Trace amounts of oxygen (s300 ppm) are typically used as the initiator, often in combination with an alkyl or acyl peroxide or hydroperoxide. Ethylene is compressed in stages with cool. ing 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 mono mer into the reactor Propane, butane, cyclohexane, propene, l-butene, isobutylene, acetone, 2-propanol, and p 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 tempera- ture). The reaction system after the start of polymerization is homogeneous (polymer swollen by monomer) 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.