Phenylacetylene monomers

Figure 4.6. Side-chain variations for phenylacetylene monomers.

Branched phenylacetylene monomers can be used to construct dendrimers on supports. This was accomplished using a triazene tether as a focal point, the reaction triad outlined above, and an AB2 monomer, as seen in Scheme 7.16 As in similar solution-phase convergent dendrimer syntheses, the first step was to prime the periphery of the dendrimer with t-butyl groups by di-coupling of a triazene-tethered dibromoaryl monomer with (di-f-butyl-phenyl)acetylene. In this and all subsequent coupling steps, an excess amount of the monodendron acetylene was used to drive the reaction to completion. The reagents and catalysts were washed away and the excess monodendron was recovered. At the end of the first step, the tri-aryl dendron I-M3-(t-Bu)4 was cleaved from the support (M represents the dendritic monomer unit). Two equivalents of this product were then coupled with the triazene-tethered di-acetylene ary] monomer to produce the heptaaryl dendron I-M7-(r-Bu)8. 

Figure 12-1 Fundamental characteristics of the phenylacetylene monomer unit. In this chapter the three basic phenylacetylene valence angles will be represented with a dot ( ) and line (—) convention, as shown in the structure key. The phenylacetylene linkage is characterized by a flexible bending force constant, F(S) [5], and an sp -sp bond order of 1.18. The indicated bond distances are taken from the crystal structure of tolane [6]. The bottom plot shows the torsional potential energy curve in tolane as determine by supersonic jet spectroscopy (adapted from [8]). The outer edges of van der Waals spheres for the two ortho hydrogens on one side of tolane are separated by nearly 2 A in the planar conformation. The result is a virtually barrierless rotation.

Figure 12-8 Plot of the number of unique cyclic geometries versus the number of phenylacetylene monomer units and structures of the unique cyclic geometries for phenylaeetylene maerocycles eomprised of between three and eight monomer units.

Figure 12-9 Examples of some unique cyclic geometries that have been realized using functionalized or branched phenylacetylene monomers.

Liquid crystalline dendrimers characterized by a well-defined molecular weight and exhibiting thermotropic columnar mesophases have been reported very recently. An example is the dendrimers based on the stiff, triconnected phenylacetylene monomer [93] is shown in Fig. 18. 

SCHEME 7.4 The synthesis of a solid phase library of pentamers using ortho-, meta-, and para-substituted phenylacetylene monomers. 

In 1969, Kern first reported that the Wilkinson catalyst, RhCl(PPh3)3, could be used to polymerize phenylacetylene monomer, and that the color of the resulting poly(phenylacetylene) was dependent on the polymerization conditions used." Recently, analogues of the Wilkinson catalyst such as [(2,5-norbornadiene)RhCl]2 (7, Figure 21.1) have been developed that selectively produce c/ -PPA even at room temperature. This is in contrast to the low temperature (—78 °C) required when using Ziegler-Natta catalysts to produce perfect c/ -polyacetylene." ... 

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