums have become very important as mixed catalysts for low-pressure polymerization of olefins (especially ethylene), a reaction discovered by Ziegler and his colleagues.263 Numerous investigations have been devoted to a study of the mechanism of this reaction e.g., it has been shown that an important role in the polymerization is played by the organotitanium compounds formed from mixtures of alkylaluminums and titanium tetrachloride.264... 

Although the low-pressure polymerization of ethylene in the presence of a Ziegler catalyst is a catalytic process, the growth reaction of aluminum hydride or a corresponding alkylaluminum with ethylene occurs as an organometallic synthesis 29 at 60-80° aluminum hydride reacts with ethylene to yield tri-ethylaluminum ... 

A. von Grosse, J. M. Mavity, J. Org. Chem. 5, 106 (1940). Recent pressure processes for aluminumtrialkyls starting with Al, Ha and olefins, as well as their applications to the low-pressure polymerization of ethylene, are given in K. Ziegler et al., Angew. Chem. 67 424 (1955). 

Low-pressure polymerizations were initiated by ultraviolet radiation in the presence of di-iert-butyl peroxide in bulk, dimethyl sulfoxide, or /ert-butanol solution at — 20°C to -I- 30°C. While the polymer precipitated out of solution at low conversion, in dimethyl sulfoxide, this precipitate was a gel which was partially transparent to light. At low conversions, the reaetion kinetics were treated as pseudohomogeneous processes [15]. 

The low-pressure polymerization of ethylene with Ziegler catalysts (Ti com-poimds/Al alkyls) is depicted in Scheme 3-9. Explain the mechanism of the polymerization. 

---