Benzocyclobutene-Acetylene Polymers

The formation of the ether linked systems 71 and 72 started with the nitro substituted anhydrides 62 and 73. Anhydride 73 was first reacted with 4- 

While there was a substantial amount of data reported on the syntheses of these acetylenic benzocyclobutene imide monomers, there was surprisingly little characterization done on the resulting homopolymers [91, 94, 95]. Table 17 details what information is available. 

The DSC of primary acetylenic monomer 68 showed the presence of two distinct exotherm peaks. The first peak of 202 °C was attributed to the acetylene reacting separately with a second acetylene while the exothermic peak of 270 °C was proposed to be a benzocyclobutene reacting with a second benzocyclobutene. The resulting polymer was believed to be a highly crosslinked network and to some extent the Tg of 380 °C would support this contention. Interestingly, this homopolymer had poor thermal stability at 343 °C in air, with a retention of 41 % of its weight after 200 h. 

Replacing the hydrogen in 68 with a phenyl group leads to the secondary acetylenic monomer 70. It was believed that this disubstituted acetylene would suppress the reaction of acetylene with itself and insure that there was an acetylene functionality available for reaction with the o-quinodimethane at 200 °G The DSC of 68 showed the presence of a single exothermic peak at 263 °C which the authors felt was adequate evidence for the occurrence of a Diels-Alder reaction between the acetylene and benzocyclobutene. Unfortunately they did not report on any control experiments such as that between diphenylacetylene and simple benzocyclobutene hydrocarbon or a monofunctional benzocyclobutene in order to isolate the low molecular weight cycloaddition product for subsequent characterization. The resulting homopolymer of 68 had a Tg of 274 °C and also had the best thermooxidative stability of all of the acetylenic benzocyclobutenes studied (84% weight retention after 200 h at 343 °C in air). 

The order of thermal stability based upon the results of the isothermal TGAs was concluded to be 70 71 68 72. This same order was proposed to reflect the relative reactivity of the monomers in a Diels-Alder reaction. 

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The use of benzocyclobutene as the source of the diene in a Diels-Alder polymerization offers a unique solution to the problems described above. Benzocyclobutene containing monomers can be stored indefinitely at room temperature without concern for further advancement of the molecular weight. It is only when benzocyclobutene is heated to temperatures of approximately 200 °C that the reactive diene, o-quinodimethane, is formed at a significant rate and enters into reaction with the dienophile. The only requirement of the dienophile is that it must be stable at these temperatures and not undergo reaction with itself. The most common dienophiles that have been successfully used in the formation of polymers from AB type benzocyclobutene monomers have been acetylenes, olefins and maleimides. 

By far most of the work reported in the literature dealing with the reaction of benzocyclobutene with acetylenes has been done by Tan and Arnold [11,91,94, 95]. They prepared several monomers and their respective polymers. Figure 35 outlines the synthetic strategies utilized in the preparation of these monomers. 

Table 17. Thermal properties of acetylenic benzocyclobutene Polymers...

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