Phenylacetylene metathesis polymerization

Figure 22 Metathesis polymerization of phenylacetylenes in the presence of in s/ffy-generated (arene)M(CO)s complexes.

Poly (acetylenes) [16], There are several catalysts available for polymerization of substituted acetylenes. Whereas Ziegler-Natta catalysts are quite effective for polymerization of acetylene itself and simple alkylacetylenes, they are not active towards other substituted acetylenes, e.g. phenylacetylenes. Olefin-metathesis catalysts (Masuda, 1985 Masuda and Higashimura, 1984, 1986) and Rh(i) catalysts (Furlani et al., 1986 Tabata, 1987) are often employed. In our experience, however, many persistent radicals and typical nitrogen-containing functional groups serve as good poisons for these catalysts. Therefore, radical centres have to be introduced after construction of the polymer skeletons. Fortunately, the polymers obtained with these catalysts are often soluble in one or other organic solvent. For example, methyl p-ethynylbenzoate can be polymerized to a brick-coloured amorph- See the Appendix on p. 245 of suffixes to structural formula numbers. 

A striking feature of the stereoregular polyacetylenes is their simple NMR spectral patterns, which facilitates elucidation of the polymerization mechanism as well as the polymer structure. A co-polymer of phenylacetylene with partly G-labeled phenylacetylene (Ph G= GH) shows two doublet carbon signals with /i3c-i3C of 72 Hz, indicating the presence of G= G bond in the polymer backbone.This is a clear indication of the insertion mechanism instead of the metathesis pathway. 

Neutral catalysts or catalyst precursors based on fluorinated ligand systems have been applied in compressed CO2 to a broad range of transformations such as Zn- and Cr-catalyzed copolymerization of epoxides and CO2 [53, 54], Mo-catalyzed olefin metathesis [9], Pd-catalyzed coupling reactions [43, 55, 56] and Pd-catalyzed hydrogen peroxide synthesis [57]. Rhodium complexes with perfluoroalkyl-substituted P ligands proved successful in hydroformylation of terminal alkenes [28, 42, 44, 58], enantioselective hydroformylation [18, 59, 60], hydrogenation [61], hydroboration [62], and polymerization of phenylacetylene... 

The ethynyl groups of ethynylbenzenes undergo polymerization by the action of Group 6, 7, and 9 transition metal catalysts, often used for olefin metathesis, to give the corresponding polyenes which are frequently called poly(phenylacetylene)s (43) [15, 47]. 

Substituted phenylacetylenes that have been polymerized by metathesis catalysts also include those with the following substituents 2- and 4-methyl- (Yamaguchi 1991 Mizumoto 1995 Vijayaraj 1995), various 2-alkyls (Abe 1989, 1994), 2,5-di-... 

In the case of the OsCb/phenylacetylene catalyst, norbomene and norbornadiene are also polymerized in a highly regular manner, and products with more than 90% cis-syndiotactic units are obtained. Most high-trans polymers produced with conventional catalysts are atactic. One of the few exceptions is poly(5,5-dimethylnorbornene) prepared with (mesitylene)W(CO)3/EtAlCl2/cJt -2,3-epoxynorbornane. This metathesis polymer contains 85% trans-vinylene enchained units that are predominantly isotactic. 

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. 

On the other hand, metathesis catalysts based on group V and VI metals effectively polymerize mono- and disubstituted alkynes to the corresponding substituted PAs. These catalysts are typically the metal chlorides, used with or without main-group organometallic cocatafysts, or metal carbonyls activated with light (Fig. 7) [110]. The latter e of catalyst is known for Mo and W only. Water can even be used as a cocatalyst with these catalysts for some monomers. For example, WQ5 I/2H2O polymerizes phenylacetylene to a soluble, powdery poly(phenylacetylene) with M = 15,000 g/mol and PDI = 2.06 [113]. 

Pfleger et al. [369] have studied photoconduction in undoped poly(phenylacetylene) which they prepared by coordination polymerization of phenylacetylene using the metathesis catalyst WOCU/Pl Sn. The polymer thus obtained was predominantly in the c/5-transoidal form, as demonstrated by IR spectra, and had a molecular weight of (A/ ) 91 000. The photoconduction threshold has been detected at 410 nm, although absorption of the film extended up to 550 nm. It is suggested that the mechanism of photogeneration is intrinsic by its nature. The formation of initial charge carrier pairs occurs by an exciton autoionization process [38]. From the temperature dependence, as well as Irom the field dependence of the quantum yield, the pair separation distance was established to be ca 2.2 nm. 

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