Xiaohua Qiu

13C NMR of polyolefins with a new high temperature 10 mm cryoprobe

Recently, a high temperature 10 mm cryoprobe was developed. This probe provides a significant sensitivity enhancement for (13)C NMR of polyolefins at a sample temperature of 120-135 degrees C, as compared to conventional probes. This greatly increases the speed of NMR studies of comonomer content, sequence distribution, stereo- and regioerrors, saturated chain end, unsaturation, and diffusion of polymers. In this contribution, we first compare the (13)C NMR sensitivity of this probe with conventional probes. Then, we demonstrate one of the advantages of this probe in its ability to perform 2D Incredible Natural Abundance Double Quantum Transfer Experiment (2D INADEQUATE) in a relatively short period of time. The 2D INADEQUATE has been rarely used for polymer studies because of its inherently very low sensitivity. It becomes even more challenging for studying infrequent polyolefin microstructures, as low probability microstructures represent a small fraction of carbons in the sample. Here, the 2D INADEQUATE experiment was used to assign the (13)C NMR peaks of 2,1-insertion regioerrors in a poly(propylene-co-1-octene) copolymer.

Error Analysis for NMR Polymer Microstructure Measurement without Calibration Standards

We report an error analysis method for primary analytical methods in the absence of calibration standards. Quantitative (13)C NMR analysis of ethylene/1-octene (E/O) copolymers is given as an example. Because the method is based on a self-calibration scheme established by counting, it is a measure of accuracy rather than precision. We demonstrate it is self-consistent and neither underestimate nor excessively overestimate the experimental errors. We also show the method identified previously unknown systematic biases in a NMR instrument. The method can eliminate unnecessary data averaging to save valuable NMR resources. The accuracy estimate proposed is not unique to (13)C NMR spectroscopy of E/O but should be applicable to all other measurement systems where the accuracy of a subset of the measured responses can be established.

Unexpected proton spin-lattice relaxation in the solutions of polyolefin and tetrachloroethane.

‘Unexpected’ proton spin‐lattice relaxation (T1) times are reported for the solutions of poly(ethylene‐co‐1‐octene) and tetrachloroethane‐d2. For the residual protons of the deuterated solvent and the methyl and vinyl protons at the polymer chain ends, their T1 relaxation times vary significantly with both the polymer concentration and molecular weight over a wide range. The T1s also decrease with increasing temperature at relative high temperatures. Such behaviors are in contrast to most reported polymer solutions in which the T1 has nearly no concentration or molecular weight dependence in the dilute and semi‐dilute regime, and normal dependence on temperature. Further investigation revealed that the paramagnetic oxygen effect did shorten the measured proton T1s, but cannot account for the unexpected T1 dependences. Spin rotation is proposed to provide a reasonable explanation. Copyright