A Review of High Resolution Liquid13carbon Nuclear Magnetic Resonance Characterizations of Ethylene-Based Polymers

The use of 13 carbon nuclear magnetic resonance (NMR) spectroscopy in the molecular characterization of macromolecules has advanced our knowledge into structural areas that have been nearly impossible to measure by other spectroscopic techniques. Innovative applications have led to determinations of polymer configurational distributions, comonomer sequence distributions, average sequence lengths, structure and distribution of short chain branches, and analyses of nonreactive end groups. As a result, the importance of 13C NMR to the field of polymer science cannot be overemphasized. The key to the success of 13C-NMR studies in defining polymer molecular structure has been a structural sensitivity which encompasses more than just a few functional groups or carbon atoms. A sensitivity to polymer repeat unit sequences of lengths from two to as many as five, seven, and even nine contiguous repeat units 1,2 has been observed. Of course, any structural technique that senses a unique response from as few as two successive repeat units will lead to a measurement of average sequence lengths 1,3 and run numbers 4. In addition to this excellent structural sensitivity, there has been an enormous improvement in the quantitative sensitivity of 13C NMR in recent years. Detection of long-chain branching in polyethylene can now be made at a level of one per ten thousand carbon atoms 5, and newer generation of high field, higher sensitivity NMR spectrometers promise to extend this detection limit another order of magnitude.