Fractionation of Partially Isotactic Polypropylene Oxide

Abstract

WE have polymerized propylene oxide in dioxan with the zinc diethyl and water catalyst¹, dissolved the resulting high-molecular-weight polymer in iso-octane at 60° C and precipitated solid phases by lowering the temperature incrementally to 40° C. In this way we were able to obtain several fractions which were identical in average molecular weight and in the distribution of molecular weights about the average. By further lowering of the temperature we obtained liquid–liquid phase separation with fractionation occurring predominantly on the basis of molecular weight. A typical result of such a fractionation is shown in Fig. 1. Further analysis of that part of the polymer undergoing liquid–liquid phase separation (some 80 wt. – %), coupled with cryoscopic measurements on the fraction of lowest molecular weight, indicates that the ratio of the weight- to the number-average molecular weight for the polymer is greater than 1,000, the consequence of a large low molecular weight tail.