Polyolefin Analyses with a 10 mm Multinuclear NMR Cryoprobe

Abstract

Polyolefins are important and broadly used materials. Their molecular microstructures have direct impact on macroscopic properties and dictate end-use applications. 13 C NMR is a powerful analytical technique used to characterize polyolefin microstructures, such as long-chain branching (LCB), but it suffers from low sensitivity. Although the 13 C sensitivity of polyolefin samples can be increased by about 5.5 times with a cryoprobe, when compared with a conventional broadband observe (BBO) probe, further sensitivity enhancement is in high demand for studying increasingly complex polyolefin microstructures. Toward this goal, distortionless enhancement by polarization transfer (DEPT) and refocused insensitive nuclei enhanced by polarization transfer (RINEPT) are explored. The use of hard, regular, and new short adiabatic 180°1 3 C pulses in DEPT and RINEPT is investigated. It is found that RINEPTs perform better than DEPTs and a sensitivity enhancement of 3.1 can be achieved with RINEPTs. The results of RINEPTs are further analyzed with statistics software JMP and recommendations for optimal usage of RINEPTs are suggested. An example of analyzing saturated chain ends in an ethylene−octene copolymer sample with a hard 180°1 3 C RINEPT pulse is demonstrated. It is shown that the experimental time can be further reduced in half because of faster proton relaxation, where the total experimental time is about 580 times shorter when compared to using a conventional method and a 10 mm BBO probe. A naturally abundant nitrogen-containing polyolefin is analyzed using 1 H− 15 N HMBC and, to our knowledge, is the first 1 H− 15 N HMBC presented in the field of polyolefin characterization. The relative amount of similar nitrogen-containing structures is quantified by two-dimensional integration of 1 H− 15 N HMBC. Two pragmatic technical challenges related to using high-sensitivity NMR cryoprobes are also addressed: (1) A new 1 H decoupling sequence Bi_Waltz_65_256pl is proposed to address decoupling artifacts in 13 C{ 1 H} NMR spectra which contain a strong 13 C signal with a high signal-to-noise ratio (S/N). (2) A simple pulse sequence that affords zero-slope spectral baselines and quantitative results is presented to address acoustic ringing that is often associated with high-sensitivity cryoprobe use.

Very Sensitive 13C NMR Method for the Detection and Quantification of Long-Chain Branches in Ethylene–Hexene Linear Low-Density Polyethylene

“…RINEPT has been widely used recently to enhance the 13 C measurement sensitivity for polyolefins. ,, This inspired us to design a new pulse sequence ainepin.2 (Figure , pulse sequence code in the Bruker format is shown in the Supporting Information). The INADEQUATE with RINEPT approach was reported before .…”

“…A model compound n-butyl-cyclohexane was first used to test the new ainepin.2 pulse sequence, together with conventional zgig, ainadigsp1d.2, 30 and RINEPT (ineptrd) 29 pulse sequences, on a 500 MHz NMR with a 5 mm BBFO cryoprobe at 25 °C. Figure 2 shows the results.…”

“…The heated sample was homogenized using a vortex machine until consistent flow is achieved. The NMR spectra were collected using a Bruker 600 MHz AVANCE III HD system equipped with 10 mm high-temperature helium cryoprobe at 120 °C − or a Bruker 500 MHz NEO spectrometer equipped with a 5 mm BBFO helium cryoprobe at 25 °C. The NMR parameters are detailed in the following Results and Discussion section.…”

“…Many efforts were devoted to this task, for example, understanding melt strength and increased zero shear viscosity, triple detector gel permeation chromatography (TDGPC) for molecular size, and 13 C NMR which directly measures the frequency of the LCB, − defined as branches with more than six carbons. − Unlike a secondary analytical method that needs calibration such as TDGPC, 13 C NMR is a primary analytical method and directly measures the methine carbon at the junction between the main chain and the long-chain branch. − Direct LCB measurement is very important not only from a practical application standpoint but also from a fundamental material science understanding angle. The recent developments of high-temperature 10 mm cryoprobe technology have overcome the traditional low sensitivity disadvantage of the 13 C NMR method. − Measuring LCB in EH LLDPE with 13 C NMR had challenges, such as “LCB signal overlap with the ethylene–hexene–ethylene branch (EHE Br) low field 13 C satellite signal when using 1-chloronaphthalene/PDCB- d 4 (9:1, w/w) mixture as a solvent to separate the LCB and the EHE Br signals and using 600–700 MHz NMR 10 mm cryoprobe to boost 13 C sensitivity”. These challenges were resolved recently .…”

Analytical Insights into the Microstructures and Reaction Mechanisms of Cationic Pd(II) α-Diimine-Catalyzed Polyolefins

“…12 The novelty of using a single photo-responsive catalyst for block copolymer synthesis prompted the investigation of this unique polymerization, which is critical for the design of more robust systems through access to diverse block copolymers from nonpolar and polar monomers. 13,14 In this follow-up study, we further elaborate on the use of ultrahigh resolution Fourier-transform mass spectrometry ■ EXPERIMENTAL SECTION NMR Measurements. Approximately 400 mg of the polymer was dissolved in 3.0 g of 1,1,2,2-tetrachloroethane-d 2 (TCE-d 2 ) with relaxation agent Cr(acac) 3 and transferred to a 10 mm NMR tube.…”