Rapid and quantitative 1D ¹³C NMR analysis of polypropylene tacticity with relaxation agent and proton polarization...

Polypropylene (PP) accounts for approximately 28.0% of the global polyolefin market, valued at $243.4 billion in 2022. Known for its lightweight, chemical resistance, cost-effectiveness, high strength and melting point, PP is widely used in various applications. Its properties and applications are closely tied to its tacticity. One-dimensional (1D) conventional ¹³C NMR has been extensively utilized to analyze PP tacticity, but its low sensitivity and longer relaxation time remain drawbacks. Typically, analyzing a single PP sample requires around 9 h of NMR time. Using of a cryogenically cooled 10 mm NMR probe can significantly reduce this measurement time; however, its high cost makes it inaccessible for most NMR laboratories. While the refocused insensitive nuclei enhanced by polarization transfer (RINEPT) technique is well-known for enhancing NMR sensitivity, there are no published studies using 1D ¹³C RINEPT to quantify PP tacticity. Relaxation agents like chromium (III) acetylacetonate (Cr(acac)₃) have also been used to reduce relaxation times in polyolefin NMR analyses. Here we introduce a straightforward and easily implementable 1D ¹³C NMR method for rapid PP tacticity quantification. This method combines Cr(acac)₃, Bruker's existing RINEPT pulse sequence (ineptrd), and our recently published ¹H decoupling sequence (bi_waltz65_256 pl) to eliminate ¹H decoupling artifacts. It is worth noting that decoupling artifacts are always present. When the signal-to-noise ratio (SNR) is low, these artifacts are obscured by noise. For example, in some two-dimensional (2D) or three-dimensional (3D) NMR spectra, decoupling artifacts are barely visible because of the low SNR. However, when attempting to observe weak signals in 1D spectra, increasing the sample concentration or the number of scans enhances the SNR, revealing the decoupling artifacts. The decoupling artifacts appeared superimpose with some other weak signals, affecting the measurements of signal intensities. Therefore, improved ¹H-decoupling methods are crucial for such data acquisition. This synergy results in a 9.4- to 9.7-fold sensitivity enhancement, equating to an 88- to 94-fold reduction (9.4² ≈ 88, 9.7² ≈ 94) in NMR acquisition time compared to conventional 1D ¹³C NMR experiment with Cr(acac)₃. The time savings are even more substantial compared to experiments without Cr(acac)₃. The faster and quantitative approach is accessible to researchers with or without cryoprobes. Beyond PP, this method can be applied to tacticity measurements of other polyolefins, such as polybutene, polyhexene and polyoctene.