Determination of Chain-Folding Structure of Isotactic Polypropylene in Melt-Grown α Crystals by 13 C– 13 C Double Quantum NMR and Selective Isotopic Labeling

Chain-folding (CF) patterns, successive CF number ⟨ n ⟩, and fraction ⟨ F ⟩ of 13 C CH 3 15% labeled isotactic polypropylene ( i PP) in melt-grown crystals at two crystallization temperatures ( T c s) of 100 and 150 °C were investigated using 13 C– 13 C Double quantum (DQ) NMR and spin-dynamics simulation. The former provided the α 1 packing structure while the latter lead to the α 2 -rich sample with an α 2 fraction of ca. 57%. It was demonstrated that the i PP chains adopt small cluster structures consisting of 6–8 stems connected via adjacent re-entry as mean structures in both α 1 and α 2 phases. There was no kinetic effect on ⟨ F ⟩ and ⟨ n ⟩ values, while kinetics did influence the folding directions of the chains under the assumptions of configurational and packing constraints. It was concluded that the CF process occurs at the growth front and dominates the chain-packing process of i PP α phases in a wide T c .