Linear acetylene oligomers

This article summarizes efforts undertaken towards the synthesis of the cyclo[ ]carbons, the first molecular carbon allotropes for which a rational preparative access has been worked out. Subsequently, a diversity of perethynylated molecules will be reviewed together, they compose a large molecular construction kit for acetylenic molecular scaffolding in one, two and three dimensions. Finally, progress in the construction and properties of oligomers and polymers with a poly(triacetylene) backbone, the third linearly conjugated, non-aromatic all-carbon backbone, will be reviewed. 

To improve the processability of PPQ, appropriate phenylquinox-aline oligomers were end-capped with acetylenic groups using 3-(3,4-diaminophenoxy)phenylacetylene (43) or 4-(3- and 4-ethynyl-phenoxy)benzil (44, 45) (Eq. 10). The processability was improved but at the sacrifice of the thermooxidative stability. In general, cured acetylene-terminated heterocyclic polymers are less stable in a thermooxidative environment than the parent linear polymer. 

Oligomerization of alkynes can lead to linear enyne dimers or afford cyclic oligomers. Dimerization of terminal acetylenes induced by Ru complexes yields stereoisomeric conjugated head-to-head enynes depending on the substituents 532... 

The reaction of a BMI with an acetylenic end-capped oligomer has been undertaken. The mechanism was thought to proceed according to the two pathways indicated in Fig. 21 either by a kind of ene synthesis with formation of an hypothetic cycloallene leading to a dihydronaphthalene derivative (pathway a) or by a condensation concerted with an hydride shift (pathway b). No experimental proof was given for the structure but the condensation of both oligomers gave an expected linear product with a better Glc than the one of BMI alone (324 J m 2 vs. 34 J m 2) [73]. 

Repetitive coupling of acetylenes with aryl halides is an effective way to directly build hyperbranched architecture in a stepwise manner. This type of polycoupling is often catalyzed by palladium complexes in the presence of amines and has been widely used for the preparation of well-defined oligomers, linear polymers, and perfectly branched dendrimers [19,20]. 

Polymerization of substituted acetylenes has often been attempted by using radical and ionic initiators since a long time ago 5). In most cases, however, the products were linear oligomers whose molecular weights (MW s) were a few thousand (Eq. (2)). Also cyclotrimers often formed as by-products. Thus it was rather difficult to synthesize selectively polymers whose MW s are higher than ten thousand. 

Linear or cyclic oligomers containing some conjugated double bonds can be obtained from allene or its superior homologs, eventually in the presence of 1,3-butadiene or acetylene (in the latter case co-oligomers result)5. Transition metal complexes, in particular low-valence complexes of Group VIII elements, were found to be the best catalysts. Dehydrogenation reactions, carried out on cis-1,4- or tram-1,4-polybutadiene and in the presence of chloranil, quantitatively yielded polyacetylene within 8—15 hours at 130 °C6. ... 

A substantial effort in this laboratory has been directed toward the synthesis and characterization of acetylene terminated oligomers for use as addition-curable, moisture-resistant, thermoset systems. Early work on the acetylene-terminated phenylquinoxalines demonstrated the moisture insensitivity of the product generated from the thermal cure. Studies of various difunctional acetylene-terminated monomers has demonstrated that the polymerization is a free radical propogation of the acetylene moiety to a linear conjugated polyene. 

Table 9.10 Examples of Polyimide Structural Adhesives Including the Main Families of Linear Polyimide Precursors as Well as Oligomers Terminated by Ethylenic Double Bonds and Acetylenic Groups...

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