Poly side-chain carbons

Spin-lattice relaxation times and 13C chemical shifts were used to study conformational changes of poly-L-lysine, which undergoes a coil-helix transition in a pH range from 9 to 11. In order to adopt a stable helical structure, a minimum number of residues for the formation of hydrogen bonds between the C = 0 and NH backbone groups is necessary therefore for the polypeptide dodecalysine no helix formation was observed. Comparison of the pH-dependences of the 13C chemical shifts of the carbons of poly-L-lysine and (L-Lys)12 shows very similar values for both compounds therefore downfield shifts of the a, / and peptide carbonyl carbons can only be correlated with caution with helix formation and are mainly due to deprotonation effects. On the other hand, a sharp decrease of the 7] values of the carbonyl and some of the side chain carbons is indicative for helix formation [854]. 

Fig. 19. Top C-NMR spectra of the amorphous component of solid poly(p-hydroxyalkanoates) as a function of temperature. Spectra were obtained without MAS. Only the aliphatic region is shown, and the backbone and side-chain carbons are labeled bb and sd, respectively. The differential dynamics of the backbone and side chains is evident in the differential line widths. Bottom Schematic behavior of the line width, under conditions of decoupling, when the...

The water content in the PMAA gel contributes to the segmental motion of the network polymer as described above. On the other hand, the restraint of chain mobility in poly(A-vinylpyrrolidone) gel, caused by the presence of crosslinking and entanglement of polymer chains, was identified [35]. The Ti for the PMAA in the gel was also measured as a function of the degree of crosslinking with constant water content. The Ti values obtained for the main and side chain carbons of the network polymer in the PMAA gel (q = 3.4), as a function of the mole ratio of MBAA to MAA (F) and uncrosslinked PMAA solution (29.4 w%), are plotted in Fig. 20.12(a, b). 

Fig. 22.6. Expanded CP/MAS NMR spectra of the main- and side-chain carbons of poly( 3-octadecyl L-aspartate) in the solid state as a function of temperature.

Most commercial polymers are substantially linear. They have a single chain of mers that forms the backbone of the molecule. Side-chains can occur and can have a major affect on physical properties. An elemental analysis of any polyolefin, (e.g., polyethylene, polypropylene, poly(l-butene), etc.) gives the same empirical formula, CH2, and it is only the nature of the side-chains that distinguishes between the polyolefins. Polypropylene has methyl side-chains on every other carbon atom along the backbone. Side-chains at random locations are called branches. Branching and other polymer structures can be deduced using analytical techniques such as NMR. 

Compression-molded devices of poly(N-palmitoyl hydroxyproline ester) (side chain length 16 carbons), poly(N-decanoylhydroxy-proline ester) (side chain length 10 carbons), and poly(JN-hexanoyl-hydrox roline ester) (side chain length 6 carbons) were prepared with dye contents of 1,5,10, and 20% of either -nitroaniline or acid orange. Release curves were obtained by placing the loaded devices into phospate buffer (pH 7.4) at 37 C. The amount of released dye was followed spectrophotometrically in the usual fashion. 

Two stable radical intermediates are observed following gamma radiolysis at 303 K. The alpha carbon radical VIII and the side chain radical IX are formed in approximately equal yields, with the total G-value for radical production equal to 3.2. This value is similar to that observed for the poly acids. The observed radicals are those which would be expected on the basis of the aliphatic carboxylic acids and previous studies of the poly amino acids with aliphatic side chains. 

Figure 50, Schematic function of the t-Bocstyrene resist, S is a sensitizer such as diphenyliodonium hexafluoroarsenate which undergoes radiolysis to produce a strong acid (A), The acid attaches the side chain of the poly-(t-Bocstyrene) where it catalyzes acidolysis of the carbonate to liberate CO2 and isobutylene and free the phenolic hydroxyl group to produce poly (p-hydroxystyrene) in the exposed areas of the resist film. The acid A is a catalyst and can cleave many carbonate groups.

An example where Independent, but similar, assignments were obtained occurs for the side-chain carbonyl carbons in polymethylmethacrylate (W) and the quaternary aromatic carbons of poly- C-methylstyrene (17). Both are sp2 carbons otherwise they are in completely different environments. The assignments, as they occur from low to high field, are ... 

The poly (amino acid)s with aromatic side chains behave somewhat differently. In poly(phenylalanine) the a-carbon radical is the major radical species observed, but radicals formed by hydrogen atom addition to the ring are also found. Benzyl radicals formed by side-chain cleavage are present, but only in very low yield. In poly (tyrosine) the only radical species observed is the tyrosyl phenoxyl radical formed by loss of the hydroxyl hydrogen. There is no evidence for formation of significant concentrations of a-carbon radicals. Thus, the nature of the substituents can strongly influence the radiation sensitivity of the backbone chain. 

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