Chemical and physical variants

Many variations from linear, regular chain formation may occur during polymerisation. The first set of variations arises from the presence of alternative reactive centres either on the monomer or on the already formed polymer chains. Thus the uniformity of head-to-tail monomer assembly  

Similar structures will occur in conjugated polymers such as polypyrroles and polythiophenes prepared from monomers with one or more different substituents at the positions on the aromatic ring not involved in the polymerisation, see Fig. 1.5. The reactivity of one particular site in the monomer usually predominates, so that the effect is relatively small. A 1 % head-to-head content may, however, seriously upset crystallisation. 

Some polymerisations do not continue in a linear fashion, and branching occurs, as shown in Fig. 1.7(a). When branching is prevalent, it can have a serious effect on properties. Thus the polymerisation of ethylene under high-pressure conditions gives a product that has so many side chains on each main chain that crystallisation is appreciably suppressed. This material is softer than the highly crystalline linear form from a catalytic low-pressure process. The two forms of polyethylene may be distinguished by a difference in density between them, the more crystalline material being denser. 

As in any other chemical compound, different geometrical arrangements of substituent groups are possible in a polymer where rigid molecular units are involved. This gives rise to trans- and cis-configurational isomerism in polymers containing double bonds in their repeat units, as in polyacetylene and natural and synthetic rubbers. The structures of the trans- and m-isomers of polyacetylene and polybutadiene are illustrated in Fig. 1.8. 

Such differences in structure can have a profound effect on the physical properties of a polymer. Thus natural rubber, which comprises cis-1,4-poly(isoprene), is a soft rubbery material at room temperature, whereas guttapercha, which comprises the corresponding ftms-isomer, is semi-crystalline and hard. The method of polymerisation determines the isomeric form of the polymer. 

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Assessment of the toxicity of aluminas has been complicated by the chemical and physical variants of the compounds and inconsistencies in the nomenclature used to describe them. The group of compounds referred to as aluminas is composed of various structural forms of aluminum oxide, trihydroxide, and oxyhydrox-ide. As these aluminas are heated, dehydration occurs, producing a variety of transitional forms temperatures between 200 and 500°C result in low-temperature-range transitional... 

To me, the most reliable nomenclature is based on actual physical and chemical operations. This is often clumsy, but should be replaced by a conceptual kind of nomenclature only when there is universal agreement that the concept is fully consistent with physical and chemical data. The topology of the faujasite framework is universally recognized, but the structural nature of chemical and physical variants is highly debatable. 

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