Configurational base unit

Smallest set of successive configurational base units that prescribes configurational repetition at one or more sites of stereoisomerism in the main chain of a regular macromolecule, a regular oligomer molecule, a regular block, or a regular chain. 

Tactic macromolecule, essentially comprising only one species of configurational base unit, which has chiral or prochiral atoms in the main chain in a unique arrangement with respect to its adjacent constitutional units. 

Note In an isotactic macromolecule, the configurational repeating unit is identical with the configurational base unit. 

Note In a syndiotactic macromolecule, the configurational repeating unit consists of two configurational base units that are enantiomeric. 

Regular macromolecule that has an equal number of the possible configurational base units in a random sequence distribution. 

Unless otherwise stated, the drawings of configurational base units, configurational repeating units, stereorepeating units, etc., provide information regarding relative configurations. 

The configurational base units (1) and (2) are enantiomeric, while the configurational units (1) and (3) cannot be enantiomeric because the constitutional units are different species, according to this nomenclature. ... 

Note As the definition above indicates, a regular polymer, the configurational base units of which contain one site of stereoisomerism only, is atactic if it has equal numbers of the possible types of configurational base units arranged in a random distribution. If the constitutional repeating unit contains more than one site of stereoisomerism, the polymer may be atactic with respect to only one type of site if there are equal numbers of the possible configurations of that site arranged in a random distribution. 

A polymer such as -[-CH=CH-CH(CH3)-CH2-hr which has two main-chain sites of stereoisomerism, may be atactic with respect to the double bond only, with respect to the chiral atom only or with respect to both centres of stereoisomerism. If there is a random distribution of equal numbers of units in which the double bond is cis and trans, the polymer is atactic with respect to the double bond, and if there is a random distribution of equal numbers of units containing the chiral atom in the two possible configurations, the polymer is atactic with respect to the chiral atom. The polymer is completely atactic when it contains, in a random distribution, equal numbers of the four possible configurational base units which have defined stereochemistry at both sites of stereoisomerism. 

In addition to isotactic, syndiotactic and atactic polymers (and other well-defined types of tactic polymers), there exists the whole range of possible arrangements between the completely ordered and the eompletely random distributions of configurational base units,... 

Tactic polymer that contains two sites of defined stereoisomerism in the main chain of the configurational base unit. 

Tactic polymer in which the main-chain double bonds of the configurational base units are entirely in the Z configuration. 

Smallest set of one or more successive configurational base units that generates the whole chain through helical symmetry. 

The nomenclature of regular polymers can denote stereochemical features if the CRU used is the configurational base unit [10], i.e. a CRU having one or more sites of defined stereoisomerism in the main chain of a polymer molecule [11]. Structure-based names are then derived in the usual fashion. The various stereochemical features that are possible in a polymer must first be defined. 

An isotactic polymer has only one species of configurational base unit in a single sequential arrangement and a syndiotactic polymer shows an alternation of configurational base units that are enantiomeric, whereas in an atactic polymer the molecules have equal numbers of the possible configurational base units in a random sequence distribution. This can be generalized as follows in zig-zag and horizontal Fischer projections ... 

There are two constitutional repeat units (Sec. l-2c) from a stereochemical viewpoint, one with R configuration for the stereocenter and the other with S configuration for the stereocenter (corresponding to la and lb). These are referred to as the two configurational base units and have an enantiomeric relationship [IUPAC, 1966, 1981, 1996]. 

A cistactic polymer is a tactic polymer in which the main chain double bonds of the configurational base units are entirely in the cis arrangement. A transtactic polymer is a tactic polymer in which the main chain double bonds of the configurational base units are entirely in the trans arrangement [134]. Geometrical cis-trans isomerism also arises from different configurations of substituents on a cyclic structure. 

An isotactic polymer is defined as A regular polymer whose molecules can be described by only one species of configurational base units in a single sequential arrangement. 17]... 

In the case of poly(cyclopentene), the configurational base units are cis-and 1,3-enchained (27) while poly(norbomene) shows cis-exo insertion [61, 62]. 

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