Hydroxybenzoic acids, copolymerization

Copolymerization can be employed in a similar fashion to modify the properties of the homopolymer of /7-hydroxybenzoic add (5-6). Poly(/ -hydroxybenzoic acid) is an infusible polymer which can be shaped only by compression sintering. A melt processable variation of this high modulus, thermally stable material can be made, however, by copolymerizing an ester of 5-6 with equimolar quantities of terephthalic acid (5-7) and biphenol (5-8) to produce an aromatic polyester which can be fabricated at temperatures near400 C but still retain many useful properties at 300°C. 

Thermally stable copolymers of 3-(trimethylsiloxyl)- and 3,5-bis(trimethylsiloxyl)benzoyl chloride (4A) or 3-acetoxy- and 3,5-diace-toxy-benzoic acid (4B) were prepared with mole ratios of AB AB2 monomer ranging from 160-5.32 Polymers containing 10-20 mole % of branching monomers were insoluble in CHC13 but soluble in polar solvents, such as A,A-dimethylformamide (DMF) or a mixture of pyridine and benzene. Compared to the linear homopolymer of 3-hydroxy-benzoic acid, the branched polymer showed lower crystallinity and slower crystallization. There was an inverse linear relationship between percent crystallinity and the number of branches in the chain. Similarly, in an attempt to improve moldability and decrease anisotropy of rigid aromatic polyesters, 0.3-10 mole % of 1,3,5-trihydroxybenzene, 3,5-di-hydroxybenzoic acid, and 5-hydroxyisophthalic acid were copolymerized with p-hydroxybenzoic acid/terephthalic acid/4,4 -dihydroxy-diphenyl.33 The branched polymer showed a lower orientation and possessed improved flex properties. 

Similarly, copolymerization of a certain amount of 4-hydroxybenzoic acid (HBA) with phenyl-1,4-hydroquinone and terephthalic acid also gives rise to a remarkable decrease in the melting point. The general structural formula and the melting points of the copolymers with different HBA content are given respectively by (3.22) and Table 3.6. In this particular case, the incorporation of 20 mol % of HBA into the polymer (3.19) has resulted in a polymer with a melting point as low as 229. 

Ph-HQ/HQ/BB (50/50) is a ciystalline polyarylate and its HDT is 297 °C. We found that copolymerization involving small amounts of tire third unit (HBA) into this system could improve its crystallinity (Figure 19.13) and HDT finally, the HDT of the copolyarylate Ph-HQ/HQ/BB/p-hydroxybenzoic acid (HBA) (47.5/47.5/5) increased to above 300 °C. 

The modification through copolymerization is illustrated with polyethyleneterphthalate (PET) modified with p-hydroxybenzoic acid (PHB). The melt viscosity reaches a minimum as the PHB content becomes 60 mol J ( ). 

Thermotropic liquid crystalline polyesters can be prepared by the use of meta-oriented modifiers to reduce the melting points of para-substituted aromatic polyesters to the processible range. Resorcinol, isophthalic acid and m-hydroxybenzoic acid are commonly used. Figure 14 shows the effect of copolymerized m-hydroxybenzoic acid on the physical properties of melt processable, modified polyesters based on terephthalic acid and methylhydro-quinone. As the m-hydroxybenzoic acid (MHB) content increases... 

In order to decrease the melting points of such polyesters and facilitate their processing through the melt, the copolymerization of the different bifrmctional monomers with their random distribution in the macromolecules are usually used. A particular good example of the efficiency of random copolymerization in reducing the melting point is provided by the system based on p-hydroxybenzoic acid (HB) and equal amounts of terephthalic acid (TA) and p-p -biphenol (B)... 

Type - 3 polyesters were synthesized by copolymerization of 4-acetoxybenzoic acid (HBA) and PET. The properties of PET/HBA copolymers with varied composition are tabulated in Table 8.6. It was found that the reaction mixture become turbid for HBA composition higher than 35%, this was due to liquid crystalline nature of copolymer. It can be observed from the table that non-liquid crystalline PET / 30-HBA is stronger than that of compositions with liquid crystalline nature. Low production cost of PET/HBA with good mechanical properties and processability made it very attractive. However, because of the tendency toward homopolymerization of the acyl ester of 4-hydroxybenzoic acid to form poly(4-oxybenzoyl)... 

Copolymerization is one of the most efficient synthetic techniques to decrease the crystallinity and melting temperature of a given polymer. The melting-point depression occurring in copolyesters (Vectra) based on p-hydroxybenzoic acid and 2,6-hydroxynaphthoic acid (HNA) is in fact relatively moderate in comparison with that of other copolymers (Fig. 6.35). The minimum in melting point occurs at about 40 mol% HNA. The decrease in isotropization temperature in the copolyesters is moderate, leading to the desired expansion of the temperature region of a nematic phase. 

Hydroxyl-functionalized cinnamic acid derivatives such as p-coumaric acid (p-hydroxycinnamic acid), ferulic acid, and sinapinic acid are attractive monomers for syntheses of high-performance polyesters. The obtained polyesters are also expected to be biodegradable in the case of copolymerization with aliphatic hydroxy acids such as lactic acid. Tanaka et al reported the thermal polycondensation of p-coumaric acid at 550 °C without any catalyst under high pressure up to 80 kbar (in the solid state) in 1975. They obtained red or brownish-red hard solids insoluble in conventional organic solvents. Higashi and his co-workers synthesized copolyesters of p-coumaric acid and 4-hydroxybenzoic acid or their methoxy substitutions (ferulic acid, vanillic acid, or syringic acid) by polycondensation using hexachlorocyclotri(phosphazene) in pyridine in 1981. The obtained polymers that exhibited UV spectra different... 

Titan is a random copolyester probably composed of two random units, ethylene-terephthalate (PET) and hydroxybenzoic acid (HNA). Incorporating the PET unit in the main chain reduces the rigidity of the molecule due to the two methylene flexible spacers and in turn decreases the melting temperature of the material. On the other hand. Zenith 6000 is fidly aromatic, where kinks were introduced by combining phenol and biphenol molecules by random copolymerization. As measured by DSC, the melting temperature of Titan was between 325 and 335 °C, while for Zenith... 

The next development in liquid crystal polyesters was the preparation by polycondensation based on terephthalic acid (TPA) and hydroquinone (HQ) or p-hydroxybenzoic acid (HBA). The polyesters are insoluble with very high melting temperatures of 600 °C for poly (TPA/HQ) and 610 °C for poly (HBA), which are by far too high to obtain stable liquid crystalline phases for melt processing. In 1972, Economy and coworkers patented several copolyester compositions, and one of these are the copolymerization of poly (4-hydroxybenzoic acid) (PHB) with 4,4 -dihydroxybiphenyl (BP) and terephthalic acid (TPA) due to the need for lower melting, melt-processable polymers. Considerable synthetic efforts have been attempted in order to decrease the melting temperatures of aromatic LC polyesters while retaining LC properties. The copolyester structure was tailored by partial substitution of TPA with isophthalic acid to produce a melt-spinnable material. 

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