Trans-Tactic polymers

Studied in ref. [26] is the effect of HFC reaction conditions on the configuration sequences in POCS-4. Since the mesomorphous state in PMCS-4 is formed in stereoregular trans-tactic polymers only [27, 32] and spatial configuration of initial monomers is not always fully preserved in poly-mers, the effect of HFC conditions on transformation of =Si-CI and =Si-OH centers in initial corn-pounds has been studied. The detected fact of cyclosiloxanes partial inversion at Cl atoms substitution at silicon was expected, as reported before [33, 34], More detailed description of reflex correlation was carried out in ref. [35], Symbols and mark projections of units and bonds to the pla-ne perpendicular to the cycle plane. 

Four cyclolinear PBPCS-6 polymers of different structures were synthesized from pure cis- and trans-isomers of dichloro- and dihydroxyderivatives of diphenyloctamethyl-cyclohexasiloxanes the first one is enriched with tt-sequences in the absence of cc-combinations (trans-tactic polymer) the second one is enriched with cosequences in the absence of tt ones (c/.v-tactic polymer) the third one is mostly enriched with tosequences from co-dichloro derivative and /ra ,v-diol (due to partial inversion of dichloride, tt-sequences are present that is why spatial structure of the third polymer is similar to the first one) the fourth one represents atactic polymer. 

Comparison of the data shown in Figure 10 with d values from the diffraction pattern in Figure 11 indicate (Figure 11a) that the mesomorphous component (a narrow reflex), d, is independent of poly(decamethylcyclohexasiloxane) chain tacticity. Hence, irrespective of trans-tactic polymer abi-lity to crystallization, the mesophase temperature range changes insignificantly. At lower tempera-ture of -73°C only, a bending on the temperature dependence curve, d(T), of the mesophase peak occurs. 

Double bonds present along a polymer chain are stereoisomeric centers, which may have a cis or trans configuration. Polymers of 1,3-dienes with 1,4 additions of the monomeric units contain double bonds along the chains and may contain up to two stereoisomeric tetrahedral centers. Stereoregular polymers can be cis or trans tactic, isotactic or syndiotactic, and diisotactic or disyndio-tactic if two stereoisomeric tetrahedral centers are present. In the latter case erythro and threo structures are defined depending on the relative configurations of two chiral carbon atoms.1... 

The butadiene polymers represent another cornerstone of macromolecular stereochemistry. Butadiene gives rise to four different types of stereoregular polymers two with 1,2 linkage and two with 1,4. The first two, isotactic (62) and syndiotactic (25), conform to the definitions given for vinyl polymers, while the latter have, for eveiy monomer unit, a disubstituted double bond that can exist in the two different, cis and trans, configurations (these terms are defined with reference to the polymer chain). If the monomer units all have the same cis or trans configuration the polymers are called cis- or trans-tactic (30 and 31). The first examples of these stereoisomers were cited in the patent literature as early as 1955-1956 (63). Structural and mechanistic studies in the field have been made by Natta, Porri, Corradini, and associates (65-68). 

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. 

A cistactic polymer is a tactic polymer in which the main chain double bonds of the configurational base units are entirely in the cis arrangement. A transtactic polymer is a tactic polymer in which the main chain double bonds of the configurational base units are entirely in the trans arrangement [134]. Geometrical cis-trans isomerism also arises from different configurations of substituents on a cyclic structure. 

In synthesized cyclolinear siloxane polymers (CLSP) the dimethylsiloxane unit was taken as a flexible spacer [74], Table 10 shows temperatures and heat transitions in relation to the flexible spacers lengths. These polymers were found in the crystalline and thermotropic states, like trans-tactic poly-(decaorganocyclohexasiloxanes), but temperature range of the... 

The chemical structure of a polymer determines whether it will be crystalline or amorphous in the solid state. Both tacticity (i.e., syndio-tactic or isotactic) and geometric isomerism (i.e., trans configuration) favor crystallinity. In general, tactic polymers with their more stereoregular chain structure are more likely to be crystalline than their atactic counterparts. For example, isotactic polypropylene is crystalline, whereas commercial-grade atactic polypropylene is amorphous. Also, cis-pol3nsoprene is amorphous, whereas the more easily packed rans-poly-isoprene is crystalline. In addition to symmetrical chain structures that allow close packing of polymer molecules into crystalline lamellae, specific interactions between chains that favor molecular orientation, favor crystallinity. For example, crystallinity in nylon is enhanced because of... 

