Ethylene, supercritical

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). 

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). 

Carbon dioxide and water are the most commonly used SCFs because they are cheap, nontoxic, nonflammable and environmentally benign. Carbon dioxide has a more accessible critical point (Table 6.13) than water and therefore requires less complex technical apparatus. Water is also a suitable solvent at temperatures below its critical temperature (superheated water). Other fluids used frequently under supercritical conditions are propane, ethane and ethylene. 

To illustrate the use of the gas sorption mode , we show in Figure 7 results of the supercritical ethylene sorption in low-density polyethylene (12,16). As seen in Figure 7, the theory is capable of fitting the ethylene sorption data. In this instance, the data at three temperatures can be fit within experimental precision using interaction parameters (p o) of 3235 atm, 3178 atm, or 3101 atm at 126°C, 140 0, and 155 C, respectively. 

Han S.J., Lohse D.J., Radosz M., and Sperling L.H. Thermoplastic vulcanizates from isotactic olypro-pylene and ethylene-propylene-diene terpolymer in supercritical propane Synthesis and morphology. Macromolecules, 31, 5407, 1998. 

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. 

This industrial process remains essentially unchanged from the 1950s [25], Here, a free-radical initiator is added to the ethylene monomer at supercritical conditions (276 MPa and 200-300 °C). The polyethylene remains in the supercritical solution until the pressure is lowered to around 5 MPa, whereupon it precipitates. A range of other monomers can be copolymerized, including carbon monoxide to give polyketones, as shown in Scheme 10.19 [26],... 

These techniques involve reactions in which the reduction of the precursor is carried out with liquid agents which can be aided with heat, plasma, light, ultrasound, microwave, or supercritical C02 [45,145-149]. Common reducing agents include NaBfb, [42], ethylene glycol [84], thiourea [48], sodium citrate [150], and formic acid [107]. 

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. 

Bach, E. Cleve, E. Schiittken, J. Schollmeyer, E. Rucker, J.W. Correlation of solubility data of azo disperse dyes with the dye uptake of poly(ethylene terephthalate) fibres in supercritical carbon dioxide. Color. Technol. 2001, 117, 13-18. 

Journal of Applied Polymer Science 11, No.7, 15th Aug.2000, p. 1478-87 MORPHOLOGIES OF BLENDS OF ISOTACTIC POLYPROPYLENE AND ETHYLENE COPOLYMER BY RAPID EXPANSION OF SUPERCRITICAL SOLUTION AND ISOBARIC CRYSTALLIZATION FROM SUPERCRITICAL SOLUTION... 

By rapid expansion of supercritical propane solution (RESS), and isobaric crystallisation (ICSS), isotactic polypropylene and ethylene-butylene copolymers were precipitated from the supercritical solution. The RESS process produced microfibres with a trace of microparticles, while the ICSS process produced microcellular products. Improvement in thermal stability was achieved by first synthesising a thermoplastic vulcanisate from polypropylene and ethylene-propylene-diene terpolymer from a supercritical propane solution, followed by RESS. 28 refs. 

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. 

In a subsequent study, they used ethylene for a dual purpose, as a substrate as well as a supercritical fluid solvent. This notoriously unreactive olefin to PKR served nicely to give 2-substituted cyclopentenones. Reaction efficiency of each alkyne substrate can be tuned by changing catalyst precursors. Not only Co2(CO)8 but also the two cobalt clusters [Co4(CO)i2] and [Co4(GO)n P(OPh)3 ] work well for some substrates (Equation (8)). The comparison with Rautenstrauch s result clearly shows the beneficial effect of this approach. 

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