NMR or Polypropylene

Introduction

¹³C NMR spectroscopy can be used to determine the tacticity of polypropylene and to determine the amount of regio errors.

NMR experiment

Sample preparation

Use a normal NMR-tube (cleaning afterwards is quite tedious). Put 25-30 mg polypropylene in the tube and an appropriate amount of deuterotetrachloroethane (CDCl₂CDCl₂). Replace the atmosphere above the solution by nitrogen (purge, using a needle) and put on the cap (the polypropylene will decay in time, but this will slow it down enough for a couple days). Heat the tube in an NMR-tube oven @ 80°C until a homogeneous, viscous solution is obtained.

Experiment setup

Use the 500 MHz NMR for this experiment. Tune the ¹H channel (to make sure that decoupling will be OK) and shim, program a ¹³C NMR experiment at @ 75°C. Tune and shim the spectrometer again after changing the temperature. Record an NMR, to see is all is OK (solvent shows up at 74.12 ppm; McKnight et al. Organometallics 1997, 16, 2879). With 100 pulses you should be able to see the signals of the polymer (three larger sets, one large singlet at approx. 46 a multiplet (CH₂-groups), a large ‘singlet’ at 27 (CH-groups) and a multiplet at approx. 20 ppm (CH₃-groups). Sometimes also small signals between 46 and 27 can be seen, and signals below 20, these are regioerrors (see below). Adjust the spectral window to the regio incorporating the solvent and all signals (say: 80 to 0 ppm), expand and type ‘movesw’
Settings:

• Pulse and decoupler power, sufficient
• d1: 10 seconds
• pw: 90 degree pulse
• nt: as many as possible considering the time (with 5000 pulses you get a reasonable spectrum).

Assignment

The signals between 22 and 19 ppm are from the methyls of the polymers. Typically triplet of triplet (at 125 MHz, 7000 pulses). Sometimes more finestructure can be observed. The left triplet is the mm-triad; middle mr/rm triad; right rr triad.
Signals between 18 and 14 ppm are ‘head to head’ methyl groups (regioerrors). Dividing the half of the intensity of the signals in this region by the total methyl region is the amount of regioerrors (in ‘formula’: 0.5 * integ(‘18-14 ppm’)/integ(‘22-14 ppm’)).