Nature of the Backbone

H-shaped dimer in which the mesogenic units are laterally attached 

T-shaped dimer containing a terminal and a laterally linked mesogenic unit. 

---

However, the significant changes in the electronic spectrum of PDHS at the transition temperature suggest that the nature of the backbone chromophore is also being altered during the process. 

As these results and Fig. 2 show, three structural components may be defined in lipid A (/) the lipid A backbone consisting of a pyranosidic HexN disaccharide and phosphate groups, (ii) substituents of the backbone phosphate residues (polar head groups), and (iii) fatty acids. Therefore, lipid A of different bacteria may be classified according to the nature of the backbone constituents (GlcpN or GlcpN3N), the type and nature of the polar head groups, and features of the acylation pattern. In a few instances, other backbone substituents have been encountered. These will be described later in conjunction with individual lipid A forms. 

It is known that increased char yield is usually associated with improved flammability behavior ( 1). This can be understood if one considers that the volatile flammable products can only diffuse with difficulty through the char, and that the thermal conductivity of a porous char layer is relatively poor (2). The structure of the polymer can contribute to the amount of char formed based on the character of the functional groups present and the nature of the backbone (2,3). Ritchie ( ) found that for a series of unsaturated polyesters and their copolymers, the temperatures at which carbon dioxide is eliminated was in the range of 280 to 345°C depending on the structure of the polyester. Aliphatic polyesters and their copolymers have less thermal... 

The carbenes derived from six- [20-22] and seven-membered heterocycles [22-24, 70] of type 14 can be obtained both with an aliphatic or an aromatic backbone. The characteristic NMR resonances for the C2 carbon atoms in six-membered carbene heterocycles (8 236-248 ppm) [20, 21] have been recorded in the same range as the Ccarbene resonances for saturated imidazolidin-2-ylidenes of type 7. Dimerization to enetetramines occurs for some carbenes derived from a six-membered heterocycle [21] and this behavior is independent from the nature of the backbone (saturated or unsaturated), but depends strongly on the steric bulk of the N,N -substituents. 

Within each of these zones, the temperatures can be changed depending on the exact nature of the backbone, isocyanate, and curative. 

Nature of the backbone Nature of the branches Method useda Reference... 

The thermomechanical properties of an organic material mainly depend on two factors. Firstly the molecular relaxations (crystalline melting point and glass transitions) which determine the temperature upper limit for applications, and secondly the chemical nature of the backbone which is responsible for the stability in a harsh environment. 

Covalent linking remains the most popular approach. The important variables are the degree of cross-linking of the polymer, the chemical nature of the backbone, the location and number of attachment points on the complex (one or two), and the presence of a spacer between the support and the complex. Since reviews of this subject have appeared recently, only the most illustrative examples are discussed (87, 88). 

Graft polymers consist of a main polymer chain, the backbone, with one or more side chains attached to it through covalent bonds, the branches. Graft copolymers are comb-shaped polymers where the chemical nature of the backbone and the branches differs. The chemical nature and composition of the backbone and the branches differ in most cases. Branches are usually distributed randomly along the backbone, although recently advances in synthetic methods allowed the preparation of better defined structures.88,90... 

For symmetrical polymers, the chemical nature of the backbone chain is the important factor determining the chain flexibility and hence Tg. Chains made up of bond sequences which are able to rotate easily are flexible, and hence polymers containing -(-CH2-CH2-)-, -(-CH2-0- H2-)-, or -(-Si-O-Si-)- links will have correspondingly low values of Tg. For example, poly(dimethyl siloxane) has one of the lowest Tg values known (-123°C) presumably because the Si-0 bonds have considerable torsional mobility. 

The proportion of salt groups that resides in either of the two environments in a particular ionomer is determined by the nature of the backbone, the total concentration of salt groups, and their chemical nature. Despite considerable research by various groups the details of the local structure of these materials remains somewhat obscure, as does the question of how low molecular weight polar impurities such as water affect the local structure. 

The ferrichromes, Figure 11, have natural optical activity associated with the ligand accordingly, the metal complexes have overall optical activity. An examination of molecular models of the ferrichromes indicates that the trans-isomers are improbable due to the cyclic nature of the backbone of the ligand however, both A and A cis-isomers are possible. 

Moreover, the synthesis of cyclic oligocarbamates was successfully achieved [32], First, the linear oligomers, three to six residues in length, were assembled on a solid support. Then, they were cyclized in solution. The resulting constrained conformation and the lipophilic nature of the backbone make the cyclooligocarbamates very interesting for lead structure search in drug discovery. 

An objective in synthesizing poly (butadiene-g-styrene) by cationic grafting onto was to study the morphology of such grafts by transmission electron microscopy and the effect of composition on graft morphology. The unsaturated nature of the backbone permitted such a study. 

The nature of the charged groups (sulfate or sulfonate) and the nature of the backbone of the polyelectrolyte have little effect on the phase diagram of polyelectrolyte/counterion mixtures. Therefore the electrostatic attractions can be considered as the principal driving force for phase separation of highly charged strong polyelectrolytes in the presence of multivalent counterions. A universal behavior is observed whatever the nature of polyelectrolyte or counterions. 

Recently the study of the dilute solution behavior of polymacromonomer, a limiting case of graft copolymer where each repeat unit carries a grafted chain, has been initiated. The main interests are focused on the dependence of conformation and size of the whole molecule on factors such as nature of the backbone and side chain, molecular weight of backbone and grafts, solvent interactions with respect to both components, and chain stiffness induced on the backbone due to the high grafting density and its dependence on the aforementioned factors [310-313]. 

Some studies dealing with the solid state properties of graft copolymers with liquid-crystal side chains have appeared in the literature [353-355]. The main attention is focused on the influence of molecular parameters (nature of the backbone and the side chains, molecular weight, etc.) on the mesomorphic properties of the liquid-crystal side chain. Salt complexes of PEO grafted onto various backbones were also considered [356] and the influence of the nature of thebackbone and salts on the conductivity of the resulting materials was studied. 

The effects of the polymer geometry and of the nature of the backbone are interdepending. Considering Fig. 29, only a hydrophobic backbone will provide a hydrophobic interior of polymeric micelles for polysoaps of the tail end geometry, whereas only a hydrophilic backbone will provide a favourable polarity profile for polysoaps of the head type . Accordingly, optimized ... 

The properties of polymers are dependent on many factors including inter-and intrachain bonding, the nature of the backbone, processing events, presence/absence of additives including other polymers, chain size and geometry, and molecular weight distribution. 

Figure 2. Structures for the mannoproteins whose acetolysis patterns are shown in Figure 1. The structures illustrate the nature of the backbone and sidechains but they are not intended to indicate the order or number of sidechains.

---