Constitutional base units

It is, however, possible - by means of modem X-ray analysis - to determine the crystal system of the crystalline domains and also the dimensions of the unit cell as well as the number of the constitutional base units in the unit cell. From these data the real crystalline density can be exactly calculated . 

Ditactic polymers possess two steroisomeric centers per constitutive base unit, and tritactic polymers possess three. Ditactic polymers may be formed by the polymerization of 1,2-disubstituted ethylene derivatives, as, for example, for pentene-2 ... 

For the regular polymers of multipeptides, a cyelie permutation of the individual peptide residues in the constitutional base unit was performed to permit alphabetical ordering without loss of directional or sequential sense. Thus, poly(alanylglycy 1-proline) is the name given to polymers prepared form (ala-gly-pro), (gly-pro-ala), and (pro-ala-gly), but not from (ala-pro-gly) or from either of its eyclic permutations. 

Included as part of the polymer name column is the molecular weight of the ehemieal repeat unit of the chain, that is, of the constitutional base unit. This is given in parentheses just below the polymer name. The values reported are in grams per mole. 

The dimensions of the unit cell (a, b, c) are given next, in Angstroms (1 A = 0.1 run), c is the fiber axis. Unless otherwise indicated, the rmit cell angles are all 90 . Where cnfy one angle differs fiom 90°, its value is given When all three angles differ from 90°, their values are fisted in the order a, fi, 7. The next column contains the munber of constitutional base units in the rmit cell described. 

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... 

In the following table of melting points, entries are alphabetical according to the basic structure of the polymer, ignoring substituents. Substituted polymers are listed alphabetically (according to the substituent) under the entry for the unsubstituted polymer. The moleeular weight is that of the chemical repeat (the constitutional base unit) in the polymer. Melting points (in C) are taken... 

The rules of this scheme are simple - coimt the number of groups/atoms in the constitutional base unit in the following order... 

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