Threo-disyndiotactic

As a consequence, eight stereoisomeric forms are foreseeable for the 1,4-polymers of the CHR=CH—CH=CHR monomer cw-l,4-(or trans-1,4-) erythro-diisotactic, cz s-l,4-(or trans-1,4-) threo-diisotactic, cw-l,4-(or trans-1,4-) eryz7zro-disyndiotactic, and cA-l,4-(or trans-1,4-) threo-disyndiotactic. For instance, Figures 5.1 and 5.2 illustrate the stereoisomerism of cis-1,4- and trans-1, 4-polymers of CHR=CH CH=CHR monomers respectively. 

Figure 5.5 Schematic presentation of the formation of cis- 1,4-polymers of terminally disubstituted butadienes (a) erythro-diisotactic, (b) threo-disyndiotactic, depending on the orientation of the incoming monomer. The monomer is above the plane, the if-butenyl group is below and Mt is on the plane of the figure...

Early studies on the homopolymerization of E-l,3-pentadiene yielded polymers with a high cis-1,4-content and an isotactic structure, whereas E-2-methyl-l,3-pentadiene resulted in a polymer with a mixed czs-1,4/transit-structure [487-492]. Investigations on the polymerization of E-1,3-pentadiene with the system NdN/TIBA/DEAC partially support these findings as a poly(l,3-pentadiene) with a cis- 1,4-threo-disyndiotactic structure was obtained [492]. A somewhat lower cis- 1,4-content of 70% was obtained when the polymerization of E-l,3-pentadiene was catalyzed by (CF3COO)2NdCl/TEA [493,494]. When 2,3-Dimethyl-1,3-butadiene is polymerized with the catalyst NdN/TIBA/EtAlC the resulting poly(2,3-dimethyl-butadiene) predominantly contains cis-1,4-units [495,496]. 

Acenaphthylene has been homopolymerized by radical emulsion polymerization to a product having predominantly a threo disyndiotactic structure (209). 

Very recently, these views have been definitively supported by the very elegant work of Porri (31), using cis-cis-1,4-dideute-rio-butadiene as moncmer, and correlating the different stereoregularities of the polymers obtained (trans-l,4-threo-diisotactic and cis-1,4-threo-disyndiotactic structures). 

These monomers produce a single disyndiotactic polymer characterized by alternating erythro and threo relationships between adjacent substituents, 18 or 19. In such a polymer it is not possible—unless one turns to selective isotopic labeling—to determine whether the erythro and threo relationship refers to the substituents A and B of the same monomer unit or to those of two successive monomeric units. 

Cyclic olefin monomers such as indene, benzofuran, and so on, can give rise to two diisotactic polymers eiythro and threo, illustrated in the Fischer projection in 26 and 27, and to two different disyndiotactic polymers 28 and 29 (Scheme 5), as each monomeric unit is clearly defined and quite distinct from its neighbors (58, 61). For polymers of this type the terminology eiythro and threo is used also. 

The combination of cis-trans isomerism with iso-syndio and erythro-threo dispositions gives complex stractures as exemplified by the 1,4 polymers of 1-or 4-monosubstituted butadienes, such as 1,3-pentadiene (72, 73), and 2,4-pentadienoic acid (74, 75) and of 1,4-disubstituted butadienes, for example, sorbic acid (76). This last example is described in 32-35 (Scheme 6, rotated Fischer projection). Due to the presence of three elements of stereoisomerism for each monomer unit (two tertiary carbons and the double bond) these polymers have been classed as tritactic. Ignoring optical antipodes, eight stereoregular 1,4 structures are possible, four cis-tactic and four trans-tactic. In each series (cis, trans) we have two diisotactic and two disyndiotactic polymers characterized by the terms erythro and threo in accordance with the preceding explanation. It should be noted that here the erythro-threo relationship refers to adjacent substituents that belong to two successive monomer units. 

The relative configuration of adjacent, constitutionally non-equivalent, carbon atoms can be specified as erythro or threo, as appropriate, by adding the required prefix to the terms diisotactic and disyndiotactic , as necessary (see Section 2.2). 

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 . 

All four diisotactic polymers (cis and trans, erythro and threo) are chiral and possess optical activity. Each of the four disyndiotactic polymers possesses a mirror glide plane and is achiral. For symmetric 1,4-disubstituted 1,3-butadienes (R = R ), only the cis and transthreo-diisotactic structures are chiral. Each of the erythrodiisotactic and threodisyndiotactic polymers has a mirror glide plane. Each of the erythrodisyndiotactic polymers has a mirror glide plane. 

Fig. 4. (A) Threo (or gauche) disyndiotactic copolymer (B) erythro (or trans) disyn-...

Fig. 6. Three stereoregular polymers have been obtained from cis- and trans-UI, -pro-pylene viz., threo-diisotactic (ti), erythro-diisotactic (ei), and disyndiotactic (ds).

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 ... 

In this case, the occurrence of two types of stereoisomeric form of disyndiotactic polymers, erythro and threo, should be emphasised let us recall that there were no differences in the stereostructure (erythro or threo) of disyndiotactic deuterated poly (a-olefins), e.g. polymers of [> - mo n od e u t e r a t e d 7-olefins, — -CH(R)-CH(2H)-]n—, when omitting different chain end groups. ... 

Figure 5.1 Stereoisomerism of cis-, A polymers of 1,4-disubstituted butadienes, poly(3,4-dialkyl-cw-l-butenylene) CH=CH CH(R) CH(R) n- or [-CH(R>-CH=CH—CH(R)—] —. Diisotactic and disyndiotactic erythro and threo polymers only one of the enantiomorphic forms of the polymers is shown...

In the case of poly(l,2-cycloalkylene)s containing symmetrical rings such as poly(l,2-cyclobutylene)s, the erythro and threo structures are also referred to as meso (M) and racemo (R) structures for denoting the stereochemistry of the rings the relative stereochemistry between the rings is denoted by meso (m) and racemo (r) configurations corresponding to diisotactic and disyndiotactic structures respectively [21,22]. 

Note that stereoregular polymers of disubstituted epoxides with a threo-diisotactic and eryt/zro-disyndiotactic structure have not been synthesised. 

A more complicated picture emerges when the polymerization of 1,2-disubstituted ethylenes (CHR=CHR ) is considered because now each carbon atom in the chain becomes a chiral center. The resulting ditactic structures are illustrated in Figure 6.1(d,ed). Two isotactic structures are obtained, the erythro, in which all the carbon atoms have the same configuration, and the threo, in which the configuration alternates. Only one disyndiotactic structure is possible. The differences arise from the stereochemistry of the starting material if the monomer is cis-substimted the threo form is obtained, whereas a trans monomer leads to the erythro structure. 

Several possibilities exist for ditacticity in macromolecules formed by polymerizing 1,2-disubstituted ethylenes of the type RHC=CHR, structures (XVIII)-(XX). It can be seen that each unit contains two different asymmetric carbon atoms in the chain. The original definitions of tacticity have been extended to include these modifications and Newman s (1956) definitions of erythro and threo structures. Structures (XVIII)-(XX) illustrate ifereo-diisotactic, erythro-diisotactic, and disyndiotactic poisoners, respectively. 

The terms isotactic and syndiotactic refer as above to the structure of each diasteric atom relative to comparable atoms in the other repeating units. The prefix erythro indicates that when a pair of adjacent diasteric atoms is rotated into an eclipsed conformation, at least two similar substituents can be superimposed. Threo denotes the nonsuperimposable isomer. In a disyndiotactic polymer each diasteric center is erythro to one of its neighbors and threo to the other, so there is only one isomer. 

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]. 

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.

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