Cis/trans content

EPM > CO > TP > CB. The highly crystalline TB had an etch rate about six times that of CB, ascribable to a morphology difference, while the partially crystalline TO had an etch rate somewhat higher than that of amorphous CO. Cis/trans content had little or no effect on the etch rate of the polyalkenamers. A mechanism involving crosslinking through vinyl units is proposed to explain the unexpected protection imparted to vinylene-rich polybutadienes by the presence of 1,2 double bonds. 

The data of Table II indicate that the etch rates for CB and its "homologues"—TP, CO (or TO), and EPM—tend to increase monotonically with a decrease in vinylene (-CH=CH-) unsaturation. The elastomeric EPM was chosen instead of crystalline polyethylene as a model for the fully saturated CB to avoid a morphology factor in etch rates, as was observed with crystalline TB. The difference in etch rates for the partially crystalline TO and the elastomeric CO (ratio of about 1.2 1.0) is attributable more to a morphology difference between these polyoctenamers than to the difference in their cis/trans content. Cis/trans content had likewise no perceptible effect on etch rates in the vinyl-containing polybutadienes (see Table I) if there was a small effect, it was certainly masked by the dominant effect of the vinyl groups. 

Figure 1. cis/trans content versus copolymer composition. 

Stereoselective ROMP has been reported with monomer (213). Initiator (211) affords highly stereoregular polymer with >98% trans C=C bonds in the polymer backbone.534 However, when (210) is used, >98% cis-poly-(213) is obtained.535 A similar situation occurs for the diester monomer (214). Furthermore, a rapidly equilibrating mixture of (210) and (211) can be used to allow intermediate cis/trans contents to be manipulated by the stoichiometry of the initiator mixture. 13C NMR536 and dielectric analyses537 suggested that trans-poly-(213) is highly syndio-tactic (92% r dyad content). The ROMP of other fluorinated olefins has been recently reviewed.538... 

The cis-trans content of polyacetylene formed with coordination catalysts depends on the polymerisation temperature the increasing trans content with increasing polymerisation temperature has been explained in terms of a thermally induced cis-trans isomerisation which occurs before crystallisation of the chain segment [10,76 78],... 

The mechanism is complicated by the possibility of anti-syn-isomerization and by n - a-rearrangements (it - r 3-allyl Act - r 1 -allyl). In the case of C2-unsubstituted dienes such as BD the syn-form is thermodynamically favored [646,647] whereas the anti-isomer is kinetically favored [648]. If monomer insertion is faster than the anti-syn-rearrangement the formation of the czs- 1,4-polymer is favored. A higher trans- 1,4-content is obtained if monomer insertion is slow compared to anti-syn-isomerization. Thus, the microstructure of the polymer (czs-1,4- and frazzs-1,4-structures) is a result of the ratio of the relative rates of monomer insertion and anti-syn-isomerization. As a consequence of these considerations an influence of monomer concentration on cis/trans-content of BR can be predicted as demonstrated by Sabirov et al. [649]. A reduction of monomer concentration results in a lower rate of monomer insertion and yields a higher trans-1,4-content. On the other hand the czs-1,4-content increases with increasing monomer concentration. These theoretical considerations were experimentally verified by Dolgoplosk et al. and Iovu et al. [133,650,651]. Furthermore, an increase of the polymerization temperature favors the formation of the kinetically controlled product and results in a higher cis- 1,4-content [486]. l,2-poly(butadiene) can be formed from the anti- as well as from the syn-isomer. In both cases 2,1-insertion occurs [486]. By the addition of electron donors the number of vacant coordination sites at the metal center is reduced. The reduction of coordination sites for BD results in the formation of the 1,2-polymer. In summary, the microstructure of poly(diene) depends on steric factors on the metal site, monomer concentration and temperature. 

Pristine polyacetylene is a typical organic semiconductor irrespective of its cis-trans content. The room-temperature conductivity measured by a conventional two-probe method and the energy gap determined by equation 19 are 1 x 10 S cm and 0.56 eV for 92.5% trans-polymQv, and 4 x 10" Scm and 0.93 eV for 80.0% cw-polymer, respectively In equation 19 (T is the electrical conductivity at T and ACg is the energy gap... 

Cell similar to that given by Baughman et al. [28) with b and c values exchanged chain axis along the longer cell axis Evaluated from polyacetylenes with various cis/trans contents 0 from [36)... 

