Triple resonance Poly

Ruhnau and Veeman [143] showed that the abundant N spin can be used to examine miscibility. They applied the rotational echo N—triple resonance technique [144] to observed signals with and without N irradiation for PMAA/poly(V-vinylimidazole) (PVIm). They observed a significant loss of spin-echo intensity when " N is irradiated. The interpolymer distance was estimated to be less than 0.5 nm by a numerical model, which was also supported by Tip results. 

Three-dimensional (3D) NMR is an upcoming area that holds a lot of promise. Peter Rinaldi, who is an acknowledged leader in 2D and 3D NMR, has written a timely and authoritative review in a special Invited Paper (48). In addition, he and his coworkers have used 3D NMR and triple resonance methods to study poly(dimethylsiloxanes) (50), and fluoropolymers (49). 

Li et al. (10) used HCF triple resonance 3D NMR to assign the H, and " F resonances of poly(l-chloro-l-fluoroethylene) (PCFE). Figure 8 shows the ID NMR spectra of this polymer. The H spectrum contains very little useful information and the C spectrum shows essentially two clusters of resonances firom C-F and CHa groups. The F spectrum contains three groups of resonances in ca. 1 2 1 ratio, fi om mm, nor/rm and rr triad stereosequences. Application of HCF triple resonance 3D-NMR is particularly usefiil in this case, since has a natural abundance of 100% and an enormous chemical shift dispersion. The sensitivity of this erqreriment is comparable to that of a simple double resonance H C -HMQC experiment. 

Figure 9. Slices corresponding to flJ3 planes (a-c) from the HCF triple resonance 3D-NMR spectrum (d) of poly(1-chloro-l-fluoroethylene) (PCFE) at the shifts of the three F resonances. (Reproducedfrom reference 10. Copyright 1997 American Chemical Society.)...

Similarly, H/ C/ Si triple resonance 3D-NMR was used to study the structure of poly(l-phenyl-l-silabutane) (PPSB).f72 In this polymer, stereogenic centers are present at the Si atoms. While the H ID NMR spectrum only revealed two sets of resonances fix)m protons a and 3 to Si (Figure 10a), the ID Si NMR spectrum exhibited three peaks from mm, mr/rm and rr stereosequences (Figure 10c). As with PCFE, a H- C- Si chemical shift correlated 3D-NMR spectrum (Figure 11) permitted assignment of the resonances from the three triad steieosequences. 

H/ C/ Si Triple Resonance 3D NMR Study of Poly(dimethyisiloxane) MDjM ... 

The concentration dependence of the and NMR chemical shifts of tetramethyldisiloxanediol in CDCI3 has been investigated. The dynamics and structure of polydimethylsiloxane emulsions have been studied by pulsed field gradient NMR spectroscopy.A H- C- Si triple resonance three-dimensional NMR study of poly(dimethylsiloxane) has been reported. Orientation of compressed siloxane elastomers has been investigated using NMR spectroscopy. Transverse relaxation measurements have been used... 

The miscibility of PMMA and poly(vinylidene flu oride) (PVDF) has been established by various meth ods. Both F and F H CP and molecular miscibility in blends of PVDF and isotactic, syndio tactic and atactic PMMAs were investigated by triple resonance H, i F, 13C solid state CP MAS NMR. The fraction of nonmixed PMMA was determined for these blends, and it was found that this fraction was smaller for isotactic than for atactic and syndiotactic PMMAs. 

Not all large molecules are spherical they can also be long random coils with a certain degree of flexibility along the chain. If the chain is flexible, there are rapid local fluctuations between monomers and the dipole-dipole interactions are effectively averaged. An example of this is polyuridylic acid (poly U), which has a molecular weight of 100,000 daltons but is nonetheless very flexible. The line widths for poly U as a random coil are only a few hertz. If the chain is incorporated into a rigid double or triple helix, these local motions are lost and the resonances are broadened beyond detection (see Section 14.7.2). 

Pristine poly-DCH. Fig. 2 shows the C CP-MAS spectra of unreacted N-dicarbazolyl-2,4-hexadiyne and of the polymerized material. The appearance of a new peak at 129 ppm in the spectrum of poly-DCH, resulting from the formar tion of a double bond, is evident from the spectra. Furthermore, polymerization causes a marked downfield shift of the triple bond resonance(s) from 68.3 and 75.0 ppm in the diacetylenic monomer to 104.3 ppm in poly-DCH. This effect is well-documented for many polycoi jugated systems containing triple bonds (12,21), and presumably reflects an increasing cumulene contribution to the original enyne electronic structure of such systems. 

Nitration of poly-DCH. In contrast to the bromination reaction, the nitration reaction cannot be as easily controlled, and the resultant products are all highly nitrated. The results of some preliminary studies are shown in Fig. 5, where the spectra of a sample of composition poly-DCH(N02)7.6 and the model compound N-ethyl-3,6-dinitrocarbazole are compared. The sites of nitration, unlike those of bromination, can be directly observed in the CP-MAS NMR spectra, although they are somewhat broadened by the dipolar interaction with the quadrupolar nitrogen nucleus as discussed above. No triple bond resonance is detected in the nitrated polymer, while some poorly resolved absorption in the region near 130 ppm in the spectra obtained with delayed decoupling indi-... 

On the other hand, the other direction of addition (yd-addition) produces a monosubstituted alkylidene that is sterically comparable to the initiating alkylidene, making this the desired direction of addition. Formation of a six-membered ring in a reaction involving a more reactive terminal alkylidene would transform 43b into 44b. The NMR spectrum of poly-6 reveals two carbonyl carbon resonances. Therefore it was speculated that poly-6 consist of a random distribution of five- and six-membered rings formed through what is nominally tail-to-tail and head-to-head cyclopolymerization of the two acetylenic bonds in the same monomer (Scheme 1). Other uncertainties in catalyst systems of the type are the rate at which alkylidene rotamers interconvert, the extent to which the reactivities of the two alkylidene rotamers differ, and the degree of selective formation of one rotamer when a triple bond reacts with a Mo=C bond. ... 

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