MOLECULAR WEIGHT DISTRIBUTION OF POLYETHYLENE POLYMERIZED BY γ-RAY IRRADIATION UNDER VARIOUS POLYMERIZATION CONDITIONS

The molecular weight distributions of polyethylene polymerized by γ-ray irradiation in gas phase under various conditions were investigated by GPC method. In the high polymerization temperature the distribution curve shows a single sharp peak, whereas in the low polymerization temperature it shows characteristic broad double peaks. The peak, corresponding to the high molecular weight species in the latter, becomes more intense with increasing polymerization pressure. The molecular weight distributions of the polymers obtained by a two-step polymerization also reveal double peaks characteristic of the low temperature polymerization. In the second step polymerization, the rate of increase in the number of the polymer molecules is larger for the high molecular weight species. This tendency, which applies also to the usual one-step polymerization, suggests the existence of the long-lived radicals during the polymerization. The average molecular weight of the polyethylene sample obtained by shielding partly the irradiation with the lead block was always higher than that obtained without shielding, and the peak in the molecular weight distribution shifts towards the higher molecular weight region by shielding. All these results suggest that the addition of the monomer to the long-lived radicals trapped in the polymer precipitated would give rise to further growth of the chains and branching reaction (which leads to the formation of long-chain branches).