Polyacetylene metathesis polymerization

A drawback to the Durham method for the synthesis of polyacetylene is the necessity of elimination of a relatively large molecule during conversion. This can be overcome by the inclusion of strained rings into the precursor polymer stmcture. This technique was developed in the investigation of the ring-opening metathesis polymerization (ROMP) of benzvalene as shown in equation 3 (31). 

Much yet remains to be done as regards the choice of the pendent radicals. The ring-opening metathesis polymerization of cyclooctatetraenes (Gins-burg et ai, 1989 Klavetter and Grubbs, 1988) has not yet been applied to the construction of high-spin polyacetylenes. 

Durham route, the metathesis polymerization of 7,8-bis(trifluoromethyl)tricyclo[4.2.2.0]deca-3,7,9-triene gives a high-molecular weight soluble precursor polymer that is thermally converted to polyacetylene (equation 19.6). The precursor polymer is soluble in common organic liquids and easily purified by reprecipitation. The end product can be aligned giving a more compact material with bulk densities on the order of 1.05 —1.1 g/cm. ... 

Grubbs and others have used the ring-opening metathesis polymerization to produce thick films of polyacetylene and polyacetylene derivatives (equation 19.7). 

Substituted polyacetylenes may be produced through the ring-opening metathesis polymerization of substituted cyclooctatetraenes.127... 

The Durham precursor route to polyacetylene is an excellent example of the application of organic synthesis to produce a precursor polymer whose structure is designed for facile conversion to polyacetylene. Durham polyacetylene was first disclosed by Edwards and Feast, working at the University of Durham, in 1980 227). The polymer (Fig. 6 (I)) is effectively the Diels-Alder adduct of an aromatic residue across alternate double bonds of polyacetylene. The Diels-Alder reaction is not feasible, partly for thermodynamic reasons and partly because it would require the polymer to be in the all m-conformation to give the required geometry for the addition to take placed 228). However, the polymer can be synthesised by metathesis polymerization of the appropriate monomer. 

Precursor Polyacetylene Via Ring Opening Metathesis Polymerization (ROMP)... 

Partially substituted derivatives of polyacetylene are synthesized via the ring-opening metathesis polymerization (ROMP) of cyclooctatetraene (COT) and its derivatives. Certain poly-COT derivatives afford soluble, highly conjugated poly acetylenes. These materials exhibit large third-order optical nonlinearities and low scattering losses. 

Extending the metathesis polymerization methodology to other cyclooctate-traene derivatives, provides a convenient route to a variety of substituted polyacetylene derivatives. For example, soluble conjugated polyacetylene derivatives can be prepared through the ROMP of trimethylsilylcyclooctatetraene (40) Eq. (41) [60 a d]. 

The third route involves metathesis polymerization of cyclooctatetraene with tungsten catalysts, yielding polyacetylene as an insoluble film along with oligomers (iOi). By first polymerizing cyclooctene and then adding cyclooctatetraene, a soluble, red block copolymer was obtained. On the basis of the visible absorption spectrum, at least two or three cyclooctatetraene units were concluded to have been added to the polymer chain forming a short polyacetylene block. No conductivity data were reported for this copolymer. 

The Durham route to polyacetylene 103, 104) involves the metathesis polymerization of 1 to give a soluble but thermally unstable high polymer (Scheme 5.2). Slowly at room temperature, or more rapidly at 80 °C, the polymer undergoes a retro-Diels-Alder reaction. This reaction results in elimination of a substituted benzene and formation of amorphous polyacetylene. An enormous weight loss accompanies the conversion, but high-density films were produced with no apparent voids. The kinetics of the transformation reaction were extensively studied (JOS). 

In principle, conjugated materials may either be directly synthesized via metathesis polymerization of acetylene or 1-alkynes, via ROMP of various cyclooctatetraenes (COTs) or via ROMP of polyene precursors as realized in the Durham route [107-111]. The first direct polymerization of acetylene to yield black untreatable unsubstituted polyacetylene was achieved with W(N-2,6-i-Pr2-C6H3)(CH-t-Bu)(OC-t-Bu)2 [112]. In order to obtain soluble polymers, polyenes were prepared via the ROMP of a polyene-precursor, 7,8-bis(trifluoromethyl)tricyclo[4.2.2.02 5]deca-3,7,9-triene (TCDTF6), using initiators such as W(N-2,6-i-Pr2-C6H3)(CH-t-Bu)(OC-t-Bu)2) (Scheme 5.10) [113, 114]. 

A potential drawback of all the routes discussed thus far is that there is little control over polydispersity and molecular weight of the resultant polymer. Ringopening metathesis polymerization (ROMP) is a living polymerization method and, in theory, affords materials with low polydispersities and predictable molecular weights. This methodology has been applied to the synthesis of polyacetylene by Feast [23], and has recently been exploited in the synthesis of PPV. Bi-cyclic monomer 12 [24] and cyclophane 13 [25] afford well-defined precursor polymers which may be converted into PPV 1 by thermal elimination as described in Scheme 1-4. 

When well-defined, less Lewis-acidic metathesis polymerization catalysts are used to polymerize COT, a lower level of detectable sp defects are formed. Also, although the polyacetylene produced is still insoluble, the reaction proceeds slowly enough to allow manipulation of the liquid reaction solution before hardening. In this way, one can obtain films in a desired shape and location, e.g., on a semiconductor [123]. This procedure was found to result in better electrical contact than can be obtained when a free-standing film prepared via the Shirakawa route is simply pressed against an electrode. 

Klavetter and Grubbs presented a versatile and convenient route to polyacetylene films by condensed-phase metathesis polymerization of 1,3,5,7-cyclooctatetraene, using the tungsten-based complexes 3 and 4. ... 

The direct metathesis polymerization of acetylenes is not the only route to polyacetylenes using olefin metathesis chemistry. Below are summarized some of the other methods that have been developed in recent years. 

Gorman, C.B., Ginsburg, E.J., Grubbs, R.H., 1993. Soluble, highly conjugated derivatives of polyacetylene from the ring-opening metathesis polymerization of monosubstituted cyclooctatetraenes synthesis and the relationship between polymer structure and physical properties. I. Am. Chem. Soc. 115, 1397-1409. 

Additional improvements in preparations of polyacetylene came from several developments. One is the use of metathesis polymerization of cyclooctatetraene, catalyzed by a titanium alkylidene complex. The product has improved conductivity, though it is still intractable and unstable. By attaching substituents it is possible to form soluble and more stable materials that can be deposited from solution on various substrates. Substitution, however, lowers the conductivity. This is attributed to steric factors introduced by the substituents that force the double bonds in the polymeric chains to twist out of coplanarity." Recently, a new family of substituted polyacetylenes was described. These polymers form from ethynylpyridines as well as from ethynyldipyridines. The polymerization reaction takes place spontaneously by a quatemization process ... 

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