NMR applications in polyolefins: beyond resolution

13C NMR spectroscopy is the elective technique of polyolefin microstructure analysis, and the progress in this spectroscopy parallels that in our stereochemical understanding of the polymers (which is quite obvious) and the reactions to make them.[1] In the latter respect, one should realize that each macromolecule is like a tape where the story of the polymerization is faithfully and sequencially recorded. In order to know that story, of course, one must be able to read the tape. In the 1990s, our group demonstrated that the use of high-field 13C NMR can lead the resolution of polypropylene spectra to an unprecedented level of detail, with substantial advances in the determination of polymer configuration.[1] Some years later, we achieved similar advances with olefin copolymers.[2] On the other hand, the intrinsically poor sensitivity of natural abundance 13C NMR spectroscopy remained an obstacle preventing evaluations of many important microstructural features such as e.g. chain ends, regiodefects, long-chain branches etc. It was only recently that the introduction of high-temperature cryoprobes improved the picture dramatically.[3] With a signal-to-noise (S/N) ratio higher by roughly 1 order of magnitude compared with conventional ones, the new probes can be applied to speed-up spectral acquisition (by roughly 100 times to achieve a given S/N) and/or lower the threshold for detectability. The former application is especially valuable in combination with High Throughput Experimentation (HTE); the latter, in turn, makes it possible to ‘see the invisible’. This talk will highlight both aspects for a number of representative case histories.