Cis-cisoid polymers

Polymerization of phenylacetylene in compressed (liquid or supercritical) CO2 has been studied using a Rh catalyst, [(nbd)Rh(acac)l2 (167). Higher polymerization rate is obtained in CO2 than in conventional organic solvents such as THF and hexane. Polymerization in the presence of a phosphine ligand, p-[F(CF2)6(CH2)2]-CeH4 3P, predominantly produces cis-transoidal polymers, while, without the ligand, both cis-transoidal and cis-cisoidal polymers are formed. 

The yields of the polymerization of three alkynes by all rare earth catalyst systems in chlorobenzene are given in table 18. The scandium and neodymium systems show the highest activities. Clearly, these new Ziegler-Natta catalysts are capable of promoting high-molecular-weight, cis-cisoid polymers in high yields. 

Stereospecific polymerization catalysts. The poly(phenylacetylene) prepared with Ziegler catalyst 28 possesses mainly the cis-cisoidal structure (as evidenced by the C—H out-of-plane deformation at 740 cm" in the IR spectrum), and is insoluble in all solvents owing to its high crystallinity. Rhodium catalysts such as 29 and 30 provide a soluble, cis-transoidal poly(phenylacetylene) . This polymer exhibits a sharp peak due to the olefinic proton at b 5.8 in the NMR spectrum. 

As shown in Figure 21.2, four steric (geometric) structures are theoretically possible for polyacetylenes, that is, cis-cisoid, cis-transoid, trans-cisoid, and trans-transoid, because the rotation of the single bond between two main chain double bonds in the main chain is more or less restricted. Polyacetylene can be obtained in the membrane form by use of a mixed catalyst composed of Ti(0-n-Bu)4 and EtsAl, the so-called Shirakawa catalyst (1) both the cis- and trans-isomers are known, which are thought to have cis-transoidal and trans-transoidal structures, respectively (Table 21.1). Phenylacetylene can be polymerized with a Ziegler-type catalyst, Fe(acac)3/Et3Al (2) (acac = acet-ylacetonate), Rh catalysts (7), and metathesis catalysts (3-5) that contain Mo and W as the central metals, to provide cis-cisoidal, cis-transoidal, cis-rich, or trans-rich polymers, respectively. 

NMR, substituted poly(phenylacetylene) polymers obtained from either Fe or Rh catalysts have virtually an all-cis structure, although it is difficult to distinguish between the cis-cisoidal and cis-transoidal structures. On the other hand. Mo- and W-derived substituted poly(phenylacetylene)s have, respectively, cis-rich and trans-rich structures. 

Polyacetylenes are the most typical and basic r-conjugated polymers, and can ideally take four geometrical structures (trans-transoid, trans-cisoid, cis-transoid, cis-cisoid). At present, not only early transition metals, but also many late transition metals are used as catalysts for the polymerization of substituted acetylenes. However, the effective catalysts are restricted to some extent, and Ta, Nd, Mo, and W of transition metal groups 5 and 6, and Fe and Rh of transition metal groups 8 and 9 are mainly used. The polymerization mechanism of Ta, Nd, W, and Mo based catalysts is a metathesis mechanism, and that of Ti, Fe, and Rh based catalysts is an insertion mechanism. Most of the substituted polyacetylenes prepared with W and Mo catalysts provide trans-rich and cis-rich geometries respectively. Polymers formed with Fe and Rh catalysts selectively possess stereoregular cis main chains. 

When acetylene is polymerized by the Shirakawa technique at low temperature, c. — 70°C, the product film has a yellow-gold appearance, it is an insulator with a conductivity of c. 10 (Qcm) and it has a low free spin density. Diffraction and spectroscopic studies establish that there is regular single-double bond alternation and that the material is predominantly the cis-cisoid homopolymer. However, this material is stable only at low temperature, and when the polymerization is conducted at room temperature a different material with a silvery appearance and a predominantly trans-transoid structure is obtained essentially the same material is produced when the cis polymer is allowed to warm to room temperature. This form of polyacetylene is a semiconductor with conductivity in the to... 

Encapsulation of any of these c/s-conformers into libraries of columnar supra-molecular dendrimers eliminates the intramolecular electrocyclization and replaces the hehx-coil transition with an unprecedented helix-helix transition and a reversible transition from cw-transoidal to cis-cisoidal. When the repeat unit of the dendronized polymer also contains a stereocenter, this reversible process can be monitored by circular dichroism (CD) and visualized by different methods [104-111]. This concept was used to elaborate molecular machines that were interfaced for the first time with the real world to lift heavy objects [111]. 

Once prepared [14], the two enantiomeric MPA polymers, poly-(f )-2 and poly-(5)-2, showed null CD spectra in a number of solvents, suggesting the presence of analogous populations of both helical senses. Thus, despite the presence of stereogenic centres at the pendants, the resulting polymer was racemic in its axial chirality. NMR, Raman and differential scaiming calorimetry (DSC) [15-18] studies pointed to cis-cisoid configurations at their polyene backbones. 

