Terephthalic acid, reaction with ethylene

Combined 1993 production figures for pure terephthalic acid and dimethyl terephthalate were about 3 5 Mt in the U.S.A., 2Mt in western Europe and l-5Mt in Japan. Over 95% is converted to polyethylene terephthalate by reaction with ethylene glycol. While fibre production remains the largest use, the major area of activity is the production of PET bottle resins, with world capacity scheduled to grow rapidly (presently over 2 Mt per annum, of which 0 9 Mt per annum is in the U.S.A.). 

PET is the most widely used polyester. As shown by the first step of reaction 8.4.3, terephthalic acid reacts with ethylene glycol to give bis(hydroxyethyl) terephthalate, 8.17. In the second step, 8.17 is converted to PET by a Sb203/Ti(0R) -catalyzed transesterification or self-condensation reaction. 

Prior to polymerization, p-xylene is first oxidized to terephthalic acid (TA) or dimethyl terephtalate (DMT). These diacid or dimethyl ester monomers are then polymerized via a condensation reaction with ethylene glycol to form the polyester. Prior to the development of a method to purify TA to make purified terephtahc acid (PTA, >99% pure) by the Mid-Century Corporation in the 1950s [10], DMT was the primary way to obtain the purified dicarboxylate. The Amoco Oil Company, now part of BP International, made several improvements to the PTA process since its inception [11]. Since the advent of the availability of PTA, it has become the monomer of choice over DMT. PTA avoids the complications of including methanol to enable purification and handling the methanol evolved during the polymerization to polyester. 

FIGURE 4.5 Plot of average chain length DP, as a function of reaction time for the acid-catalyzed condensation of ethylene glycol with terephthalic acid producing poly(ethylene terephthalate) (PET). 

All these plastics are essentially the same compound, composed of terephthalic acid (para-phthalic acid) esterified with ethylene glycol. This polyester is made by a base-catalyzed transesterification of dimethyl terephthalate with ethylene glycol at a temperature around 150 °C. At this temperature, methanol escapes as a gas, driving the reaction to completion. We will study polyesters and other polymers in more detail in Chapter 26. 

Polyethylene terephthalate) in short PET is a polyester. It is mainly used in the garment industry with or without natural cotton and has trade names such as Terylene , Dacron , etc. As the name indicates, it is a polymer between terephthalic acid (PT) and ethylene glycol. Both terephthalic acid and dimethyl terephthalate (DMT) can be used to make the polymer. A majority of the modem plants tend to use PT as the starting material because of the availability of high-purity PT on a large scale. Both PT and DMT are first converted to bis(hydroxy ethyl) terephthalate 8.17 (see reaction 8.26). For PT this is effected by a straightforward esterification reaction. For DMT a transesterification reaction catalyzed by zinc and manganese acetate is used. 

Other procedures to obtain the polymer include the use of terephthalic acid esterified to its dimethyl ester, which by a transesterification reaction with ethylene glycol generates the polymer. In addition to ethyleneglycol, other diols can be used in the esterification readion, such as 1,4-butanediol, 1,4-cyclohexanedimethanol, 1,6-hexanediol, etc. 

PET is a semi-crystalline thermoplastic polymer used in the manufacture of fibres, packaging films, bottles, electrical insulators, etc. As shown in Scheme 2.1, PET can be produced by two different routes by condensation between terephthalic acid (TPA) and ethylene glycol (EG) or through the reaction of dimethyl terephthalate (DMT) with ethylene glycol. Both alternatives lead to the monomer bis(hydroxyethyl) terephthalate (BHET), which is further polymerized into PET. 

Reaction of PET with water allows the polyester chains to be broken down into terephthalic acid (TPA) and ethylene glycol. The process can be carried out under neutral, acidic or basic conditions. A crucial aspect of this chemical recycling method is the purity and properties of the obtained TPA in order to achieve the specifications normally required for direct esterification to produce fresh PET polymer. TPA is usually purified by crystallization from solvents such as acetic acid, whereas a variety of procedures have been reported to remove the different impurities present in the hydrolysis product. 

