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with an alkyl or acyl peroxide or hydroperoxide. Ethylene is compressed in stages with cooling between stages and introduced into the reactor. Initiator and chain-transfer agent are added during a late stage of compression or simultaneously with the introduction of monomer into the reactor. Propane, butane, cyclohexane, propene, 1-butene, isobutylene, acetone, 2-propanol, and propionaldehyde have been used as chain-transfer agents. The initial reaction temperature is typically 140 180 C, but this increases along the length of the reactor to peak temperatures as high as 300 325 C before decreasing to about 250 275 C, due to the presence of cooling jackets. Polymerization occurs in the highly compressed gaseous state where ethylene behaves much as a liquid (even though ethylene is above its critical temperature). The reaction system after the start of polymerization is homogeneous (polymer swollen by monomer) if the pressure is above 200 MPa. Some processes involve multizone reactors where there are multiple injections of initiator (and, often, monomer and chain-transfer agent) along the length of the tubular reactor. The conversion per pass is usually 15 20% and 20 30% per pass for single-zone (no additional injections of reactants) and multizone reactors, respectively. After leaving the reactor, polyethylene is removed from the reaction mixture by reducing the pressure, usually in two stages in series at 20 30 MPa and 0.5 MPa or less. Molten polyethylene is extruded, pelletized, and cooled. The recovered ethylene is cooled to allow waxes and oils (i.e., very-lowmolecular-weight polymer) to be separated prior to recycling of ethylene to the reactor. Overall, ethylene polymerization is carried out as a bulk polymerization (i.e., no added solvent or diluent) in spite of the inherent thermal and viscosity problems. Control is achieved by limiting conversion to no more than 30% per pass. The polymerization is effectively carried out as a solution or suspension process (with unreacted monomer acting as solvent or diluent) depending on whether the pressure is above or below 200 MPa. Autoclave reactors differ from tubular reactors in two respects. Autoclave reactors have much smaller length-to-diameter ratios (2 4 for single-zone and up to 15 18 for multizone reactors) and operate at a much narrower reaction temperature range. The polyethylene produced by radical polymerization is referred to as low-density polyethylene (LDPE) or high-pressure polyethylene to distinguish it from the polyethylene synthesized using coordination catalysts (Sec. 8-11b). The latter polyethylene is referred to as high-density polyethylene (HDPE) or low-pressure polyethylene. Low-density polyethylene is more highly branched (both short and long branches) than high-density polyethylene and is therefore lower in crystallinity (40 60% vs. 70 90%) and density (0.91 0.93 g cm 3 vs. 0.94 0.96 g cm 3 ). Low-density polyethylene has a wide range and combination of desirable properties. Its very low Tg of about 120 C and moderately high crystallinity and Tm of 105 115 C give it exibility and utility over a wide temperature range. It has a good combination of strength, exibility, impact resistance, and melt ow behavior. The alkane nature of polyethylene imparts a very good combination of properties. Polyethylene is highly resistant to water and many aqueous solutions even at high temperatures. It has good solvent, chemical, and oxidation resistance, although it is slowly attacked by oxidizing agents and swollen by hydrocarbon and chlorinated solvents at room temperature. Polyethylene is a very good electrical insulator. Commercial low-density polyethylenes have number-average molecular weights in the range 20,000 100,000, with X w =X n in the range 3 20. A wide range of LDPE products are produced with different molecular weights, molecular-weight distributions, and extents of branching (which affects crystallinity, density, and strength properties), depending on the polymerization temperature, pressure (i.e., ethylene concentration), and reactor type. Long-chain branching increases with increasing temperature and conversion but decreases with increasing pressure. Short-chain branching decreases with increasing temperature and
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