Copolymer Characterization with Benchtop 60 MHz NMR

Garbage bags, car tires, pantyhose, your credit cards, non-stick pans, Styrofoam, spandex, hairspray, the lenses of your glasses…….the average person doesn’t realize that polymers are, well, EVERYWHERE!!  It’s hard to believe that research in this field didn’t really begin until WWII with synthetic rubber and such things.

The fundamental properties of each polymers can be varied through a number of factors (e.g., chemical compositions, structure, tacticity).  So, it goes without saying, that in order to consistently prepare the same polymers with the same characteristics and/or performance, these factors must be carefully QA/QC’d.  There are numerous, often time consuming, analytical methods used (e.g., mass spectrometry (MS), Gel Permeation Chromatography/Size Exclusion Chromatography (GPC/SEC), inherent and intrinsic viscosity (IV), melt rheology and melt flow index (MFI) to determine average molecular weight and polydispersity index (PDI)).

More recently, NMR Spectroscopy has proven itself to be a useful technique to add to the characterization repertoire because in a single experiment you can:

1)   Observe reaction completion & uniformity (i.e., unreacted monomers are easily observable due to well-defined, resolved lines among the broad polymeric resonances).
2)   Quantify the relative composition of the structural components through pre-defined integral regions.
3)   Quantify the relative percentages of structural isomers (e.g., branched vs. linear).
4)   Observe the presence of stereoisomer and determine poylmer tacticity,
5)   Determine molecular weight (Mw), molecular number (Mn) and (PDI).

Polymer characterization has been performed on traditional NMR systems for years.  This can be expensive and, depending on availability, time consuming.  Given that polymer NMR is often defined by broad, poorly defined peak areas and resolution is not an issue, we were interested in determining whether or not benchtop 60 MHz NMR could provide comparable results to high field instruments.

Soooo…..we gathered and compared the data acquired on the NMReady with that acquired at high field (400 MHz for the sake of this study).  We used three samples: polystyrene-polybutadiene (PS-PB) and two polystyrene-polyisoprene (PS-PI) with differing wt% compositions of polystyrene.

When compared with the data acquired on a 400 MHz spectrometer, it is clear that although the resolution of chemical shifts at 60 MHz isn’t quite as good, the peak area resonances are still resolved.  At either field, it is clear that the reaction is complete and that there is no monomeric material remaining.

Example 60 MHz 1H NMR spectrum for PS-PB

Example 400 MHz spectrum for PS-PB in d-CHCl3

The relative composition of the copolymer can be reliably determined by: (i) normalizing; and (ii) correlating the integrals of the aromatic region (i.e., the styrene component) and the olefin region (i.e., the butadiene or isoprene component).  Sample tabulated results are statistically equivalent and included at the end of this post for comparison.

Similarly, the internal/terminal isomers of the olefin component can be quantified by comparing the relative ratios in the 5-6 ppm range.  The difference in %internal/terminal olefins is more pronounced than % composition, although still highly comparable.

The results obtained are also consistent for the two samples of PS-PI shown below.

Example 60 MHz 1H NMR spectra for two different polymeric samples of PS-PI

Example 400 MHz spectra for two polymeric samples of PS-PI in d-CHCl3

The characterization of polymers is only potential application for the NMReady-60.  For more information on this, or for other ideas of how to include a benchtop NMR spectrometer into your daily workflow, please contact us!