Alternating units, polymer description

Polymer products synthesized in laboratories and in industry represent a set of individual chemical compounds whose number is practically infinite. Macro-molecules of such products can differ in their degree of polymerization, tactici-ty, number of branchings and the lengths that connect their polymer chains, as well as in other characteristics which describe the configuration of the macromolecule. In the case of copolymers their macromolecules are known to also vary in composition and the character of the alternation of monomeric units of different types. As a rule, it is impossible to provide an exhaustive quantitative description of such a polymer system, i.e. to indicate concentrations of all individual compounds with a particular chemical (primary) structure. However, for many practical purposes it is often enough to define a polymer specimen only in terms of partial distributions of molecules for some of their main characteristics (such as, for instance, molecular weight or composition) avoiding completely a... 

The simplest, from the viewpoint of topological structure, are the linear polymers. Depending on the number m of the types of monomeric units they differentiate homopolymers (ra=1) and copolymers (m>2). In the most trivial case molecules in a homopolymer are merely identified by the number Z of monomeric units involved, whereas the composition of a copolymer macromolecule is defined by vector 1 with components lly..., Za,..., Zm equal to the numbers of monomeric units of each type. At identical composition these molecules can vary in microstructure which is characterized by the manner of alternation of different units in a copolymer chain. Because the values of the average degree of polymerization l=lx+... +Zm in synthetic copolymers normally constitute 102-104 it becomes clear that the number of conceivable types of isomers with different microstructure turns out to be practically infinite. Naturally, a quantitative description of any polymer specimen comprising macromolecules with such an impressive number of configurations can be performed exclusively by statistical methods. 

Among the nonalternant polymers, one should separate systems that are formed from nonalternant monomeric units from the structures with alternant chains and with nonalternant end groups. Both types were considered in our previous work [88]. Here we give only a general description of the systems and we present conclusions that have been derived from the analysis of the results obtained in the calculations. 

Colmnn 13 of the tables indicate briefly the conformation of the polymer chain in the crystal (in a hehcal notation) either as reported in the reference cited or as inferred from the value of the fiber axis. The designation n p/q specifies the munber (n) of skeletal atoms in the asymmetric imit of the chain and the number of such asymmetric units (p) per q turns of the helix in the crystallographic repeat. Thus, poly(ethylene), as listed, has two carbon atoms in the backbone with one such imit per turn in the repeat - it is designated as a 2 1/1 helix. Alternatively, poly(ethylene) considered a poly(methylene) would be designated as a 1 2/1 helix - an entirely equivalent description of the eonformation. Note that n may differ from the number of ehain atoms in the constitutional base unit. On the one hand, isotactic poly(propene) has two skeletal atoms in the asymmetric unit and three units per turn (2 3/1). On the other hand, syndiotactic poly-(propene) with the same eonstitutional base unit as isotactie... 

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