Rigid Aromatic Polyester Networks

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 recent literature contains, nevertheless, very interesting novel syntheses of highly branched aromatic polyester dendrimers and of unbranched stiff aromatic polyester chains. These synthetic efforts will be described below. They indicate that conditions are presently ripe for the creation of novel aromatic polyester networks and gels. 

Prominent among these efforts are ones using the classical Schotten-Bauman [403-408] method, condensing aromatic acid chloride with aromatic hydroxyl in the presence of an organic base to create an ester group  

This technique was employed to create liquid crystalline rigid rodlike polyesters decorated with short flexible alkoxy side-chains rendering them soluble [420-422]  

Another poiyesterification using an aromatic acid chloride was developed by Higashi and associates [424,425], Accordingly, by activating in-situ carboxylic acids with diphenyl chiorophosphate, a direct poiyesterification of aromatic carboxylic acids with phenolic hydroxyl groups is achieved. This method was used by Aharoni [61] to create relatively high molecular weight aromatic poly(ester amide) 

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The polyols used include PO adducts of polyfiinctional hydroxy compoimds or amines (see Table 4). The amine-derived polyols are used in spray foam formulations where high reaction rates are required. Crude aromatic polyester diols are often used in combination with the multifunctional polyether polyols. Blending of polyols of different functionality, molecular weight, and reactivity is used to tailor a polyol for a speciflc application. The high functionality of the polyether polyols combined with the higher functionality of PMDI contributes to the rapid network formation required for rigid polyurethane foams. 

The preparation of networks from wormlike polymers such as polyesters and polyamides, and from stiifer polymers such as polyimides and poly-p-phenyl-enes, will be described in Sect. 3 below. In addition to the chemical nature of the stiff segments, attention will be paid to the rigidity or flexibility of the junction points, and to the sequence of reactions performed in order to obtain various rigid aromatic networks. 

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