Azo free radical initiator

Figure 4.1 Commercially available azo free radical initiators.

Ethylene and a few other monomers are in the gas phase at atmospheric conditions. Thus, conventional low-density polyethylene (LDPE) is produced at pressures in the range of 15,000-50,000 psi (1000-3400 atm), and temperatures of about 150-300 °C [5]. These pressures are well above the critical pressure of ethylene (731 psi), so the monomer can be considered to be in the fluid phase. Depending on the temperature, pressure, and polymer molecular weight, one or two phases may be present as the polymer forms. Peroxide and azo free-radical initiators are used, as are small amounts of oxygen. Oxygen reacts with monomer to form peroxide initiators in situ. 

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