Polytriacetylene

Fig. 15. Examples of conjugated polymers. The dotted lines indicate bonds lateral to the conjugated backbone. R and R2 denote end groups. The simplest conjugated polymer is polyene with alternating single and double bonds. By introducing triple bonds, one goes from polyene over polydiacetylene and polytriacetylene to polycarbyne. The aromatic polyphenylene and polyphenylenevinylene contain additional phenyl rings. Polythiophenes additionally possess sulfur atoms...

In polytriacetylenes (Fig. 19) the conjugation across the backbone is weaker than for polyenes. Conversely, the PTAs are stable under normal laboratory condition and show no degradation over months, which makes them an interesting candidate for applications. Furthermore, the chemical synthesis allows to have a well defined number of monomer units and to attach definite functional groups enabling the study of the structure-property relationships in nonlinear optics. 

Fig. 19. Molecular structure of polytriacetylenes (PTA). The conjugation ends are substituted by donor (dimethylaniline) and acceptor (nitrophenyl) groups to modify the nonlinear optical properties. The lateral OTBDMS ((ferf-butyl)dimethylsilyloxyl) groups serve for improved solubility in the solvent (typically chloroform)...

In the following we will first discuss examples of polytriacetylenes (Fig. 19) and then compare these results with the ones of other molecular structures. Monomers of poly triacetylenes with the three neutral end groups TMS (trimeth-ylsilyl), TES (triethylsilyl), and TIPS (triisopropylsilyl) have been investigated. By deconvolution of the absorption spectra assuming Gaussian fine shapes the wavelengths A00 for the absorption process from the ground state to the zero vibrational mode of the lowest excited state are obtained as described below. 

The pure TMS and TES polytriacetylene oligomer series were investigated including a polymer sample. The different end groups in the polymer have a negligible influence on the molecular property as the PTA backbone is long enough to dominate the molecular properties. 

J.2 Stable Soluble Conjugated Carbon Rods with a Polytriacetylene Backbone... 

Scheme 13-14 Preparation of the soluble carbon rods 58a-e which have a polytriacetylene backbone. The length shown is the distance between the two extreme tips of the end-capping phenyl rings.

The polydiacetylenes and polytriacetylenes differ from polyacetylene because preorganization of the diacetylene and triacetylene is required for a successful polymerization (7). This remarkable observation was first recognized (8,9) in 1969 and marks the beginning of modern polydiacetylene and polytriacetylene chemistry. In a few cases, this topochemically controlled polymerization occurs from a crystal of the monomer to a crystal of the polymer, giving rare examples of macroscopic single polymer crystals (9). 

Preparation of Polytriacetylene. Soon after the early understanding of the diacetylene polymerization was reported (8,9), attempts were made to polymerize a triacetylene to produce a polytriacetylene (45). However, these early attempts as well as more recent efforts (7) were not successful. The difficulty of the topochemically controlled polymerization is the organization of the triacetylene monomer with a translational repeat distance of about 0.74 nm. 

Polytriacetylenes have recently been prepared by a topochemically controlled polymerization of a triacetylene. This was accomplished using the host-guest strategy (Fig. 8). A vinylogous amide was used to establish the required translational repeat distance and y-rays were necessary to induce the polymerization (48). 

The pyridine host can be removed from the polytriacetylene by washing with acid and the polytriacetylene can be dissolved in base. The optical and Raman spectra are in accord with those that have been observed for the oligomers. 

The ene-yne polymeric structure was the result of multiple 1,4-additions between adjacent molecules. Wegner originally utilized different monomeric functional groups and determined that those capable of H-bonding, such as urethanes and diols, provided the necessary intermolecular alignment to facilitate the conjugate addition. Fowler and Lauher further elaborate on soUd-state formation and chemistry of polydiacetylene and polytriacetylene in Chapter 5. 

The preparation of new polydiacetylenes and polytriacetylenes is complicated by the fact that no one has demonstrated a direct 1,4-diacetylene or a 1,6-triacetylene polymerization in solution, 1,2-polymerizations being more favorable. However, the polymers can be prepared in the solid state as the result of a topochemical polymerization. Topochemical reactions are solid state reactions in which the product and the regio- and stereochemistry of a reaction are directly controlled by the preorganization of the reactants. 

The analysis of the diffraction data showed that the irradiated crystals consist of a solid solution of the monomer and its polymer. After 40 Mrad of radiation, the crystals polymerize to about 70%, the resulting polytriacetylene crystal structure is shown in Scheme 5.8. Further irradiation and polymerization brings about a sudden phase change in the crystals to an amorphous "glassy state that no longer gives useful diffraction data. 

The polytriacetylene was isolated by first extracting the irradiated crystals with concentrated aqueous HCl to remove the pyridine host and then by extracting the residue with methanol to remove any unreacted triacetylene. A red, amorphous solid, that does not melt up to 300 °C, was isolated in about 70% yield. It is insoluble in common organic solvents such as methanol, acetone, ethyl acetate and methylene chloride. The carboxylic add polymer slowly dissolved to form a red... 

Raman spectroscopy has proven to be an excellent method for characterizing conjugated polymers [38]. The nonpolar multiple bonds usually display intense Raman bands. The polytriacetylene described here is no exception. The Raman spectmm of the polytriacetylene (0.011 g in 0.1 m NaOH) shows only two intense bands in the 2500-500 cm region at 2148 and 1552 cm . These can assigned to the C=C and C=C bonds respectively [4]. 

Anthony, J.A., Boudon, C., Diederich, E, Gisselbrecht, J.R, GramHch, V, Gross, M., Hobi, M., and Seiler, R, Stable, soluble, conjugated carbon rods with a persilylethynylated polytriacetylene backbone, Angew. Chem. Int. Ed. Engl, 33, 763,1994. 

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