Backbone flexibility

The range of high-temperature rubbers is very small and limited to the silicones, already considered in this chapter, and certain fluororubbers. With both classes it is possible to produce polymers with lower interchain attraction and high backbone flexibility and at the same time produce polymers in which all the bonds have high dissociation energies and good resistance to oxidation. 

Before doing so, we briefly examine the influence of conformation and flexibility. Indeed, formation of succinimide is limited in proteins due to conformational constraints, such that the optimal value of the and ip angles (Sect. 6.1.2) around the aspartic acid and asparagine residues should be +120° and -120°, respectively [99], These constraints often interfere with the reactivity of aspartic acid residues in proteins, but they can be alleviated to some extent by local backbone flexibility when it allows the reacting groups to approach each other and, so, favors the intramolecular reactions depicted in Fig. 6.27. When compared to the same sequence in more-flexible random coils, elements of well-formed secondary structure, especially a-helices and 13-turns, markedly reduce the rate of succinimide formation and other intramolecular reactions [90][100],... 

The influence of backbone flexibility was seen, for example, when the stability of the linear tetrapeptide 6.70 was compared to that of a cyclic hexa-peptide derivative of the same sequence [91]. Indeed, the cyclic peptide was approximately one order of magnitude more stable than the linear peptide in the pH range of 2-7. The results at higher pH values were inconclusive since the stability of the cyclic peptide decreased dramatically due to the degradation of a disulfide bond absent in the linear peptide. 

Fig. 20 a-d. Four types of cylindrical brushes with different flexibility of the side chain and the main chain a stiff backbone - stiff side chains b stiff backbone - flexible side chains c flexible backbone - stiff side chains d flexible backbone - flexible side chains... 

Backbone flexibility enables axial shrinkage of the worm-like macromolecule compared to the contour length of the main chain. 

To do this, we define two additional multi dimensional spaces B and C (Figure 1). Space B contains the values of the catalyst descriptors that pertain to these catalysts e.g. backbone flexibility, partial charge on the metal atom, lipophilicity) as well as the reaction conditions (temperature, pressure, solvent type, and so on). Finally, space C contains the catalyst figures of merit (i.e., the TON, TOF, product selectivity, price, and so forth). Spaces B and C are continuous, and are arranged such that each dimension in each space represents one property. 

There is a considerable literature on the effect of chemical composition on Tg. A number of factors have been shown to influence Tg, including backbone flexibility, steric effects, polarity, pendant groups, crystallinity, plasticizers, crosslink density, and co-polymerization (13,5). After a brief qualitative discussion of the first few factors, we will consider the last two in more detail. 

The best evidence for metal fluxionality in Cd7-MT was provided by " Cd NMR saturation transfer experiments, which established the presence of inter- and/or intramolecular metal exchange within the 3-metal cluster of the /3-domain with a half-hfe of the order of 0.5 s. The confirmation of similar processes taking place within the 4-metal cluster, but with a half-hfe of about 16 min, was afforded by metal exchange studies using the radioactive Cd isotope. In this context, it should be noted that the intersite cadmium exchange measured in the 3-metal cluster of MT-1 was found to be much faster than that in MT-2. This finding was in line with the enhanced backbone flexibility recognized in the NMR studies of the former isoform. ... 

The silicones comprise a broad class of materials, including oils, elastomers, and resins. Because of backbone flexibility,... 

Over the years, our laboratory has generated a library of analogues of in which backbone flexibility was... 

Backbone flexibility was restricted by introducing three principal types of residue substitutions within 1)... 

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