AFM and MM studies showed that these polymers in CHCI3 presented identical handedness for the internal (polyene backbone) and the external (pendants) helices (3/1 helix), whereas in THF the internal and external helices (2/1 helix) presented opposite helical senses. DSC traces supported the cis-cisoidal and cis-transoidal helical structures associated with those structural features. 

Isomerism along the backbone of cw-poly(arylacetylene)s can arise from the different dihedral angles that are possible about the C-C bonds of the main chain. The transoid (s-trans) conformation is more extended, whereas the cisoid (s-cis) conformation is more compact and both are helical [126-129]. Degradation of the main chain structure occurs through cyclization reactions of the polyene backbone [126-129, 137-142], which require the polymer to adopt a cis-cisoidal conformation. Scheme 3 illustrates the stmctural transformations that degrade the polymer backbone. 6jr-Electrocyclization of triene segments in the backbone occurs under... 

A lot of work, mainly synthesis and elementary characterizations, have been performed on substituted polyacetylenes. It is somewhat surprising that almost no companion structural studies are ava able. The lack of structural data does not allow an accurate knowledge of the internal conformation of the polymer. Nevertheless these systems appear very interesting, the existence of a possible cis-cisoid conformatitm may involve a well defined helical structure such that we can speculate a relation between the specific elastic behavior of an helical-spring like stmcture and the optical properties of the conjugated backbone. 

Solvent polarity is also important in directing the reaction bath and the composition and orientation of the products. For example, the polymerization of butadiene with lithium in tetrahydrofuran (a polar solvent) gives a high 1,2 addition polymer. Polymerization of either butadiene or isoprene using lithium compounds in nonpolar solvent such as n-pentane produces a high cis-1,4 addition product. However, a higher cis-l,4-poly-isoprene isomer was obtained than when butadiene was used. This occurs because butadiene exists mainly in a transoid conformation at room temperature (a higher cisoid conformation is anticipated for isoprene) ... 

O j-polyacetylene shows 10 reflections in x-ray diffraction 439). The unit cell was identified as orthorhombic, a = 761 pm, b = 447 pm, c = 439 pm with a density of 1.16 g cm-3. In the original x-ray study the b axis was taken to be the chain axis but subsequent electron diffraction studies allowed fibre patterns to be obtained 440). On the principle that, for all polymers, the fibre axis is the chain axis, c was identified as the chain direction although there is some dispute The analysis cannot definitely distinguish between the cis-transoid and trans-cisoid structures. 

Catalyst complexation with a Lewis base or other electron donor may affect the polymer microstructure in different ways. If the added component occupies one coordination site, a monomer coordinates to another site of the active species with one double bond, i.e. as an s-trans-rf ligand, which gives rise to the formation of trans-1,4 monomeric units via the pathway (a)-(b) [scheme (10)]. Depending on the lifetimes of metal species complexed with the monomer and with the Lewis base or the other donor [scheme (11)], mixed cis-1,4/trans- 1,4-polybutadienes or an eb-czs-1, 1 A trans-1,4-polymer can be formed. One should mention in this connection that equibinary cis-l,A/trans- 1,4-butadiene polymers can also be formed in systems without the addition of a Lewis base or other electron donor in such cases, the equilibrium of the anti-syn isomerisation is not shifted and there are equal probabilities for the reaction pathways involving coordination of a transoid monomer and a cisoid monomer [7]. 

Polyacetylene contains conjugated double bonds in a linear structure [51-54], which leads to special electrical conductivity properties. For this reason, polyacetylene and some other related polymers have applications in the optical and electronic industries. Pojyacetylene synthesized at relatively low temperature seems to have a cis-conformation, with two possible cis- forms indicated as c/s-transoid, and frans-cisoid, as shown below ... 

A long sought goal in mechanistic polymer chemistry has been the determination of those factors which lead to cis, trans or vinyl structures in diene polymers. Various proposals have been made jnd are summarized in the comprehensive revie edited by Saltman. The simplest proposal, advanced by Cossee and Arlman, assigns the dominant role to the nature of the diene coordination. In this mechanism bidentate coordination, e.g. of necessity involving the cisoid conformation of the diene, would lead to cis polymer. 

It has also been suggested that photoexcitation of the cis-transoid skeleton with energy corresponding to its optical band gap yields the trans-cisoid moiety (Tanaka et al., 1984a). The growth of four new IR absorption peaks, found after irradiation at 802, 1062, 1112, and 1259 cm-1, was ascribed to the vibrational modes in trans-cisoid (CH)X. Hence, the trans-cisoid structure does not collapse for a considerably long time in the actual (CH)X polymer, which suggests the existence of a local potential minimum around this structure in the potential hypersurface. 

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