P-Methylene protons

The fast motion spectrum of the /-PMMA radical consists of 21 lines attributed to three separate isotropic hyperfine coupling constants. There is coupling to the methyl group to form a quartet (22.9 G) that is then split further into a triplet from one set of p-methylene protons (16.4 G) and another triplet from the other set (11.7G). Theoretically, this should lead to 36 lines (4 x 3 x 3), but a fortuitous degeneracy exists because one of the fast motion p-methylene couphng constants is almost exactly... 

ESR spectra of short-lived radicals in the liquid state. Applying it to the radical polymerization of AA, MAA, and itaconic acid (ITA), Fischer et al. <> -56) observed ESR spectra of monomer, dimer, and polymer radicals and discussed the conformations of these radicals in terms of the hyperfine splitting constants for their P-methylene protons. Ranby et al. 6i> extended its application to the radical polymerization of several monomers such as vinyl esters and butadiene and also to the copolymerization of binary monomer systems. However, the use of the thermal-redox radical-generating method has been chiefly restricted to reactions in aqueous solution. 

The well-resolved 16-line spectrum shown in Fig. 30 a was obtained in the polymerization of MA in aqueous solution at pH 1. It consists of a doublet of doublets of quartets for unequal p-methylene protons and methyl protons, in which the hyperfine splitting constant is 13.75 and 11.04 G for the former and 22.46 G for the latter. Therefore, it may be concluded that the spectrum is due to the propagating radical of MAA. Fischer et al. found this spectrum to be similar to the 9-line spectrum for MAA polymerized in the solid state, shown in Fig. 30b, and indicated that the 9-line spectrum probably resulted from the broadening of each line of the 16-line spectrum. 

Ranby et al. observed a well-resolved spectrum shown in Fig. 31 in the polymerization of VAc with Ti /HjO, and described it as a doublet of triplets of narrow quartets, in which the hyperfine splitting constant was 21.0, 12.5, and 1.4 G for the a-proton, two p-methylene protons, and methyl protons of the ester group, respectively. The spectrum was assigned to... 

The spectrum for MMA in Fig. 51 consists of 13 lines with the intensity distribution 1 2 1 4 3 3 6 3 3 4 1 2 1 for a single conformation, in which the dihedral angles of p-methylene protons with the p-orbital of the unpaired electron are 55° and 65°. Similar 13-line ESR spectra were obtained in the radical polymerization of IBMA and BzMA. However, the intensities of the 8 lines, each between the other 5 lines, relative to those of the 5 lines were weaker in these methacrylates than in MMA. The 8 lines became weaker with an increase in the bulkiness of the ester group. For TPMA, whidi has a bulky ester group, the 8 lines scarecely appeared, and hence the spectrum consisted of only 5 lines spaced at 23 G. 

The ESR spectrum Fig. 53 a was obtained in the radical polymerization of iso-propenyl acetate (IpAc), and can be compared favorably with the spectrum of Fig. 53 b, which was simulated with a doublet of doublets of quartets due to unequal p-methylene protons and p-methyl protons, in which the hyperfme splitting constant is 15.0 and 13.6 G for the former and 23 G for the latter Thus, the observed spectrum... 

First, a mixture of oligomers containing 2-7 monomer units (P = 2-7) was prepared by ATRP and model radicals with short-chain lengths were generated from the mixture without any further separation. Well-resolved ESR spectra of the model radicals were observed at various temperatures. The 12-line ESR spectrum observed at 150°C is shown in Fig. 7a the two p-methylene protons are almost equivalent in small radicals at such high temperature. This finding indicates that rotation of the radical chain end is too fast to detect differences in methylene protons on the time scale of ESR spectroscopy. In order to estimate the critical chain length that would show the 16-line spectrum, model radical precursors with... 

Fig. 6. Simulated spectra for nonequivalent 3-methylene protons (16-line spectrum) and equivalent P-methylene protons (12-line spectrum) for propagating radicals of fBMA.

Since there is no J coupling between the CH2 and a-CHs protons, the NMR phenomenon for poly(methyl methacrylate) is relatively simple. In the proton NMR spectrum, the ester methyl protons appear near 6.5r, the p-methylene protons appear near 8.0r, and the p-methyl protons appear between 8.5 and 9.0r. Figure 19.9 shows the p-methylene proton spectrum Figure 19.10 shows the p-methyl proton spectrum. Both were observed at 220 MHz. In both spectra, the ester methyl protons appear near 6.5r, but, in order to show the details of p-methylene protons and p-methyl protons, they were not shown in the two figures. In both cases, the sample was prepared by dissolving poly(methyl methacrylate) in 10-15% chlorobenzene, with the temperature of the NMR measurement at 135°C. 

FIGURE 19.9 The 220-MHz P-methylene proton spectra of poly( methyl methacrylate) in chlorobenzene at 135 C (a) syndiotactic (b) isotactic. [Source Bovey (1972). By permission of Dr. Bovey and Academic Press.]... 

The /aH,pH vicinal coupling constants can be determined from Exclusive COSY (E-COSY) type experiments and, together with intraresidue NOEs, can be used to obtain stereospecific assignments of P-methylene protons and side-chain x angle restraints (Table 2). 

Figure 9 Model for the difference electron density of CoilSerL16Pen using either (a) Cys or (b) Val as a model for the non-coded for amino acid penicillamine (Pen) in the interior of a three stranded coiled coil. Shown is a top down view bom the N-terminus of the three-stranded coiled coil (PDB 3H5F) during the building of the Pen side chain [64]. The 2Fa-Fc map (blue, contoured at 1.5a) and Fq-Fc map (green, contoiued at 3o) after one round of refinement, show that the density for the methyl groups (at the position of the P-methylene protons) is missing for the Cys model, as is the thiol group when Val is used. The other amino acid side chains have been omitted for clarity.

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