Two-Step Aromatic Networks

In two-step aromatic networks the precursor chains are much longer than the average length between crosslink points  

Unlike one-step networks, where all stiff segments projecting from each branchpoint are completely or almost completely indistinguishable from each other, in 

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There are in the literature three reported methods to prepare two-step aromatic polyamide networks. Two of them use the reduction of a pendant nitro group to a reactive amine as a means to create reactive sites along the precursor chain. A U.S. patent by Erhan [614] claims the preparation of high molecular weight linear polyamide by either interfacial or solution polymerization of a single AB-type monomer with itself ... 

The third method to create two-step aromatic polyamide networks is chemically similar to the one discussed above in Sect. 3.1.6.2. Here, a fraction of the terephthalic monomer in the reaction solution is replaced by 2,3-cyclobuta-terephthalic acid (619a). Upon polymerization a lyotropic aromatic polyamide is obtained which, when heated to 380 °C or higher, randomly crosslinks along the stiff chains to form a two-step network. Not much else is known at present about this system. 

Two-step polydiacetylene networks in which the aromatic repeat unit is other than thiophene were also prepared [628]. Aromatic (Ar) units such as... 

Aromatic networks prepared from long segments end-capped by ethynyl groups will be described in Sect. 3.1.6.3, and in Sect. 3.2.3 we shall describe the two-step preparation of substantially rigid networks using the reactions of ethynyl groups present in long linear chains to form interchain crosslinks. 

To the best of our knowledge, no fully aromatic polyester networks have been reported in the literature until now. Several reports appeared describing the preparation of liquid crystal thermosets from rodlike polyester segments and branchpoints or junctions created by the reaction of reactive end-caps with one another [417-419]. These end-capped polyester networks will be discussed, however, with all other fully or mostly aromatic networks made in two steps by reacting the end-caps of stiff segments with each other. 

The linear polyamides with pendant carboxy groups prepared by both methods are reacted with rigid aromatic diamines, such as DABA, in solution under Yamazaki conditions to yield gels of two-step rigid aromatic polyamide networks such as [619] ... 

By the use of aromatic diamines, we believe such reactions can be employed in the creation of highly crosslinked fully aromatic and conjugated two-step networks ... 

There exists in the literature at least one [603] reference describing the two-step preparation of highly aromatic networks from linear aromatic polymer chains decorated with occasional pendant acetylene group. The precursor linear polymer ... 

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