P-Acetoxybenzoic acid

Kevlar has the highest tensile strength and is often used as a reinforcing fibre in composites with, e.g., epoxy, PEEK. The thermotropic liquid crystal polymer Vectran is made by melt polymerisation of p-acetoxybenzoic acid and 6-acetoxy-2-naphthoic acid, (the corresponding hydroxy acids decompose on melting). Because of its liquid crystal properties the polymer can be spun into fibres from the melt. Kevlar is spun from a solution in concentrated sulfuric acid, and can be melt drawn to give a high modulus (stiff) polymer. Vectran ... 

Aromatic polyesters were particularly good candidates for this new field of thermotropic main-chain polymers, since the relatively low energy of association of the ester groups led to low inter-chain forces. Further research led to the discovery that incorporation of 2,6-naphthylene or of 4,4 -biphenylyl groups, in addition to p-phenylene groups, as components of aromatic polyesters, introduced a useful new degree of randomness. Particularly useful, and the basis of the commercial products Vectra (polymer) and Vectran (fibre) from Hoechst-Celanese and Kuraray, are the copolymers formed by polymerisation of mixtures of p-acetoxybenzoic acid and 6-acetoxy-2-naphthoic acid. Within a range of... 

A method was proposed for the preparation of p-hydroxybenzoic acid by oxidation of p-cresol with atmospheric oxygen in an acetic acid-acetic anhydride mixture under catalysis of cobalt acetate, manganese(II) acetate, and sodium bromide (Litvintsev et al. 1994). This procedure ensures 60% yield of p-acetoxybenzoic acid and 100% conversion of the initial p-cresol. 

Introduction of flexible spacer units such as methylene, methylene oxide, and dimethyl-siloxane groups lowers the melting point and increases the temperature range within which the mesophase is stable. Often these spacer units are introduced by copolymerization. Thus, preformed p-acetoxybenzoic acid is reacted with PET, introducing a mesogenic unit in a polymer that has flexible spacer units (from the ethylene glycol) in it. 

Treatment of p-acetoxybenzoic acid with thionyl chloride generated the acid chloride, which was used without purification to acylate diethyl L-glutamate. Deacetylation of the resulting triester (36) gave (37) [51] (Scheme 3.6). 

Both benzimidazole-containing polymer and phenylimidazole-containing polymer show the hydrophobic binding in the catalyses, obviously (35, 37). The polymers are poly(ABI-pr), poly-(ABI-am), poly(ABI-am-ap), poly(API-pr), poly(API-am) and poly(API-am-ap). Imidazole moieties in these polymers have hydrophobic binding properties themselves. Profiles of hydrolyses of p-acetoxybenzoic acid, ABA(7), show the Michaelis-Menten kinetics. Catalytic activities of those polymers are more than 30 times that of low molecular weight analogues. Several sets of data are tabulated in Table 5. 

The Michaelis-Menten kinetics were similarly observed for water-soluble co-pcdymers containing BI or PI units in the solvolysis of p-acetoxybenzoic acid (ABA) 19 (71). btterestingly, the monomeric analogs, AcBI22 and AcPI 23, do not produce substrate binding under die same condition. However, the Michaelis-Menten... 

Another possibility for obtaining imide modified thermoplastic polyesters was to use as a monomer the hydroxy acid made from trimellitic anhydride and aminoethanol. Such a poly(ester-imide) was claimed for injection molding [240]. For the same use, poly(ester-imide)s containing aminophenol/trimellitic anhydride [241],imidised polyfbutylene terephthalate) [242] and a wholly aromatic poly(ester-imide) made from trimellitic anhydride, p-aminobenzoic acid, p-acetoxybenzoic acid, diacetoxybiphenyl and terephthalic and isophthalic acids are known, which showing optical anisotropy [243]. 

L6ger (56) showed that 0-methyl-hordenine methiodide yields p-vinylanisole and trimethylamine on treatment with alkali and 0-acetyl-hordenine is oxidized by permanganate to p-acetoxybenzoic acid. Hordenine is therefore /3-p-hydroxyphenethyldimethylamine, a structure confirmed by several syntheses. 

The fact that neighbouring p-oxybenzoic acid units were detected in the composition of thermo-oxidation products allows a supposition that, beside copolycondensation, homopolycondensation reactions of p-acetoxybenzoic acid also proceed. The output of free p-oxybenzoic acid is 3-4-fold higher, than its output in the linked state shaped as end groups in oligomers. Apparently, free p-oxybenzoic acid is formed as the result of thermal reactions at decomposition of labile bonds in structural p-oxybenzoic blocks. 

Figure 2. Chemical modification of PET via a melt transesterification reaction with p-acetoxybenzoic acid, to obtain a liquid-crystalline copolyester.

Fig. 2. Schlieren texture of copolyester formed "by transesterification of poly(ethylene-1,2 - diphenoxyethane-p,p -dicarho-xylate) w th p-acetoxybenzoic acid. Recognizable singularities S = - 1 (Crossed polarizers, 280°C, x 150). From ref. 25.

