Supercritical ethylene, polymerization

Solution Polymerization. Two types of solution polymerization technologies are used for LLDPE synthesis. One process utilizes heavy solvents the other is carried out in mixtures of supercritical ethylene and molten PE as a polymerization medium. Original solution processes were introduced for low pressure manufacture of PE resins in the late 1950s subsequent improvements of these processes gradually made them economically competitive with later, more advanced technologies. 

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

A variety of technological processes arc used for polyethylene manufacture. They include polymerization in supercritical ethylene at a high ethylene pressure and temperature above the PE melting point (110-140°C), polymerization in solution at 120-150°C or in slurry, and polymerization in the gas phase... 

The first application of supercritical fluids (SCFs) to the polymer industry occurred in the 1930s, with the development of a free-radical bulk polymerization of supercritical ethylene to produce low-density polyethylene (LDPE) [ 1,2]. LDPE is synthesized by a high-pressure process, at 180-300 °C and 1000-... 

It should be noted that on an industrial scale, reactions or other processes in SCF media are not new. Many industrial reactions developed in the early part of the twentieth century are actually conducted under supercritical conditions of either their product or reagent including ammonia synthesis (BASF, 1913), methanol synthesis (BASF, 1923) and ethylene polymerization (ICI, 1937). 

Ethylene is compressed to 2,700 bar and a free-radical initiator, e.g., trace amounts of oxygen or a peroxide, is injected into the feed stream to promote the free-radical polymerization. The polyethylene polymer that is formed remains dissolved in the supercritical ethylene phase at the operating temperature, which ranges from 140 to 250°C. The heat of reaction is removed by through-wall heat transfer when the tubular reactor is used and by regulating the rate of addition of initiator when the autoclave reactor is used. 

Let us consider the application of transition state analysis to interpret the work of Ehrlich and coworkers on the reaction behavior of ethylene polymerization in supercritical ethylene (Ehrlich, 1971). Ehrlich presents experimental data on the polymerization of ethylene at 130°C and 1,500 bar. At these conditions supercritical ethylene can solubilize 5 wt% to 10 wt% high molecular weight polyethylene, which is produced during the reaction. Normally, the conversions are kept to —10% which means that the reacting supercritical ethylene-polyethylene mixture is near a mixture critical point. Ehrlich argues that the partial molar volume of M, which has volumetric... 

Much of the earliest information on ethylene polymerization can be found in the patent literature. In a 1946 patent Krase and Lawrence (1946) describe an SCF reaction process for making ethylene polymers. Ethylene is reacted in the presence of a catalyst at temperatures between 40 C and 400°C and at pressures from 800 to 4,000 bar. The polymer is then recovered using a stepwise reduction in pressure with the objective of reducing compression costs. The authors note in this very early patent that appreciable quantities of the polymer are still solubilized in the supercritical fluid phase at pressures as low as 150 bar. More than likely, the material still soluble at 150 bar consists of... 

The cationic palladium a-diimine complexes are remarkably functional-group tolerant. Ethylene polymerizations can be carried out in the presence of ethers, organic esters, and acids, but nitriles tend to inhibit polymerizations. In addition, polymerizations have been carried out in the presence of air and in the presence of an aqueous phase.Aqueous emulsion and suspension polymerizations using these catalysts have been developed as a route to microspheres of polymer for adhesives as well as for other applications.2 ° 2 Preparation of elastomers is often complicated by difficult solvent removal, so polymerizations in supercritical CO2 have been investigated. It is also possible to combine the activity of the palladium catalysts with other polymerization techniques such as living-free-radical polymerizations. One interesting observation is that the... 

Polymerization of Olefns in Supercritical CO2 Using Brookhart Catalyst 1171 Table 8.6 Experimental conditions of the ethylene polymerizations. 

I g Catalytic Polymerization of Olefins in Supercritical Carbon Dioxide Table 8.8 Experimental results of ethylene polymerizations. 

High-pressure copolymerization of ethylene with acrylic acid esters and with (meth)acrylic acid are other important technical processes that are run under supercritical conditions close to those of high-pressure ethylene polymerization (6, 6b). 

The first commercial polyolefin arrived with Imperial Chemical Industries s (ICI) production of free-radical-polymerized, low-density polyethylene (LDPE) in the 1930s. The process was carried out in supercritical ethylene at extreme... 

Solution Polymerization These processes may retain the polymer in solution or precipitate it. Polyethylene is made in a tubular flow reactor at supercritical conditions so the polymer stays in solution. In the Phillips process, however, after about 22 percent conversion when the desirable properties have been attained, the polymer is recovered and the monomer is flashed off and recyled (Fig. 23-23 ). In another process, a solution of ethylene in a saturated hydrocarbon is passed over a chromia-alumina catalyst, then the solvent is separated and recyled. Another example of precipitation polymerization is the copolymerization of styrene and acrylonitrile in methanol. Also, an aqueous solution of acrylonitrile makes a precipitate of polyacrylonitrile on heating to 80°C (176°F). 

In 1988, Terry and coworkers attempted to homopolymerize ethylene, 1-octene, and 1-decene in supercritical C02 [87], The purpose of their work was to increase the viscosity of supercritical C02 for enhanced oil recovery applications. They utilized the free radical initiators benzoyl peroxide and fert-butyl-peroctoate and conducted polymerization for 24-48 h at 100-130 bar and 71 °C. In these experiments, the resulting polymers were not well studied, but solubility studies on the products confirmed that they were relatively insoluble in the continuous phase and thus were not effective as viscosity enhancing agents. In addition, a-olefins are known not to yield high polymer using free radical methods due to extensive chain transfer to monomer. 

The polymerization of other fluoroolefins such as TFE with hexafluoropro-pylene (HFP), TFE with ethylene, and vinylidine difluoride - " further demonstrates the broad applicability of liquid and supercritical CO in the production and processing of fluorinated polymers. Many of the aforementioned advantages associated with CO2, including tunable solvent properties, integrated synthesis, separation and purification processes, negligible chain transfer in the presence of highly electrophilic species, and relative ease of recycling, make it an ideal solvent for fluoroolefm polymerization. 

In practice, fiee-radical polymerization of ethylene and other small olefins is usually carried out at very high pressures (because the rate varies approximately as P ) with a solvent such as isobutane, which forms a supercritical solution at reaction conditions. 

One of the earliest chemical applications of SCFs was the polymerization of ethylene, C2H4. As pointed out by McHugh and Krukonis, [10] this process remains an important landmark in supercritical chemistry because the operating temperatures and pressures are much higher than those proposed in most new applications. Therefore, the C2H4 polymerization demonstrates that engineering problems are unlikely to be the major obstacle to the introduction of new SCF processes. 

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