Before discussing mechanisms appropriate to some individual catalyst systems we may note that early indications (4) that there was a correlation between tacticity and cis content have not been borne out by wider studies. Thus high-cis polymers may be fully syndiotactic (ReCls with all monomers) or nearly atactic (WClg/BuijSn with 5), while high-trans polymers may be fully tactic (19) C( mes)W(C0)37EtAlCl2 with 1 and 1 or nearly atactic (RuCls with all monomers). In tactic polymers cis double bonds are always... 

Figure 2.13. Potential energy diagram showing the minimum in potential energy for the trans form for a tactic polymer chain.

Polymers with double bonds in the chains can occur in cis-tactic (ct) or trans-tactic (tt) configurations according to how the parts of the chain are arranged about the double bond. An example of this is l,4-poly(buta-diene), with the configurations... 

For a chain with completely free rotation, = 0 and equation (4-13) becomes equation (4-10). An aW-trans chain is a rigid chain with 0 = 0, which is, however, physically absurd with regard to equation (4-13). Thus, steric hindrance parameter. With tactic polymers, the relationship between 0 and (7 is more complicated. One can, however, always use an equation analogous to equation (4-13) ... 

Butene-1 is converted to iso tactic polymers with Ziegler-Natta catalysts. Alternatively, cis- or trans-butene-2 can be used as starting material since, in the presence of certain catalyst systems, they isomerize before polymerization. st-Poly(butene-l) is obtained by hydrogenation of st-l,2-poly-(butadiene). 

Obviously as the complexity of the monomer increases, greater orders of tacticity are possible. To date only one form of a tritactic polymer has been isolated, and that by polsonerizing an ester of trans-trans sorbic acid (CH3CH=CHCH=CHCOOR) (Natta et al., 1960b). The polymer, which contained three elements of stereoisomerism, two optical and one geometrical, was identified as erythro-diiso-trans tactic. 

Tritactic polymer n. An isotactic or syndio-tactic polymer that is also of the cis- or trans form because the molecules are unsaturated and have double bonds. 

Polymers with different tacticities and cis/trans olefin contents were also synthesized from racemic and enantiomerically enriched exo- and cnt/o-5-methylnorbornene. High-cis syndiotactic polymers were prepared with ReCls (in benzene or chlorobenzene) and high-trans atactic polymers were obtained with two Ru-based catalysts, RuCl3-(H20)3 in PhCl/EtOH and an in... 

O DeU, R. McConviUe, D. H. Hofmeister, G. E. Schrock, R. R. Polymerization of enantiomeric-aUy pure 2,3-dicarboalkoxynorbornadienes and 5,6-disubstituted norbornenes by well-characterized molybdenum ring-opening metathesis polymerization initiators. Direct determination of tacticity in cis, highly tactic and trans, highly tactic polymers. /. Am. Chem. Soc. 1994, 116, 3414-3423. 

This analysis was made possible because the chemical shifts of the various olefinic carbon double bonds in these unsymmetrically substituted norbomene derivatives are very sensitive to whether they are in an HH, TT or HT/TH unit, as can be seen for the case of poly(l-methylnorbomene) [63] in Figure 1.12. It was therefore possible to examine a range of metathesis catalysts for their ability to produce tactic polymers. In fact, a range of tacticities was observed, with extremes in behaviour being represented by the ReClj catalyst, which produced an all-cis syndiotactic polymer [65] and the W(mesityl) (CO)j catalyst, which produced a high trans isotactic polymer [71]. 

Most properties of the substituted polyacetylenes (PA), such as permeability to penetrants, solubility, and thermal stability are strongly related to the number and/or steric hindrance of die substituents 1-4). However, these properties cannot be related in a strai tforward fashion to the kind of substituents or to the mono- or doublesubstitution along the main chain. In fact, the configuration of the repeat units and, consequently, the chain conformation and packing have to be taken into account and might be responsible for the properties of Afferent polymers. For instance, it has been shown that structural differences originating from different cis/trans tactic ratio in... 

If all the units along the chain are trans or if all the units along the chain are ois the polymer is called tactic in the first case trans-tactic, in the second case cis-tactic. It may be noted that, in the case of 1,4 polyisoprene, these two possibilities correspond to guttapercha and natural rubber respectively tactic polymers of isoprene are naturally occurring polymers. 

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