The electrical resistivity of films obtained with various cis/trans contents by the above preparation was measured by the conventional two-probe method under vacuum in a temperature range of -120 to 20 °C [40], The resistivity and energy gap of trans-rich polyacetylene were 1.03 x 10" cm and 0.56 eV, respectively, whereas the values of a cis-rich (80%) one were 2.43 x 10 Q. cm and 0.93 eV, respectively. Hatano et al. [41] reported that the resistivity and energy gap measured on compressed pellets of powder... 

The cis isomer may be converted to the trans isomer, which is the thermodynamically stable form, by heating at 150°C for 30 min to 1 hr. The cis-trans content is conveniently determined from the relative intensities of bands in the IR spectra. 

The morphology of all polyacetylenes made with various Ziegler-Natta catalysts is fibrillar. The conductivity of films of polyacetylene depends on the cis-trans content, varying from 10 (ohm-cm) for the trans material to 10 (ohm-cm) for the cis isomer. Doping increases the conductivity of both cis- and frons-polyacetylene drastically. 

Both structures have a glass transition temperamre T, which is lower than that of natural rubber, although the for the ds-polymer is the lowest recorded so far for a hydrocarbon elastomer. Variation of the catalyst composition can be used to control the cis-trans content of the chain. For molar ratios (A1 W) of up to 2 1, a predominantly ds-polypentenamer is formed, but as the molar ratio is raised, increasing amounts of the trans placements are found in the chain, and at 6 1 this amounts to =90% trans content. 

The isomerisation between syn and anti rotamers of a large number of M(NR)(CHR XOR )2 (M = Mo (H W) complexes has been studied. Rate constants and acdvaticHi energies for this process are dependent on the nature of R, R, R2 and the solvent. The result of bulk polymerisations with 2,3-b/s(trifluoromethyl)norbom Kliene suggests a relationship between alkylidene rotamer isomerisation rates and the polymer cis/trans content.33 The molybctenum complex, Mo OC(CF3)2Me 2 N(2,6-P r 2C6H3) (CHCMe2Ph), catalyses the cross-metathesis of aryl- and alkyl-substituted terminal alkenes to unsymmetrical alkenes widi trans selectivities of >95%.335 The molybdenum alkylidene... 

A monomer such as butadiene opens up another stereochemical option—that is, whether the remaining double bond is cis or trans. Not surprisingly, the differing stereochemistries have different properties, with the trans tending to have higher transition temperatures. Conditions have been found that allow some, but not complete, control over the cis / trans content of the polymer. 

The catalyst used for polymerization influences several properties such as the solubility of PTMSP in various solvents (7). Even though the cis-trans content of PTMSP has not been quantitatively determined, Costa et al (5) and Izumikawa et al. (6) reported that the polymerization catalyst NbClj produced a more regular chain confi ation than TaClj. In addition, PTMSP synthesized using NbClj had a cis-rich structure relative to that prepared with TaClj. In this study, Sie effects of physical aging on gas permeability and molecular motion of PTMSP membranes synthesized using various catalysts are reported. 

The electrical conductivity of polyacetylene has been much improved since 1960 as shown in Fig. 7.1 [12]. Data reported by earlier investigators were obtained with nondoped compressed pellets prepared from the intractable black powder [13-15] therefore, the values were between 10 and 10 S/cm depending on the processing and manipulation of the specimens. The conductivities of as-prepared free-standing films were in the same range as those of the compressed pellets, and it become clear that values changed almost linearly with cis-trans content. For example, the values of 76 and 15% trans films were 1.3 x 10 and 6.0 x 10 S/cm, respectively [16]. At the first report of doping in 1977, the values were 0.5 and 38 S/cm for films doped with bromine and iodine [2], respectively, and 560 S/cm for films doped with AsF.< [3]. 

Poly acetylene, the simplest among the linear ir-conjugated polymers, is a semiconductor the conductivity is dependent on the crystallinity [1], the oxygen content [2,3], and the cis-trans content [4]. The electrical conductivity of crystalline films of poly acetylene varies from 10 S cm for the cw-isomer to 10 S cm for the ra/75-isomer [4]. Either cis- or trans-polyacetylene films can be doped with various electron-accepting species, such as halogens (L or Br.), metal halides (AsE, or FeCl,), protonic acids... 

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