Ditacticity

Complications arising from other types of isomerism. Positional and geometrical isomerism, also described in Sec. 1.6, will be excluded for simplicity. In actual polymers these are not always so easily ignored. Polymerization of 1,2-disubstituted ethylenes. Since these introduce two different asymmetric carbons into the polymer backbone (second substituent Y), they have the potential to display ditacticity. Our attention to these is limited to the illustration of some terminology which is derived from carbohydrate nomenclature (structures [IX]-[XII]) ... 

Examples of ditactic polydienes are provided by a class of crystalline polymers derived from the alkyl-esters of the trans-trans isomer of sorbic acid.101,102 For example trans-poly (methyl sorbate) (-CH(CH3)-CH =... 

Note 1 Terms referring to the tacticity of polymers (tactic, ditactic, tritactic, isotactic, cistactic, etc.) can also be applied with similar significance to chains, sequences, blocks, etc. 

FIGURE 2.6 Simulated formulas of ditactic isomers where R2 are chain extensions and R and Ri are not hydrogen. 

The polymerization of 1,2-disubstituted ethylenes, RCH=CHR, such as 2-pentene (R = — CH3, R = —C2H5), presents a different situation. Polymerization yields a polymer structure II in which there are two different stereocenters in each repeating unit. Several possibilities of ditacticity exist that involve different combinations of tacticity for the two stereocenters. Various stereoregular structures can be defined as shown in Fig. 8-2. Diisotactic structures occur when placement at each of the two stereocenters is isotactic. 

Little is known about the R/S isomerism (i.e., erythro and threo ditactic structures are possible) at the stereocenters that result from double-bond polymerization. Cycloheptene and higher cycloalkenes undergo only ROMP double-bond polymerization does not occur because the larger rings can accommodate the double bond without being highly strained. 

The simplest optically active ditactic polymers prepared so far are isotactic cis and trans 1.4-polypenta-l.3-diene (93, 95). 

It should also be mentioned that atactic polymers may be formed from monodeuteropropylene and pentadeuteropropylene, similarly to the case of the polymerisation of non-deuterated propylene. However, atactic structures with respect to one stereogenic tertiary carbon atom, with the other possibly arranged to form tactic polymer sequences, are not known. Also, ditactic polymers, which might be characterised by the appearance of two distinct series of tacticity as regards both stereogenic tertiary carbon atoms, i.e. the isotactic with respect to one stereogenic carbon atom and syndiotactic with respect to the other one, and vice versa, are not known. 

Show by adopted Fischer projections the various (if any) tactic (ditactic) polymers that might possibly be obtained from each of the olefins ... 

Indicate tactic or ditactic polymers that can be obtained by the coordination polymerisation of oxiranes. 

The polymers feature two chiral centers per monomer unit and therefore are ditactic. While polymers produced by achiral Pd catalysts seem to be atactic, using chiral metallocene catalysts highly tactic crystalline materials can be produced, featuring extraordinary high melting points (in some cases above the decomposition temperature) and extreme chemical resistance. 

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. 

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. 

Ditactic polymers possess two stereoisomeric centers per constitutional monomeric unit, and tritactic polymers possess three. Ditactic polymers may be formed by the polymerization of 1,2-disubstituted ethylene derivatives, as, for example, with pentene-2 ... 

Figure 3-7. The four configurations of ditactic poly(2-pentene). eit = erythrodiisotactic, tit = threodiisotactic, est = erythrodisyndiotactic, tst = threodisyndiotactic R = CH3, R = C2H5 or vice versa.

Figure 3-8. The four configurations of ditactic poly(cyclohexene).

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 versatihty of these catalysts was further illustrated, when ring-opening metathesis polymerizations of norbornene were carried out in hquid carbon dioxide at high pressure, using Ru (H20)6-(Tos)2. The product was reported to be c/s-ditactic polynorbomene [172]. It should be noted, however, that stereoselective polymerizations of norbornene are not confined to these catalysts only. 

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