Solid-State NMR Characterization of the Chemical Defects and Physical Disorders in α Form of Isotactic Poly(propylene) Synthesized by Ziegler–Natta Catalysts

Abstract

The order–disorder phenomenon and spatial heterogeneity of chain packing, partitions of stereodefects, and molecular dynamics of α form of isotactic polypropylene (iPP) samples, which are synthesized by Zieglar–Natta catalysts, are investigated by solid-state (SS) NMR. High-resolution 13C NMR under high-power TPPM decoupling at field strengths of 110 kHz allows observation of the order–disorder phenomenon in the chain-packing structures of α form. High isotacticity samples (isotacticity at pentad level, ⟨mmmm⟩ = 99.4%) give a maximum ordered packing (α2) fraction of 66% at crystallization temperature (Tc) of 155 °C while low stereoregularity samples (⟨mmmm⟩ = 91.0%) have only 47% at the same Tc. However, Mw (58.7–982 kg/mol) does not play a significant role in ordered packing formation. Using 13C-labeled CH3 of iPP, direct spatial correlations between the α2 and α1 structures are investigated by 13C detection of two-dimensional (2D) 1H–1H spin-diffusion (CHHC) experiments. The time dependence of the spin-diffusion polarization transferred signal intensities determines the average domain size of the α1 and α2 structures of iPP crystallized at 150 °C, which was found to be 40 nm under an assumption of 2D spin diffusion. Additionally, the 13C filter CPMAS NMR spectrum on 13C CH3-labeled iPP demonstrates that chemical defect is almost excluded from the crystalline region at Tc = 150 °C (defect free crystal) while ca. 2% is in melt quench sample. Moreover, 13C centerband-only detection of exchange experiments on α2-rich sample with highest ⟨mmmm⟩ = 99.4% indicate that crystalline dynamics follows a single Arrhenius plot with an activation energy of 116 kJ/mol across reported order–disorder transition temperatures (157–159 °C).