Disyndiotactic polymer

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. 

Turning to propylene, cis addition was found also for syndiotactic polymers (4(X), 401). This result deserves additional comment. It is known that only one disyndiotactic polymer is obtained from a CHA=CHB olefin (see Sect. II-B) but this is no longer true when one considers the syndiotactic copolymers between two differently labeled monomers. The syndiotactic copolyriKr between perdeuteropropylene and propylene-l-d, can have either of the two structures 99 Ot 199. Hew 9ve motvomet mil deti ixv% from Ihe second mononvei (present in small quantity) can be clearly identified as to its stereochemistry. 

The stereocenters in all three stereoregular polymers are achirotopic. The polymers are achiral and do not possess optical activity. The diisotactic polymers contain mirror planes perpendicular to the polymer chain axis. The disyndiotactic polymer has a mirror glide plane of symmetry. The latter refers to superposition of the disyndiotactic structure with its mirror image after one performs a glide operation. A glide operation involves movement of one structure relative to the other by sliding one polymer chain axis parallel to the other chain axis. 

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. 

Figure 3.8 Stereoisomerism of polymers of -substituted ot-olefins -[-CH(R)— CH(R )-]h-. Diisotactic and disyndiotactic polymers...

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. ... 

The validity of the proposed mechanism is borne out by the results of the polymerisation of (E,E)-1 -(2H)-1,3-pentadiene with Nd(OCOR)3—AlEt2 Cl A1 (z-Bu)3 and Co(Acac)2 AIEt2Cl—H20 catalysts [20-22,195], The Nd-based catalyst gave a czj-1,4-eryt/zro-diisotactic polymer, while the Co-based catalyst yielded a cA-l,4-t/zra>-disyndiotactic polymer. The formation of such polymers is shown by the schemes in Figure 5.5 [7,41],... 

The polymerization of cis oxides gives, in contrast to trans monomers, polymers of different stereochemistry, namely cis-2,3-butene oxide forms racemic (RR, SS) disyndiotactic polymers whereas cis-1,4-dichloro-2,3-butene oxide gives racemic mixtures of (RR, RR) and (SS, SS) diisotactic polymers. 

Tanaka, T. and Matsumoto, A. (2002) First disyndiotactic polymer from a 1,4-disubstituted butadiene by alternate molecular stacking in the crystalline state. J. Am. Chem. Soc., 124, 9676-9677. 

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. 

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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