Protons, aromatic backbone

The NMR spectra of copolymers prepared by simultaneous oxidation of the two phenols and those prepared by sequential oxidation, in either order, are almost identical. The methyl peak is broadened, as is the peak caused by the protons of the pendant phenyl rings centered at 8 7.20 ppm, and all show the same peaks for aromatic backbone protons in about the same intensity ratios. The polymer obtained by oxidizing a mixture of DMP and the separately prepared homopolymer of MPP with a cuprous bromide-tetramethylbutanediamine catalyst, the procedure considered to have the best chance of producing a block copolymer, was completely random. 

The soluble and insoluble fractions were examined separately. The insoluble fraction, which made up 35% of the total, had the NMR spectrum expected of a DPP-rich block copolymer, with a sharp methyl proton signal and only one strong signal, at 8 6.46 ppm (PPP), in the aromatic backbone region. The composition, from comparison of the integrated intensities of the methyl and backbone proton signals, was 82 mole % DPP and 18% MPP. The soluble fraction had the spectrum expected of a block copolymer with about 65% MPP units. Since a coprecipitated blend was separated almost quantitatively into the pure homopolymers with m-xylene under these conditions, the copolymer is characterized as a block copolymer. 

When we determine values of 7 for benzene rings attached to polymers, we find that they are much shorter than for benzene itself they are in fact at the lower limit of usefulness of the direct method. We have found (3) that for the phenyl resonance of a low molecular weight polystyrene in carbon disulfide 7 is only about 0.4 0.1 sec. at 25°. For the backbone protons, 7 is even shorter and cannot be measured by the direct method. For the phenyl groups of poly-y-benzylglutamate in trifluoroacetic acid, 7 is 0.7 0.1 sec. Recently, more reliable values have been obtained for polystyrene by the spin echo method (72). For a polymer of molecular weight 63000 in tetrachloroethylene, three 7 X values could be resolved. At 25°, these were 0.033 sec. for the aliphatic protons, 0.076 sec. for the ortho protons and 0.20 sec. for the meta-para protons. The temperature coefficients for all three Tt values correspond to a heat of activation of about 3 kcal/mole. The differences in the absolute Tx values at the aromatic positions can be explained in terms of of differences in the sums of r e in equation (56). The shorter aliphatic Tj value probably reflects also a longer correlation time for backbone... 

The direct observation ofmolecuiar l backbones., 7 - The direct observation of carbon 5 containingfunctionalgroupsthat have no. attached protons (e.g.-hCN)T, The direct observation of carbon Reaction sites. The ease of quantitative analysis. / The rapidity of aruilysis time. The direct observation of OHandNH % groups (undetedtable by UCNMR). The separation of olejmic and aromatic f protons (olefinic and aromatic carbon resonances overlap). ... 

Proton Sponges with Other Aromatic Backbones... 

After the discovery of OMAN (1), the search started for other, more basic proton sponges possessing aromatic backbones with optimal, even shorter N... N distances to form N H+... N bridges with the optimal linear geometry. Staab has found that fluorene series... 

As an introduction, our previous studies on the conformations of maleic acid copolymers with aromatic vinyl monomers are summarized. To characterize the compact form and the pH-indueed conformational transition of the maleic acid copolymer with styrene in aqueous NaCl, 400 MHg H-NMR spectra were measured. The spectral form depended on the molecular conformation. Because each of proton resonance peaks could not be separated, the spin-lattice relaxation time T was estimated by using the inversion recovery technique (tf-t-tf/2). The T s for both side chain and backbone protons reflected the transition, and the protons were considered to be in a more restricted motional state in the compact form than in the coil form. Also, from temperature dependence of each Tj, motion of the copolymer in the coil form was described in terms of the local segmental jump (D) combined with the isotropic rotational motion (O), when a ratio between both the correlation times tq and Tq was about 0.07. For the compact form, the ratio was found to be about 10. By referring to theoretical diagram of Tj vs. tq for the methylene protons on the backbone, value of Tn for the compact form was compared with that for the coil form at 35 C. 

Among these, acidity of ionic groups and membrane morphol-ogy appear to be crucial, and they are inter-related. Kreuer et al. [23] reported that typical sulfonated aromatic polymers are unable to form defined hydrophilic domains, as the rigid aromatic backbone prevents the formation of continuous conducting channels. Thus, various strategies have been pursued to obtain efficient ionic networks for enhancing the proton conductivity. 

The MI NMR solution spectrum of SAN copolymers. Figure 1, shows that the aromatic and backbone protons are well separated and the peak areas are easily quantified to provide the composition of the copolymers. However, it is not possible to obtain sequence distribution information from the Mi NMR spectrum of these polymers. 

The rigid aromatic backbone chemical structure and the strong intermolecular H-bond interactions between the -NH and -N=C groups of PBI polymers (Figure 1.25) [215] make the PBI membranes brittle, which restricts the H3PO4 doping level and limits the applications of PBI membranes to HT-PEMFCs. Several efforts have been exerted in modifying PBI membranes to improve their proton conductivity and mechanical properties for HT-PEMFC applications, which have been... 

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