Characterization of hyperbranched aliphatic polyesters and their trimethylsilylated derivatives by GPC-viscometry

Gel permeation chromatography (GPC) viscometry with universal calibration (UC) has been assessed as a means of characterizing the absolute molecular weight distribution of hydroxyl terminated hyperbranched aliphatic polyesters (HBPs) of aliphatic compounds. To generate a valid UC curve, it is necessary to work under conditions of strict size-exclusion separation. This requirement was met when both dimethyl formamide (DMF) and tetrahydrofuran (THF) were used as eluants. All the HBPs were highly soluble in DMF at room temperature, but its use may necessitate special columns. Dissolution of the HBPs in THF on the other hand, which is compatible with a variety of stationary phases, was relatively difficulty requiring prolonged heating above 50degreesC. As an alternative to direct characterization, the HBP hydroxyl end groups were end-capped with trimethylsilane prior to the GPC measurements. Near quantitative replacement of the hydroxyl groups was achieved and the modified HBPs remained stable for several weeks under dry nitrogen. This permitted straightforward GPC analysis in THF at room temperature, giving results consistent with those obtained for the unmodified HBPs.