They proposed a pure radial splay model to describe the molecular arrangement aro d the lines of strength S = + 1. In investigations Mackley et al. made on a copolyester prepared from Ho mol % poly(ethylene terephthalate) and 60 mol % p-acetoxybenzoic acid, a number of integer disclinations were also observed. However, according to Viney and Windle who carried out some optical observations of Refects in the same copolyester, if disclinations of strength - 1/2 are not. seen below 3H0°C, they are more numerous than those of strength"1 above this temperature. These observations are at variance with results reported by Mackley et al. but seem to corroborate well with optical observations in polyester I (n = 5) made by Kleman et al.. One of the clearest conclusions of their study is obtained from the predominance of s = 1/2 lines ... 

Table 4.4 Processing parameters and mechanical properties for injection molded thermotropic copolyester of 60 mol% p-acetoxybenzoic acid, 20 mol% terephthalic acid and 20 mol% naphthalene. (Reprinted with permission from [21], copyright 1983 Society of Plastics Engineers, Inc.)...

Random copolymerization is an effective way to disturb the regular molecular structure of the polymer chain. The lack of periodicity along the chain inhibits crystallization and, thus, reduces the crystal size and perfection and depresses Tm. If the chosen comonomer is essentially linear, losses of chain mesogenicity can be minimized. This approach has been followed in designing thermotropic copolyesters, such as Vectra A by Celanese (now Ticona), which continues to enjoy commercial success. Vectra A is the copolyester of p-acetoxybenzoic acid (ABA) and 2,6-acetoxynaphthoic acid (ANA) with a mole ratio of 73/27. The melting point of this LCP is around 280°C, which is much lower than the melting points of the either poly (ABA) or poly (ANA) [6]. 

ANA and ABA are among the most commonly used monomers for main-chain thermotropic LCPs. A typical example is the commercially available LCP Vectra A for which the monomer composition is 73/27 ABA/ANA [6,7]. p-Acetoxybenzoic acid (ABA) provides the benzene ring that potentially can form a mesogenic unit. Phenylene units in its homopolymer can rotate and, thus. 

Fumta et al. (1994) have prepared compatibUized blends of LCP with an amine-functionalized PPE. For example, an LCP derived from p-acetoxybenzoic acid, terephthalic acid, isophthalic acid, and 4,4 -diacetoxydiphenyl was blended with an amine-functionalized PPE to provide a blend with improved properties compared to those with unfunctionalized PPE. PPE was melt functionalized with either p-aminostyrene, diallylamine, allylamine, or vinylimidazole. Other LCP were also used. 

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 aliphatic chain, was reported (8). Shortly thereafter, at the Tennessee Eastman Co. thermotropic polyesters were synthesized by the acidolysis of poly(ethylene terephthalate) by p-acetoxybenzoic acid (9). Copolymer compositions that contained 40-70 mol% of the oxybenzoyl unit formed anisotropic, turbid melts which were easily oriented. 

Industrial Thermotropic LCPS. Vectran (Celanese), poly(6-hydroxy-2-naphthoic acid-co-4-hydroxybenzoic acid) [81843-52-9], was the first thermotropic fiber to become commercially available (58). Vectran is S5mthesized by the melt acidolysis of p-acetoxybenzoic acid and 6-acetoxy-2-naphthoic acid. 

Acetyl p-acetoxybenzoic acid. Plates from CgHg. M.p. 187-187-5°. 

Figure 2 (4,5) summarizes some key points on what was the first reported and wel 1-characterized thermotropic polymer. The Eastman workers melt copolymerized p-acetoxybenzoic acid with poly(ethylene terephthalate) (PET) to form a series of thermotropic aliphatic-aromatic polyesters (X7G polymers). Some of these compositions showed the characteristic melt optical anisotropy typical of nematic small molecules, that is, the melt between crossed polaroids on a hot stage microscope transmits light. This nematic order was persistent. Of course, shear birefringence is also shown by isotropic polymer melts but light transmission decay is quite rapid. 

Terephthalic acid (TEA), 2,3 diacetoxynaphthalene (2,3-DAN), p-acetoxybenzoic acid (ABA) and the organically modified mont-morillonite and the reactants mixture can be heated in four phases. Initial phases of heating are at 240°C and 260°C respectively under a steady stream of gas. Continuous generation of acetic acid takes place during the progress of condensation. The reaction temperature can be raised further to 280°C and maintained for extended period under reduced pressure. Finally, the temperature can be raised to 290°C and the pressure reduced further. Up to 3% clay content the dispersion is better without any sign of agglomeration [70]. 

The following illustrates preparation of a typical liquid crystalline copolyester from reaction of PET with terephthalic acid, p-acetoxybenzoic acid, and hydroquinone diacetate. The following components were placed into a one-neck 500-ml round-bottom flask which was fitted with a stainless steel stirrer and a short head with a nitrogen inlet and condensate take-off. 

Irregular LC polymers can be obtained by esterification of the prepared polymer with a low-molecular-weight reagent. The reaction between polyethylene terephthalate and p-acetoxybenzoic acid in a melt can serve as an example [26] ... 

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