摘要
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,costeffectiveness,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 ^(1)H 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 ^(1)D 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 acquisitio n.This synergy results in a 9.4-to 9.7-fold sensitivity enhancement,equating to an 88-to 94-fold reduction(9.4^(2)≈88,9.7^(2)≈94) in NMR acquisition time compared to conventional 1D ^(13)C 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.
