High-resolution solid-state 13C nuclear magnetic resonance study of isotactic polypropylene polymorphs

Abstract

High-resolution 13C nuclear magnetic resonance spectra are reported for three solid samples of isotactic polypropylene (i-PP): (i) the α-crystalline form (monoclinic), (ii) the β-crystalline form (hexagonal), and (iii) the mesomorphic or smectic form of i-PP. These spectra were obtained using magic angle spinning, high-power proton dipolar-decoupling and cross-polarization techniques, and for the β- and smectic forms of i-PP constitute the first reported high-resolution solid-state spectra. The spectrum of the α-crystalline form shows well resolved splittings (1 ppm) of the methyl and methylene carbon resonances, as was reported previously by Bunn et al. These splittings are absent in the spectra observed for the β- and smectic forms of i-PP. Based on a comparison of the chemical shifts and T 1 relaxation times observed for the carbon resonances in these three forms of solid i-PP, we reach the following conclusions: (i) i-PP adopts the same 3 1 helical conformation in each crystalline polymorph, (ii) the packing of i-PP helices in the β-form crystal is closer to the more distant of the two interhelical packings in the α-crystalline form, and (iii) the local packings of 3 1 helices are very similar in the β- and smectic forms of i-PP.