Separation of polymers by molecular weight through inclusion compound formation with urea and α-cyclodextrin hosts

alpha-Cyclodextrin (alpha-CD) and urea (U) hosts were used in order to separate a mixture of poly-(ethylene glycols) (PEGs) with two different molecular weights (M-w = 600 [PEG(600)] and M-w = 20 000 [PEG(20000)]) by forming the respective inclusion compounds (ICs). Differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, and wide-angle X-ray diffraction (WAXD) have been used to verify the formation and successfully characterize all inclusion compounds. The high melting point of the urea inclusion compound (PEG*-U IC) formed from the solution containing both polymers was the first evidence that urea preferentially included poly(ethylene glycol) with the higher molecular weight. The PEG*-U IC and PEG(20000)-U IC X-ray diffraction patterns and FTIR spectra were very similar to each other, confirming that urea preferentially complexes PEG(20000) in their mutual solution. Since the above-mentioned techniques were not as useful in the case of alpha-CD ICs, viscosity measurements were made in order to demonstrate the molecular weight selectivity of alpha-CD host. The efflux time of PEG*-alpha-CD IC solution was much closer to that of a PEG(20000)-alpha-CD IC solution, which suggested that alpha-CD also included poly(ethylene glycol) having a high molecular weight rather than the low molecular weight PEG(600). The efflux time of PEG*-U IC solution is almost identical with that of the PEG(20000)-U IC solution indicating that urea has better molecular weight selective properties than alpha-CD.