Disyndiotactic polymer configurations

Stereoregular polymers that can be afforded by 2,4-hexadiene and other symmetric terminally disubstituted butadienes (of the CHR CH CH CHR type) exhibit still more complex stereoisomerism, since each monomeric unit in these polymers possesses three sites of isomerism. The formation of these polymers involves 1,2- and 1,4-polymerisation. The 1,2-polymers derived from the CHR=CH—CH=CHR monomers exhibit the same type of stereoisomerism as polymers with a 3,4 structure obtained from monomers of the CH2 CH CH=CHR type. However, owing to the presence of the R substituent at the double bond in the side group of the polymer derived from a monomer of the CHR=CH—CH=CHR type, two types of eryt/zro-diisotactic, t/zraz-diisotactic and disyndiotactic polymer are foreseeable, each type with either cis or trans configuration of the double bond, as in the 1,2-polymer derived from a monomer of the CH2 CH CH CHR type. Thus, six stereo-isomeric forms of 1,2-polymer are possible for the CHR CH CH CHR monomer. The 1,4 monomeric units in the polymers formed by the polymerisation of CHR CH CH CHR monomers contain one double bond (in either cis or trans configuration) and two tertiary carbon atoms and therefore can exist as two sets of enantiomers, erythro and threo ... 

Figure 2.3 Threo and erythro relative configurations in monomeric units containing two adjacent tetrahedral stereoisomeric centers and two different substituents A B, and succession of (+) and (-) bonds in r/zr o-diisotactic, ryt/zro-diisotactic, and disyndiotactic polymers. When A = B the relative configurations are defined racemo and meso.

The term "racemo" is introduced here as the logical prefix for the designation of an arrangement that is analogous to racemic, in the sense defined above. It is unfortunate that the meaning of the term racemic current in organic chemistry is not directly applicable to polymers, but the use of the prefix "racemo proposed here should not cause confusion because of the special context. To achieve a full configurational description, it may be necessary to preface the name of a polymer with a compound adjective that combines a term such as erythro, threo, meso or racemo with a term such as diisotactic or disyndiotactic . 

Tactic polymers are classified as isotactic, syndiotactic, or heterotactic according to how the configurational monomeric units are bonded to each other. If two defined stereoisomeric positions per configurational monomeric unit occur, then this is known as a ditactic polymer, which may, for example, be diisotactic or disyndiotactic. If the configurational monomeric unit has three defined stereoisomeric positions, then the polymer is a tritactic polymer. Polymers of higher tactic number are similarly defined. 

With the 3,4- polymers there are no geometrical isomers but three stereoisomers, disyndiotactic (each r-carbon opposite to that of the corresponding carbon of the preceding repeat unit), e/y thro-diisotactic and rAreo-diisotactic (the latter pair differing in the relative configuration of adjacent r-carbon atoms). 

By addition polymerization, the carbon arbon double bond can be opened via a trans mode resulting in t/zreo-polymers of diisotactic, disyndiotactic or atactic structure or via a cis mode to form erythro- o y-mers of diisotactic, disyndiotactic or atactic configuration [5] [Eq. (2)]. 

CAS represents stereochemistry in polymers by text descriptor terms when the necessary information is reported such terms include isotactic, syndiotactic, threo-diisotactic, erythro-diisotactic, and disyndiotactic. The term atactic (for a random configuration) is not employed by CAS in indexing specific polymers. In addition to the special terms above, polymer stereochemistry is defined, when appropriate, by the stereo descriptors E, Z, R, S, R, and S. ... 

Double bonds present along the chains of polymers of 1,3-dienes with 1,4 additions of the monomeric units are stereoisomeric centers that may assume cis or trans configurations. Polidienes may also contain up to two stereoisomeric tetrahedral centers. Stereoregular polydienes can be cis or trans tactic, isotactic or syndiotactic, and diisotactic or disyndiotactic if two stereoisomeric tetrahedral centers are present. In the latter case, erythro and threo structures are defined depending on the relative configurations of two carbon atoms [1]. 

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