Macromolecular skeleton

Screw structures or helices (helix Greek = winding, convolution, spiral) are encountered in various variations in nature and technique. Propeller-shaped, helical structures play an important role in architecture, physics, astronomy and biology. Screw-shaped macromolecular skeletons of nucleic acids, proteins and polysaccharides are important structural elements in biochemistry. Their helix turns often are stabilized through hydrogen bonds, metal cations, disulfide linkages and hydrophobic interactions. 

In the area of molecular recognition, many attempts have been made to design host molecules capable of complexation of guest molecules. Host molecules that can recognize and stabilize transition states would behave as effective artificial enzymes. Attempts to design artificial enzymes based on host molecules with two or more catalytic elements will be briefly mentioned here before discussing construction of active sites of artificial enzymes on macromolecular skeletons. 

Thus, the density of chemical crosslinking points cannot serve as an index for the cormectivity of the macromolecular skeleton of network polymers. This makes it impossible to use to characterise the structure of network polymers in a computer simulation, which follows from the results presented previously. The d value, which provides determination of elastic properties, may serve as a suitable parameter. However, to estimate other properties, one more parameter is required, which would characterise the degree of thermodynamic nonequilibrium of the structures of vitreous polymers. This role can be played by dfOr the density of the cluster network of physical entanglements [48], or by the proportion of clusters (p [140] For instance, the necessity to take into account d, V i or [Pg.334]

This paper is a preliminary report on the potential of Py-MS and Py-GC/MS to characterize ambers, copals, and other resins in microgram scale. A wide range of analyzed samples make this publication especially attractive. Py-MS methods employed by these authors compared with the earlier work done by Poinar and Haverkamp permitted them to discriminate thermally desorbable compounds from fragments of the macromolecular skeleton resulting from pyrolytic dissociation. Using Py-GC/MS, the researchers overcame an important disadvantage of the Py-MS approach — the lack of isomer information, which is a particular drawback in the case of terpenoids. 

For oriented polymers, the Uo parameter was observed to be close to the activation energy of the accumulation of free radicals and of new ends of the macromolecules. Both are caused by rupture of macromolecules [9,10]. Therefore, Uo was proposed to be directly related to the rupture of a chemical bond in the macromolecular skeleton. This proposal is confirmed by the numerical coincidence of Uo with the activation energy of thermal destruction, U,a, as evaluated from mass spectrometry data [9]. Nevertheless, this interpretation of Uo cannot explain the other experimental data ... 

ARTIFICIAL AND NATURAL FUNCTIONALIZED BIOPOLYESTERS FROM MACROMOLECULAR SKELETON SELECTION TO PROPERTY DESIGN BY ESTER PENDANT GROUPS.301... 

Artificial and Natural Functionalized Biopolyesters From Macromolecular Skeleton Selection to Property Design by Ester Pendant Groups... 

By covalently attaching reactive groups to a polyelectrolyte main chain the uncertainty as to the location of the associated reactive groups can be eliminated. The location at which the reactive groups experience the macromolecular environment critically controls the reaction rate. If a reactive group is covalently bonded to a macromolecular surface, its reactivity would be markedly influenced by interfacial effects at the boundary between the polymer skeleton and the water phase. Those effects may vary with such factors as local electrostatic potential, local polarity, local hydrophobicity, and local viscosity. The values of these local parameters should be different from those in the bulk phase. 

This work demonstrates that functionalization of the internal cavities of various dendrimers can be done via a post modification of the skeleton. Various functional groups can be selectively introduced aminophosphite, aldehyde, hydrazone, dichlorophosphane sulfide. Therefore all the chemistry reported on the surface of dendrimers can be now envisaged to be done into the cavities and it is demonstrated for the first time that a macromolecular chemistry can be performed into the internal voids of a dendrimer. 

Turning to macromolecular inorganic compounds, say ZnS, the two hypothetical ionic extremes are Zn2+S2- and Zn6-S6+ (an inverted, unusual formulation). We can imagine a continuous array of possible electron distributions between these extreme limits, one of which is the electron-pair covalent bonding state. The association of covalency with = Ay in Eqn. III.3 warrants non-polar formal MOs. However, a different situation arises when electrons are permitted to enter the empty MO skeleton. The electron-pair "covalent state corresponds to... 

Some intimate that macromolecular chemistry has become a mature science and that it is no longer at the frontier of scientific endeavor. In actuality, if we use the analogy of a human body, macromolecular science has only developed its skeleton, composed largely of homopolymers such as polyethylene, polyesters, and polyamides. The body is just beginning to develop. 

A close connection exists between the presence of a flexible polymer skeleton and the flexibility of the bulk material. Macromolecular flexibility is often defined in terms of the glass-transition temperature, Tg. Below this temperature, the polymer is a glass, and the backbone bonds have insufficient thermal energy to undergo significant torsional motions. As the temperature is raised above the Y g, an onset of torsional motion occurs, such that individual molecules can now twist and yield to stress and strain. In this state the polymer is a quasi-liquid (an elastomer) unless the bulk material is stiffened by microcrystalfite formation. Thus, a polymer with a high Tt is believed to have a backbone that offers more resistance to bond torsion than a polymer with a low 7 g. 

Immature kerogens and asphaltenes contain thiophene units in their macromolecular structure, which have mainly linear, isoprenoid, branched and steroidal side-chain skeletons. These units and possibly other sulfur-... 

Optical Materials. The polyphosphazene skeleton is electron-rich, which means that it provides a refractive index increment compared to conventional saturated organic backbones. In addition, the macromolecular substitution synthesis aUows highly unsaturated organic side groups to be linked to the skeleton in ways that allow the refractive index, the color, the liquid crystalline, and nonlinear optical characteristics of the polymer to be finely tuned. Thus, the use of these polymers in opto-electronic (photonic) switches and lens systems is a subject of growing interest. 

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