Calculation of average molecular weights of oligomers in high-performance gel permeation chromatography

Abstract

Most chromatograms of synthetic oligomers in high-performance gel permeation chromatography consist of two parts: one represents a low-molecular-weight fraction where oligomers are separated into individual peaks and the other a high-molecular-weight fraction with one broad curve. An example is the chromatogram of polystyrene 600 (nominal molecular weight 600), which was separated into 13 peaks from the dimer to the tetradecamer when recycle operation was applied or longer column lengths were employed. Average molecular weights in this instance were M − w = 688 and M − n = 598. Average molecular weights calculated from the norma chromatogram of polystyrene 600 by using several methods were M − w = 691–742 and M − n = 605–648. The preferred and most practical procedure involved the calculation of average molecular weights for the low-molecular-weight fraction of the chromatograam by measuring each peak area and by knowing its molecular weight, calculation of those for the high-molecular-weight fraction in a similar manner as in high polymers, and then calculation of the overall average molecular weights.