In recent years methods have been developed to produce terephthalic acid with satisfactory purity, and direct polycondensation reaction with ethylene glycol is now the preferred route to this polymer. 

Phthalocyanines with M = Si(OH)2 or Ge(OH)2 were eovalently incorporated into polyesters during the polycondensation of terephthalic acid dimethylester and ethylene glycol [182]. With only lO" molar amounts of dye, the polyesters are intensely blue-colored. Good solubility can be achieved in water by axial substitution at the central metal or with hydrophilic polymers. Phthalocyanines with the tetravalent M = SiCb were reacted with the sodium salt of methoxypoly(oxyethylene) (M = 5000 Da) to give the blue-colored polymer 68 which is soluble in water and some organic solvents [183]. The reaction of phthalocyanines with the trivalent M = AlCl with poly-(oxyethylene) or poly(vinylalcohol) also led to water-soluble polymers having covalent bonds of the polymers at the Al(III) [184]. These water-soluble materials have been tested in the photodynamic therapy of cancer. 

PREPARATIVE TECHNIQUES Synthesized by condensation/step-growth polymerization between ethylene glycol and terephthalic acid. Low-viscosity and easily spinnable PET are synthesized by ester interchange. Dimethyl terephthalate is reacted with ethylene glycol in a 1 1.7 ratio at 0.020 atm and 160-230°C. Final reaction occur at 260-300°C under vacuum at 0.001 atm. Synthesis of PET is done by using aromatic sulphonates as catalysts ... 

Polyesters are synthesized by either direct esterification (Eq. 1) or transesterification (Eq. 2) reactions. In direct esterification, terephthalic acid is reacted with ethylene glycol to produce polymer and water as a by-product. Reaction conditions involve an esterification step in the presence of catalyst (usually antimony trioxide) and a polycondensation step. In transesterification, dimethyl terephthalate is reacted with ethylene glycol to produce polymer and methyl alcohol as a by-product the transesterification step in the presence of catalyst, (usually metal carboxylates), is followed by a polycondensation step in the presence of catalyst (usually antimony trioxide). 

Polyesters are also made by transesterification reactions. Terephthalic acid reacts with methanol to give dimethyl terephthalate (DMT) which then reacts with ethylene glycol to give PET. Again a Lewis acid is used as a transesterification catalyst. Common catalysts are tetraalkyl titanates or antimony oxide. 

Condensation polymerization differs from addition polymerization in that the polymer is formed by reaction of monomers, each step in the process resulting in the elimination of some easily removed molecule (often water). E.g. the polyester polyethylene terephthalate (Terylene) is formed by the condensation polymerization (polycondensation) of ethylene glycol with terephthalic acid ... 

Poly(ethylene Terephthalate). Poly(ethylene terephthalate) is prepared by the reaction of either terephthalic acid or dimethyl terephthalate with ethylene glycol, and its repeating unit has the general structure. 

Ester interchange reactions are valuable, since, say, methyl esters of di-carboxylic acids are often more soluble and easier to purify than the diacid itself. The methanol by-product is easily removed by evaporation. Poly (ethylene terephthalate) is an example of a polymer prepared by double application of reaction 4 in Table 5.3. The first stage of the reaction is conducted at temperatures below 200°C and involves the interchange of dimethyl terephthalate with ethylene glycol... 

Reactions of the Methyl Groups. These reactions include oxidation, polycondensation, and ammoxidation. PX can be oxidized to both terephthahc acid and dimethyl terephthalate, which ate then condensed with ethylene glycol to form polyesters. Oxidation of OX yields phthaUc anhydride, which is used in the production of esters. These ate used as plasticizers for synthetic polymers. MX is oxidized to isophthaUc acid, which is also converted to esters and eventually used in plasticizers and resins (see Phthalic acids and otherbenzenepolycarboxylic acids). 

