Erythro-diisotactic

Later, Tieke reported the UV- and y-irradiation polymerization of butadiene derivatives crystallized in perovskite-type layer structures [21,22]. He reported the solid-state polymerization of butadienes containing aminomethyl groups as pendant substituents that form layered perovskite halide salts to yield erythro-diisotactic 1,4-trans polymers. Interestingly, Tieke and his coworker determined the crystal structure of the polymerized compounds of some derivatives by X-ray diffraction [23,24]. From comparative X-ray studies of monomeric and polymeric crystals, a contraction of the lattice constant parallel to the polymer chain direction by approximately 8% is evident. Both the carboxylic acid and aminomethyl substituent groups are in an isotactic arrangement, resulting in diisotactic polymer chains. He also referred to the y-radiation polymerization of molecular crystals of the sorbic acid derivatives with a long alkyl chain as the N-substituent [25]. More recently, Schlitter and Beck reported the solid-state polymerization of lithium sorbate [26]. However, the details of topochemical polymerization of 1,3-diene monomers were not revealed until very recently. 

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).

It should be added that alternating ethylene/2-butene copolymers can exhibit stereoregularity namely the ethylene/cA-2-butene copolymer, which possesses an erythro-diisotactic structure and is a crystalline polymer. It may be interesting to note that from the formal point of view the alternating eryt/zro-diisotactic ethylene/cA-2-butene copolymer, i.e. erythro-diisotactic poly[ethylene- //-(c/.v-2-butene)], can be treated as isotactic head-to-head and tail-to-tail polypropylene. Isomeric trans-2-bu. ene gives atactic amorphous copolymers with ethylene [2,82]. 

Only cycloolefins with rings containing an odd number of carbon atoms, such as cyclopentene (x = 3) and cycloheptene (x = 5), yield crystalline copolymers with an erythro-diisotactic configuration. Cycloolefins with an even number of carbon atoms in the ring, such as cyclobutene (x = 2) and cyclooctene (x = 6), give amorphous copolymers [241]. 

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...

To rigorously confirm the microstructure of the oligomers, and by analogy the polymer, crystals of dimer 5 were grown from cold methylene chloride and subjected to single crystal X-ray diffraction analysis. An ORTEP plot of 5 is presented in Fig. 4.21 along with appropriate labehng. The tacticity of this diad was determined to be erythro-diisotactic. 

Sorensen and Campbell [110] indicate that, depending on the nature of the initiator that has been used, either an erythro-diisotactic or a threo-diisotactic polymer forms. The catalyst is formed from aluminum chloride and either (-I-)-/ -phenylalanine (to produce a polymer with ( + ) rotation) or phenylalanine (to produce a (-) rotating polymer) in toluene. Other initiators include aluminum chloride with (-)-menthoxytriethyltin, -germaniiun, or -silicon [111]. The following preparation is a brief summary of the detailed procedure given in Ref. [110]. 

In the erythro-diisotactic configuration (eit), the substituents R and R all lie on the same side in a Fischer projection, whereas in a flying wedge projection, all the R substituents are found on one side, but the R substituents on the other side of the plane of the paper. In the Newman projection of the eclipsed conformation, R lies above R and H above H in the eit configuration. The characteristic features of the three other configurations can be seen in Figure 3-7. 

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. 

It was difficult until recently to assess the relative merits of these mechanisms owing to the lack of information on the opening of the double bond. Such information was reported recently by Natta et al. (1962c). In these studies, raws-l-chloro-2-butoxy-ethylene and cis-l-chloro-2-butoxyethylene were polymerized in toluene at —75°C using ethylaluminum dichloride as catalyst. The trans monomer afforded a polymer having a fereo-diisotactic structure (VII), whereas the cis monomer gave an erythro-diisotactic structure (VIII). 

Fig. 16. Schematic drawing s of the two diisotactic structures of polybenzo-furan (only one of the enantiomers is shown for each structure) (a) erythro-diisotactic, (b) tfereo-diisotactic.

Only recently, examples of a lattice controlled polymerization of butadiene deri-vatieves have been reported 24,25.167-171) yy y-irradiation of butadienes crystallized in perovskitetype layer structures yielded erythro-diisotactic 1,4-trans-polymers Furthermore, crystalline 1,4-trans-polymers could be obtained upon UV-or y-irradiation of native halide salts of unsaturated primary amines and long chain butadienes in lipid layer structures... 

Both the carboxylic acid and aminomethyl substituent groups are in an isotactic arrangement, causing an erythro-diisotactic configuration of the polymer chains. Indeed, the 1,4-addition induces two centers of chirality per monomer unit. However, due to the center of symmetry of the space group the unit cell contains a racemic mixture of the polymer. 

Polymerization of cis-l-deuterio-propylene gave exclusively erythro-diisotactic PP, whereas the frans-l-deuterio-propylene afforded exclusively the threo PP stereoisomer (Figure 10). This investigation clearly demonstrates that insertion takes place via cis addition of the transition metal alkyl to the double bond [19, 20]. 

Hexadiene was found to polymerize in DCA and apoCA canals on heating over 100°C for 10 to 20 days after Y-ray irradiation. Particullary trans, trans-2,4-hexadiene polymerized in an inclusion state via radical mechanism for the first time. The polymers from the monomer prefer erythro structure to threo structure in a DCA canal, while they do slightly threo to erythro in an apoCA canal. It is considered that the polymerization proceeded preferentially in the canals through trans opening to yield erythro diisotactic structure in a DCA canal [ll]. 

Stereoregular polymers with more than one stereoisomerism site per monomeric unit in the main chain have been identified in nature (polysaccharides and nucleic acids), and have been synthesized (e.g. erythro-diisotactic, trans-polymethylsorbate (19)). 

In the case of isotactic polymerization it has been observed that the overal stereochemical mechanism of addition to the double bond is cis (8,9) Indeed NMR analysis of polymers obtained in the presence of isotactic catalysts from cis- and trans-ld -propene indicated that erythro-diisotactic polymers are obtained from the former and threo-diisotactic polymers from the latter. 

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. ... 

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