Acetoxybenzoic acids, synthesis

In 1975, the synthesis of the first main-chain thermotropic polymers, three polyesters of 4,4 -dihydroxy-a,a -dimethylbenzalazine with 6, 8, and 10 methylene groups in the aHphatic chain, was reported (2). Shortly thereafter, at the Tennessee Eastman Co. thermotropic polyesters were synthesized by the acidolysis of poly(ethylene terephthalate) by/ -acetoxybenzoic acid (3). Copolymer compositions that contained 40—70 mol % of the oxybenzoyl unit formed anisotropic, turbid melts which were easily oriented. 

Problem 24.15 Aspirin is acetylsalicylic acid (o-acetoxybenzoic acid, 0-CH3COO-CftH4COOH) oH of wintergrcen is the ester, methyl salicylate. Outline the synthesis of these two compounds from phenol. 

The most widely studied example is the SCCG of poly(4-oxybenoate), FOB, also called poly(4-hydroxybenzoic acid).PHB. Since the abbreviation PHB is also used for poly(/ -hydroxybutyric acid), this abbreviation is avoided in this text. Early attempts to polymerize 4-hydroxybenzoic acid, 4-HBA are discussed in Chap. 2. In the twentieth century several derivatives of 4-HBA were successfully used as monomers (see Formula 13.4), but polymerization of 4-HBA itself was also conducted in solution by means of dehydration agents, such as trifluoroacetic acid [60], thionyl chloride [61], poly(ethyl phosphate) plus imidazole [62], and various other phosphorous reagents [63]. However, these polycondensations were typically conducted at temperatures <150 °C and yielded oligomers. More than 90 % of all publications dealing with synthesis and characterization of POB are based on polycondensations of 4-acetoxybenzoic acid, 4-ABA, at temperatures around or above 300 °C. A first attempt to polymerize 4-ABA in bulk at temperatures up to 300 °C was published in 1959 [64]. The resulting polyester was described as intractable and thermally unstable above 350 °C. 

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