Formation and Characterization of Polypropylene−Urea Inclusion Compounds

Abstract

Isotactic (i) and syndiotactic (s) polypropylenes (PP) have been successfully incorporated into the channels of their inclusion compounds (IC) with urea (U). The IC's were obtained by cocrystallization following the addition of a warm toluene solution of polymer to a warm solution of urea in methanol. The white crystalline precipitates formed upon cooling were filtered and analyzed using X-ray diffraction, differential scanning calorimetry (DSC), and Fourier transform infrared (FTIR) and 13C NMR spectroscopies. The results were compared with the components (urea and polypropylene) in their pure crystalline forms, the known hexagonal poly(ε-caprolactone)−urea IC, and the known trigonal poly(ethylene oxide)−urea IC. X-ray diffraction data indicated that no free polymer was present in either sample, while FTIR showed that the i-PP−U and s-PP−U IC's were not the hexagonal and trigonal structures commonly observed. Instead the structure appears similar to a “large tetragonal” structure observed for the poly(ethylene glycol)−urea IC. DSC indicated a melting point for the i-PP−U IC at 138 °C, while 13C NMR showed that the i-PP and s-PP polymer chains are included in their respective 31 and 21 helical, crystalline conformations. Previous modeling has indicated that the crystalline conformations are unable to fit in an IC channel with a diameter less than 7 Å. This again confirms the nonhexagonal and nontrigonal structure of the IC, because the channel diameters for these structures are ca. 5.5 Å. Similarities have been found with the partially characterized “large tetragonal” structure of the low molecular weight poly(ethylene glycol)−urea IC, whose channel diameter is not yet known.