The largest commercial use of ethylene glycol is its reaction with dicarboxyUc acids to form linear polyesters. Poly(ethylene terephthalate)... 

Small amounts of polymer-grade terephthaHc acid and dimethyl terephthalate are used as polymer raw materials for a variety of appHcations, eg, adhesives and coatings. They are also used to make high performance polymers or engineering resins. Poly(ethylene terephthalate) is itself an engineering resin, although one more widely used is poly (butylene) terephthalate, formed by reaction with 1,4-butanediol as the comonomer. 

The phthalic acid and benzoic acid are reacted to form a reaction intermediate. The reaction intermediate is dissolved in sulfuric acid, which precipitates terephthalic acid (TPA). Fifty percent of the TPA is sold as a product and 50 percent is further processed at your facility into polyester fiber. The TPA Is treated with ethylene glycol to form an intermediate product, which is condensed to polyester. 

Polyesters are the most important class of synthetic fibers. In general, polyesters are produced by an esterification reaction of a diol and a diacid. Carothers (1930) was the first to try to synthesize a polyester fiber by reacting an aliphatic diacid with a diol. The polymers were not suitable because of their low melting points. However, he was successful in preparing the first synthetic fiber (nylon 66). In 1946, Whinfield and Dickson prepared the first polyester polymer by using terephthalic acid (an aromatic diacid) and ethylene glycol. 

Using excess ethylene glycol is the usual practice because it drives the equilihrium to near completion and terminates the acid end groups. This results in a polymer with terminal -OH. When the free acid is used (esterification), the reaction is self catalyzed. However, an acid catalyst is used to compensate for the decrease in terephthalic acid as the esterification nears completion. In addition to the catalyst and terminator, other additives are used such as color improvers and dulling agents. For example, PET is delustred hy the addition of titanium dioxide. 

An alternative route to PET is by the direct reaction of terephthalic acid and ethylene oxide. The product bis(2-hydroxyethyl)terephthalate reacts in a second step with TPA to form a dimer and ethylene glycol, which is released under reduced pressure at approximately 300°C. 

PET is the polyester of terephthalic acid and ethylene glycol. Polyesters are prepared by either direct esterification or transesterification reactions. In the direct esterification process, terephthalic acid is reacted with ethylene glycol to produce PET and water as a by-product. Transesterification involves the reaction of dimethyl terephthalate (DMT) with ethylene glycol in the presence of a catalyst (usually a metal carboxylate) to form bis(hydroxyethyl)terephthalate (BHET) and methyl alcohol as a by-product. In the second step of transesterification, BHET... 

The principal solvolysis reactions for PET are methanolysis with dimethyl terephthalate and ethylene glycol as products, glycolysis with a mixture of polyols and BHET as products, and hydrolysis to form terephthalic acid and ethylene glycol. The preferred route is methanolysis because the DMT is easily purified by distillation for subsequent repolymerization. However, because PET bottles are copolyesters, the products of the methanolysis of postconsumer PET are often a mixture of glycols, alcohols, and phthalate derivatives. The separation and purification of the various products make methanolysis a cosdy process. In addition to the major product DMT, methanol, ethylene glycol, diethylene glycol, and 1,4-cyclohexane dimethanol have to be recovered to make the process economical.1... 

This new monomer is separated from the excess glycol and polymerized. The monomer has two hydroxy endgroups but with catalysis and temperature, it will self-condense to give ethylene glycol as the by-product. The overall result is a one-to-one reaction of terephthalic acid with ethylene glycol, but a substantial amount of glycol is internally recycled. 

Polymerisation of a diol with a dicarboxylic acid is exemplified by the production of a polyester from ethylene glycol and terephthalic acid either by direct esterification or by a catalysed ester-interchange reaction. The resulting polyester Terylene) is used for the manufacture of fibres and fabrics, and has high tensile strength and resiliency its structure is probably ... 

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