Density of polyethylene^†

The density of polyethylene produced by free-radical-initiated batch polymerization at elevated pressures and temperatures was found to be determined by the amount of short-chain branching and the molecular weight. The short-chain branching was shown to depend solely upon the pressure and temperature of polymerization. This branching increased as the pressure was reduced or as the temperature was increased. The molecular weight was shown to have an important influence on the density at much higher molecular weight levels than previously reported (up to 50,000 M_n). The density increased as molecular weight decreased. At a constant polymerization pressure and temperature, the type and amount of initiator and chain-transfer agent had no effect on short-chain branching. Inasmuch as the type and amount of these substances do have an effect on molecular weight, they did affect density indirectly through the molecular weight. An equation relating the density, molecular weight, and polymerization pressure and temperature is given.