Synthesis conditions and structure of low density polyethylene. II. Average molar mass and molar mass distribution

Abstract

AbstractThe first part of this publication dealt with the effect of synthesis conditions on short and long‐chain branching of low density polyethylene; this second part reports on the effect on the molar mass, the molar mass distribution and the polydispersity that can be determined from these parameters. The average molar mass was measured with a membrane osmometer and the molar mass distribution by gel permeation chromatography, using the same samples as those used for the branching analysis. These samples were produced in continuously operating stirred autoclaves under widely differing polymerization conditions.The number‐average molar mass falls as the polymerization temperature and the initiator concentration rises and the pressure decreases. When the polymerization temperature is high, the molar mass distribution is narrower and less asymmetrical. When the polymerization temperature is low, there are broader, more asymmetrical distributions with a larger higher molar mass fraction. The polydispersity calculated from the molar mass distribution falls accordingly as the polymerization temperature rises. The polyethylene samples produced in a squat stirred autoclave have a higher polydispersity than those produced in a narrow autoclave. These findings have been confirmed on commercial products produced in various types of reactors.