Thin-layer chromatography of oligomers

The mechanism of the thin-layer chromatography of oligomers based on the different adsorption activities of the end groups and the central units, characterized by adsorption energies −ϵe and −ϵc, is discussed. It is shown that the application of the concept of the critical adsorption energy, −ϵo, to the central oligomer units makes it possible to distinguish three cases of the molecular weight dependence of RF values: at −ϵo > −ϵc and −ϵo < −ϵc we have a negative and positive molecular weight dependence of RF values, respectively, while at −ϵo = −ϵc the RF values do not depend on molecular weight. In the last instance, thin-layer chromatography can be used for the separation of oligomers on the basis of functional groups and degree of branching independent of their molecular weights. For the separation of oligomers on the basis of molecular weight thin-layer chromatography, hydroxyl end groups should be blocked in the case of poor adsorption activity of the central units. In this instance the second type of molecular weight dependence of RF values is observed. A quantitative method for the determination of low-functionality impurities monools and poly(propylene oxide)diols in oligomers based on the chromatographic spot length is suggested.