Determination of the molecular weight and other physicochemical properties of poly(bubylene glycol) 1000 by gradient RPHPLC and measurement of the ELSD, UV, and fluorescence responses

Molecular weight and other physicochemical data for poly(butylene glycol) 1000 (PBG 1000) have been determined by use of a new, highly efficient, gradient reversed-phase high-performance liquid-chromatographic (RPHPLC) procedure. Separation of the native material or its α, ω-bis(1-naphthylurethane) derivative was achieved on a C₁₈ stationary phase with a ternary acetonitrile-water-tetrahydrofuran mobile phase. Detection was achieved by measurement of the signal response from evaporative light scattering (ELSD), UV, and fluorescence (FD) detection.Proof that all the oligomers contained in the sample had been separated by the method was obtained by liquid chromatography coupled with electrospray ionization time-of-flight mass spectrometry (LC-ESI-TOFMS). It was also confirmed by this technique that di(butylene glycol) is the lowest homologue in the sample. Although the dimer was also observable in the HPLC-UV trace of the PBG 1000 α, ω-bis(1-naphthylurethane) derivative, it was obviously too valatile to be seen in the ELSD trace; tri(butylene glycol) was, nevertheless, still recognizable with sufficient signal intensity.Because all the homologues were separated to baseline, the method was used to calculate the number- and weight-average molecular weights,M ₙ andM w, both from peak areas and from peak heights. The best fit to data obtained from end-group titration were obtained from calculations based on the HPLC-UV response;M ₙ values of 948 and 937 were obtained from peak heights and areas, respectively.M ₙ andM w values calculated from the ELSD trace obtained from native PBG 1000 were substantially (ca 10%) lower that those calculated from the UV trace obtained from the α, ω-bis(1-naphthylurethane) derivative. Similar differences were also discovered by comparing theM ₙ andM w values obtained from UV and FD-values were approximately 20% higher for FD.When the retention times of individual oligomers, from either ELSD of the native sample or from UV-FD of the α,ω-bis(1-naphthylurethane) derivative, were plotted against the number of repeat units,P, the ELSD curve approaches the UV-FD curve at values ofP of approximately 60. This observation can be explained by the pronounced contribution of the hydrophobic end-groups to the overall retention of PBG 1000; the effect of this decreases with increasing chain length and becomes nearly independent ofP only for a very high degree of polymerization.