Polymers cis-transoid

Joo et al. utilized the highly water-soluble rhodium complex [RhCl(CO)(TPPTS)2] for the polymerization of terminal alkynes (phenylacetylene and (4-methylphe-nyl)acetylene for the structure of TPPTS cf. Section 7.2.2.3) [150]. This catalyst selectively produces cis-transoid polymers at room temperature in homogeneous solution in water/methanol mixtures, as well as in biphasic mixtures of water and chloro-... 

Polymerization of phenylacetylenes is feasible even in aqueous media by using water-soluble catalysts. For example, (cod)Rh+(mid)2PF6 (mid = N-methylimidazole) provides cis-transoidal poly(phenylacetylene) (cis 98%) in high yield (98%) (166). Other catalysts, (cod)Rh(S03C6H4-p-CH3)(H20) and (nbd)Rh(S03C6H4-p-CH3)(H20), work as water-soluble catalysts to produce cis-transoidal polymer (166). The polymerizations can be done under air thus, a poly(phenylacetylene) thin film (thickness ca 250 nm) is readily obtained by dropping a dilute chloroform solution of phenylacetylene onto the water surface of a dilute aqueous solution of (cod)Rh(S03C6H4-p-CH3)(H20) in an open beaker (166). 

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. 

Oligomerization and polymerization of terminal alkynes may provide materials with interesting conductivity and (nonlinear) optical properties. Phenylacetylene and 4-ethynyltoluene were polymerized in water/methanol homogeneous solutions and in water/chloroform biphasic systems using [RhCl(CO)(TPPTS)2] and [IrCl(CO)(TPPTS)2] as catalysts [37], The complexes themselves were rather inefficient, however, the catalytic activity could be substantially increased by addition of MesNO in order to remove the carbonyl ligand from the coordination sphere of the metals. The polymers obtained had an average molecular mass of = 3150-16300. The rhodium catalyst worked at room temperature providing polymers with cis-transoid structure, while [IrCl(CO)(TPPTS)2] required 80 °C and led to the formation of frani -polymers. 

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. 

Poly(phenylacetylene) derivatives 104—106 bearing achiral functional side groups have been synthesized. The polymers possess a stereoregular cis-transoidal structure. Excess single-handed helicity of the main chain can be induced for the polymers by the interaction with chiral molecules.26b,184 188 For example, 104 shows intense CD bands in the presence of optically active amines and amino alcohols including 107... 

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. 

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. 

The cationic cyclization of polyisoprene with acid catalysts is well documented. The same reaction in polybutadienes requires much more severe conditions, higher temperatures and more acidic catalysts, and until recently has received much less attention. A cyclized polymer with a reduction of 35—40% of the initial unsaturation, can be prepared by treating cis-l,4-polybutadiene with an alkyl aluminium chloride-organic halide catalyst in xylene solution at >100 C."- Such polymers, containing polycyclic sequences apparently at random within the chains, have better skid resistance and tensile properties than the parent polymer. Cyclization has been reported to accompany other reactions in polydienes, for example the radiation-induced addition of carbon tetrachloride to 1,2-polybutadiene, and the direct addition of a o j unsaturated carboxylic acids (acrylic and cinnamic) to polydienes and polypentenamers. It is reported that the thermal isomerization of cis-transoidal poly(phenylacetylene) is accompanied by cyclization, and additionally chain scission and aromatization at temperatures >120°C. ... 

Polyacetylene is expected to be an electroconductive polymer. Polymerization with organorhodium compounds easily affords a highly stereoselective polymer. For example, the polymerization of phenylacetylene with [(NBD)RhCl]2 in a triethylamine solvent affords a polymer having a cis-transoid structure with mol. wt. 520000 in 93% yield [124,125]. 

Polyacetylene the simplest example of this class of poly-conjugated systems was synthesized in 1958 by Natta et al.(77) as the trans form by means of typical Ziegler catalysts. Cis and trans configurations of polyacetylene were reported first by Watson, McMordie and Lands (78) in 1961. Shirakawa and Ikeda (79 81) synthesized an all-cis and an all-transpolyacetylene and pointed out the cis-transoid(IV)and trans-transoid(V)structure of these polymers. 

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. 

Simionescu Cl, Percec V (1980) Thermal cis-trans isomerization of cis-transoidal polyphenylacetylene. J Polym Sci Polym Chem Ed 18 147-155. doi 10.1002/pol.l980. 170180114... 

Karim SMA, Nomura R, Masuda T (2001) Degradation behavior of stereoregular cis-transoidal poly(phenylacetylene)s. J Polym Sci A Polym Chem 39(18) 3130-3136. doi 10.1002/pola.l294... 

Recently, a few heterogeneous Rh catalysts have been reported. Kopaczynska et demonstrated that rhodium nanoparticles stabilized by polyvinylpyrrolidone exhibit catalytic activity in the polymerization of PA. The stereochemistry of the polymer produced with this catalyst is purely cis-transoidal. The progress in polymerization can be monitored by atomic force miaoscopy (AFM) and transmission electron microscopy (TEM). This report includes the first detection of a spectacular helical poly(PA) using AFM imaging. Son and co-workers reported that the nanopartides composed of the (benzoquinone)Rh(cod) complex and aluminum compounds catalyze the polymerization of PA. The catalyst nanopartides can he recovered hy centrifugation, and the recovered nanopartides show almost the same artivity. 

---