NMR Spectroscopy for polymer chemists - Tom Wilks

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FT NMR • Since all environments are simultaneously excited, we actually see a superposition of lots of cosines. • The magnetisation also decays over time (i.e. it returns to being along the z axis). • Both of these give us the Free Induction Decay (FID) spectrum: FT NMR • The FID is then converted to the spectrum with which we are familiar by a Fourier transform: A little demonstration… http://www.youtube.com/watch?v=7aRKAXD4dAg&feat ure=player_detailpage Things to bear in mind • Higher temperature  lower sensitivity • High viscosity  poor results • Try increasing the number of acquisitions rather than the concentration • Possible to use non-deuterated solvents, but you must add a small amount of deuterated to provide a lock for the machine Things to bear in mind • Magnetization of the nuclei by a pulse begins to return to its original equilm value along the z axis and the xy plane immediately after the pulse • Return of the Z-component (Mz) to equilm value is longitudinal relaxation • Return of Mxy to zero is tranverse relaxation • Both are 1st order processes, characterised by the times T1 and T2 Things to bear in mind • Good data depends on use of appropriate T1 (spin lattice) and T2 (spin-spin) relaxation times • Width of line in NMR spectra determined by T2 • Short T2 broad lines • Maximum repetition rate during NMR acquisition is given by T1 (the delay time should be at least 5 times the longest T1) T1 always greater than or equal to T2 In polymers T1 much greater than T2 For small molecules… Relaxation mechanisms • T2 relaxation is caused by fluctuations in any direction • ie molecular motion • Define a correlation time (tc) for a molecule • This is the average time it takes one molecule to rotate by one radian Small molecule (Mn < 1000 Da), tc ca. 10-12 sec For a proton at 300 MHz u0 = 108 (Larmor procession frequency) Small molecules move too fast to relax by spin-spin relaxation But polymers do not…. For small molecules… Chemical shift • Rapid rotation Brownian motion of small molecules averages out dipolar and other anisotropic magnetic interactions • Called molecular tumbling • Leads to narrow lines in the NMR spectra • If tumbling (or rotation) is slow relative to the timescale of the NMR then the signal is broadened • Because different chemical shifts are observed • Brownian motion of molecules decreases with increasing size • Line width of an NMR signal increases as size increases Polymers… NMR signals always broad in the 1D NMR of polymers (each monomer is similar to its neighbours, therefore signals overlap) Interactions usually broaden the NMR resonance lines of a spectrum. In (non-viscous) liquids these interactions are averaged by rapid isotropic motions of the molecules – in polymers by bond rotations of the backbone and the side- chain or groups. The fast isotropic motions average the interactions to zero or to a single finite value representing the average interaction. Line broadening due to slow relaxation as a result of larger mass, therefore slower tumbling How is NMR useful in polymer science • Measurement of polymerisation conversion • End group analysis • Molecular weight determination • Stereochemistry • CMC determination ATRP O Conversion by 1H NMR CH3 H CH3 CH3O (CH2 )2O CH2 CH2 O Br + m x CH3 H O (1) O CH3 Selection of 1H NMR spectra N Cu(I)Br N Toluene recorded during the C5H13 polymerization of MMA on O CH3 COOCH3 COOCH3 MeOPEG-IX (X = 12, 45, 113). CH3O (CH2 )2O CH2 CH2 O CH2 CH2 Br x m CH3 CH3 CH3 (2) Alkene signals 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 Compare integration of monomer and polymer signals and work out conversion… Conversion by NMR Hp = 1.75 Hm = 1 Bromo-styrene Conversion = Hp Hp + Hm = 1.75/2.75 = 63.5% Conversion by NMR Error on this measurement? Copolymerizations are more tricky… example in the assessment Copolymerizations are more tricky… example in the assessment Conversion by NMR RAFT monomer (1-0.31)*100 = Ethyl monomer (1-(0.72-0.31)*100 = Conversion by NMR Error in A0 End Group Analysis • NMR = easiest way to get Mn for your polymer • For reliable numbers you need to ensure that: 1. Polymer signals do not overlap with the end group 2. The end group signal is well resolved 3. The integration is reliable Note: really only works well for polymers with Mn up to 30,000 Da End group analysis – RAFT example Ideally, use both end groups for comparison – this may also help you see if the Z group is stable. TMS can even be incorporated into the CTA!1 Other nuclei may also be useful…2 1) Päch, M.; Zehm, D.; Lange, M.; Dambowsky, I.; Weiss, J.; Laschewsky, A. J. Am. Chem. Soc. 2010, 132, 8757 2) Godula, K.; Rabuka, D.; Nam, K. T.; Bertozzi, C. R. Angew. Chem., Int. Ed. Engl. 2009, 48, 4973 End group analysis – RAFT example Ideally, use both end groups for comparison – this may also help you see if the Z group is stable. TMS can even be incorporated into the CTA!1 Other nuclei may also be useful…2 1) Päch, M.; Zehm, D.; Lange, M.; Dambowsky, I.; Weiss, J.; Laschewsky, A. J. Am. Chem. Soc. 2010, 132, 8757 2) Godula, K.; Rabuka, D.; Nam, K. T.; Bertozzi, C. R. Angew. Chem., Int. Ed. Engl. 2009, 48, 4973 End group analysis • Compare the α and ω signals. S O N O S 31 S C A B 10 O O C - can be affected O by grease A B 2.00 2.16 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 ppm (t1) End Group Analysis d g h f j a i b c m = 25 b d d e f, g c h e j i a ppm End Group Analysis Commercial polymer end group modification End Group Analysis Polymer end group modification Norbornene signals End Group Analysis - ATRP 2 DIFFERENT METAL CATALYSTS, 2 DIFFERENT END GROUPS End Group Analysis • Isotopic enrichment of the end groups is an option (albeit expensive) but this makes determination of Mn trickier. • If you have a protic end group you can derivatise it to make the protons easier to see.3 • It is possible to use 13C but bear in mind that this is: a) much less sensitive, and b) the thiocarbonyl signal usually lies outside the range of a normal scan. 3) Postma, A.; Davis, T. P.; Donovan, A. R.; Li, G.; Moad, G.; Mulder, R.; O'Shea, M. S. Polymer 2006, 47, 1899 Molecular weight - Mn • 1H NMR can also be used to calculate DP by comparing the integration of polymer to end group • This can in turn be used to calculate Mn, NMR S O N O S S C Set integration of A 31 B 10 O O A=2 D C - can be affected O Integration of D by grease = DP D Mn = (DP x Mr of monomer) + Mr of CTA A B 2.00 30.85 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 ppm (t1) Ref: J Chem Edu, 2011, 88, 1098 Molecular weight - ATRP Mn = 5,000 Da Molecular weight Poly(ethylene glycol) diacrylate, commercially available from Aldrich Copolymerisation composition LCST measurement A graph to show the ratio of 1H NMR signal intensities for PNIPAM C-H next to methyl groups (assigned as e in inset, at 3.8 ppm) as a function of temperature in D2O solution (6 mg/mL) for the PMA27-b-PNIPAM47 diblock copolymer.