Threo-diisotactic

By the presence of cyclic monomer units obtained, for example, from cyclic olefins (benzofuran, indene, etc.) (58, 254). The erythro- and threo-diisotactic stractures (26, 27 or in a different representation 77, 78) are chiral. If B is equal to A (cyclobutene or analogous monomers) only the threo-diisotactic structure 27 is chiral. 

Of the possible chiral eiythro- and threo-diisotactic forms (77 and 78), the threo appears more probable on a conformational basis (334). The polymer has a rigid skeleton and is generally amorphous but can be converted into a crystalline material by a nonconventional annealing process (335, 336). 

Analogous remarks can be generally made upon polymers of monomers (VIII) (7, 33) even supposing that the main chains are of infinite length, monomeric units existing in two enantiomeric forms can be foreseen in the case both of the erythro and the threo diisotactic macromolecules when R and R" are different. On the contrary when R = R", only the monomeric units of the threo diisotactic macromolecules can exist in two enantiomeric forms. Some stereoisomers of the polymers of monomers containing one double bond in a ring (5, 127), such as for... 

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).

On the other hand, the polymerisation of trans-1 -(2 H)-propene with heterogeneous Ziegler-Natta catalysts leads to the formation of the respective polymer of threo-diisotactic structure [scheme (49)], which proves the m-insertion of the monomer [26,275] ... 

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. 

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

When the approaching monomer is in syndiotactic conformation and is added as such (c), the / carbon is fixed after addition, but the a carbon can rotate (d) and assume isotactic conformation after the addition of the next unit. The effect of the lithium cation is again assumed to be the driving force for the rotation of the potentially syndiotactic unit to the isotactic conformation. Repeating steps (a) and (b) leads to a threo-diisotactic chain, and repeated steps (c), (d) and (e) to an erythrodiisotac-tic chain. The addition of tetrahydrofuran reduces the number of threo-diisotactic units because it competes with carbonyl groups for coordination to Li+. This enhances the amount of monomer approaching the centre by the syndiotactic route. 

Helix-sense-selective polymerization has also been attempted for several bulky monomers including an acrylonitrile derivative105 and a-substituted acrylates.106107 Triphenylmethyl crotonate (TrC, 35) affords optically active, helical polymers by the polymerization using DDB—FILi and PMP—FILi complexes.108109 The polymers possess a nearly completely threo-diisotactic structure. Although the polymers indicate relatively small specific rotation ([(x]d +5.6° and +7.4° for the samples with DP =15 and... 

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]. 

When 1,2-disubstituted olefins are polymerized with Ziegler-Natta catalysts, the ditacticity of the products depends on the mode of addition. It also depends on the structure of the monomer, whether it is cis or trans. A threodiisotactic structure results from a syn addition of a trans monomer. A syn addition of a cis monomer results in the formation of an erythrodiisotactic polymer. For instance, cis and /ra/i -l- Z-propylenes give erythro and threo diisotactic polymers, respectively. To avoid 1,2-interactions in the fully eclipsed conformation, the carbon bond in the monomer units rotate after the addition of the monomer to the polymeric chain. 

The Cossee-Arlman theory requires attack from the same side of the molecule as well as cis opening of the double bond. Miya-zawa and Ideguchi (1963) have showed this experimentally. From cis-l-deuteriopropylene they obtained erytfero-diisotactic poly (propylene-1 di) and from tra s-l-deuteriopropylene, threo-diisotactic poly (propylene-ldi). 

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. 

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). 

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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