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 (<i>i</i>PP) 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, ⟨<i>mmmm</i>⟩ = 99.4%) give a maximum ordered packing (α₂) fraction of 66% at crystallization temperature (<i>T</i>_c) of 155 °C while low stereoregularity samples (⟨<i>mmmm</i>⟩ = 91.0%) have only 47% at the same <i>T</i>_c. However, <i>M</i>_w (58.7–982 kg/mol) does not play a significant role in ordered packing formation. Using ¹³C-labeled CH₃ of <i>i</i>PP, 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 <i>i</i>PP 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 <i>i</i>PP demonstrates that chemical defect is almost excluded from the crystalline region at <i>T</i>_c = 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 ⟨<i>mmmm</i>⟩ = 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)