Nuclear magnetic resonance spectroscopy of polypropylene homopolymers

The main advantage of NMR in the characterization of polymers is the unique molecular structured information which can be obtained. For instance, the relative configurational distribution (tactidty) and regio-irregularities (defects) in homopolymers of polypropylene (PP) can be mapped out and represent a unique microfingerprint of the polymer. In particular, this information contains indirect and vital information about the stereospecificity of the catalyst, which in turn enables one to improve the design of the catalyst. 

Spectral resolution is also limited by sample characteristics as mirrored in the line width of the resonance peaks which is dependent upon polymer concentration and acquisition temperature. Dissolving the polymer at relatively high temperature followed by temperature cycling improves the resolution, i.e. reduces the line width. Experience suggests that a polymer concentration of approximately 10-15 wt.% gives the narrower lines with an acceptable signal-to-noise ratio. 

When comparing the pentad integral areas of the methyl resonances two necessary relations (equations (1) and (2)) should always be considered [2]. 

The potential use of solid-state CPMAS NMR to detail the molecular structure of PP is demonstrated by many authors and is nicely reviewed in a recent paper by ToneUi [3]. The key parameter in such an analysis is the -y-gauche effect which enables the NMR chemical shifts of vinyl polymers to be assigned and provides an opportunity to test or derive rotational isomeric state (RIS) model descriptions of their conformational characteristics. 

The isotropic chemical shifts of 7-trans/7-trans (tt) and 7-gauche/7-gauche (gg) conformations known from sPP and of 7-trans/ 7-gauche (tg) from the 3j helix of iPP give the basis for the interpretation of the spectra of aPP. At 20° above the glass transition the exchange of conformations is rapid. 

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Nuclear magnetic resonance spectroscopy of polypropylene homopolymers... 

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