Polymers, Natural Organic structure

A large part of organic and macromolecular chemistry starts with the chemical functionalization of benzene, and benzene units serve us building blocks for important polymers. Naturally, benzene-based aromatic materials also represent an important subclass of jt-conjugaled architectures. Despite some synthetic difficulties related to the generation of structurally well-defined oligo- and poly(phenyl-... 

The overwhelming majority of synthetic polymers is organic in nature, and it is on these that we will concentrate. The simplest and most common synthetic polymer is polyethylene, which will be our first example. Figure 1.1 shows the basic chemical structure of polyethylene. Pairs of hydrogen atoms are attached to the carbon atoms that make up the backbone. The repeat unit in this structure contains two carbon atoms and is derived from the ethylene monomer. In the case of polyethylene, the number of monomer residues, which is known as the polymerization... 

The technique is currently not used as widely as UV, visible and infrared spectrometry partly due to the high cost of instrumentation. However, it is a powerful technique for the characterization of a wide range of natural products, raw materials, intermediates and manufactured items especially if used in conjunction with other spectrometric methods. Its ability to identify major molecular structural features is useful in following synthetic routes and to help establish the nature of competitive products, especially for manufacturers of polymers, paints, organic chemicals and pharmaceuticals. An important clinical application is NMR imaging where a three-dimensional picture of the whole or parts of a patient s body can be built up through the accumulation of proton spectra recorded over many different angles. The technique involves costly instrumentation but is preferable to... 

The formation ofC—C bonds between aromatic rings is an important step in many organic syntheses and can be accomplished by chemical, photochemical, or electrochemical means. As was noted earlier, fundamental considerations of the parameters for a dielectric which must be dealt with in designing a thermally stable, low-dielectric-constant polymer naturally lead one to consider rigid-rod, nonconjugated aromatic polymers containing no lossy functional groups. A structure such as poly(naphthalene) is a likely candidate. 

Before we get to some of the details of how polymer molecules organize themselves into structures such as the spherulites shown in Figure 8-1, it is useful to review a few fundamental things. First, what are the basic states of matter, in the sense of solid/liquid/ gas, found in most materials and do polymers behave the same way as smaller molecules Second, we should review the nature of intermolecular forces between molecules, because it is the magnitude of these relative to thermal energy (kT or RT) and hence molecular motion that determines the state of a polymer at a particular temperature. Once these fundamentals have been established we will discuss structure. 

The usefulness of analytical pyrolysis in polymer characterization, identification, or quantitation has long been demonstrated. The first application of analytical pyrolysis can be considered the discovery in 1860 of the structure of natural rubber as being polyisoprene [10]. This was done by the identification of isoprene as the main pyrolysis product of rubber. Natural organic polymers and their composite materials such as wood, peat, soils, bacteria, animal cells, etc. are good candidates for analysis using a pyrolytic step. 

Eliminations and other reactions do not necessarily take place only on the polymeric chain or only on the side groups. Combined reactions may take place, either with a cyclic transition state or with free radical formation. The free radicals formed during polymeric chain scission or during the side chain reactions can certainly interact with any other part of the molecule. Particularly in the case of natural organic polymers, the products of pyrolysis and the reactions that occur can be of extreme diversity. A common result in the pyrolysis of polymers is, for example, the carbonization. The carbonization is the result of a sequence of reactions of different types. This type of process occurs frequently, mainly for natural polymers. An example of combined reactions is shown below for an idealized structure of pectin. Only three units of monosaccharide are shown for idealized pectin, two of galacturonic acid and one of methylated galacturonic acid ... 

If a mixture of L- and D-alanine were caused to polymerize, nearly all the polymer molecules would have different structures because their sequences of D-alanine and L-alanine monomer units would differ. To create polymers with definite structures for particular roles, there is only one recourse to build all polypeptides from one of the optical isomers so that the properties will be reproducible from molecule to molecule. Nearly all naturally occurring a-amino acids are of the L form, and most earthly organisms have no use for D-a-amino acids in making polypeptides. Terrestrial life could presumably have begun equally well using mainly D-amino acids (all biomolecules would be mirror images of their present forms). The mechanism by which the established preference was initially selected is not known. 

Of course, the nature of the organic functional group exerts an influence on the structure and, as a consequence, on the stoichiometry of the eventual polymer. The real structure... 

Natural organic polymers - such as structural proteins (ceratin, collagen),... 

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