Ester-exchange

Of these four routes the first and third have been studied intensively, in particular in the preparation of the bis-phenol A polycarbonates. Since this polycarbonate is at present the only one of major commercial importance the following remarks will apply to this only unless otherwise stated. 

In this method the reaction is typically carried out at 180-220°C at 20-30 mmHg pressure until 80-90% of the phenol of condensation has been removed. The temperature is then gradually raised to 290-300°C and the pressure reduced to 1 mmHg or below. The melt viscosity increases considerably 

The high melt viscosity limits the molecular weights obtainable and although number average molecular weights of 50000 can be obtained it is difficult to attain values of above 30000 without special equipment. 

This variation has the obvious disadvantage that the less volatile diphenyl carbonate is more difficult to remove than phenol. 

A number of basic materials such as hydroxides, hydrides and amides of alkaline and alkaline earth metals and metal oxides such as zinc oxide and antimony oxide are useful catalysts for the reaction. Acid ester-exchange catalysts such as boric acid, p-toluene sulphonic acid and zinc chloride are less 

Polymerisation then proceeds by splitting out of diphenyl carbonate to give the 

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There is no danger that the tertiary alcohol group will form an ester under these conditions. Ester exchange is described in Norman p. 134-5. 

The ester 322A is made by ester exchange with ethyl acetoacetate and a suitable alcohol. The product 322B decarboxylates spontaneously on heating. Draw out the whole sequence starting from ethyl acetoacetate. 

Liquid crystal polyesters are made by a different route. Because they are phenoHc esters, they cannot be made by direct ester exchange between a diphenol and a lower dialkyl ester due to unfavorable reactivities. The usual method is the so-called reverse ester exchange or acidolysis reaction (96) where the phenoHc hydroxyl groups are acylated with a lower aHphatic acid anhydride, eg, acetic or propionic anhydride, and the acetate or propionate ester is heated with an aromatic dicarboxyHc acid, sometimes in the presence of a catalyst. The phenoHc polyester forms readily as the volatile lower acid distills from the reaction mixture. Many Hquid crystal polymers are derived formally from hydroxyacids (97,98) and thein acetates readily undergo self-condensation in the melt, stoichiometric balance being automatically obtained. 

Catalytic amounts of mercuric chloride are usually employed in this preparation. Aluminum isopropoxide is a useful Meerwein-Potmdorf-Verley reducing agent in certain ester-exchange reactions and is a precursor for aluminum glycinate, a buffering agent (see Alkoxides, metal). 

Chemistry. Poly(vinyl acetate) can be converted to poly(vinyl alcohol) by transesterification, hydrolysis, or aminolysis. Industrially, the most important reaction is that of transesterification, where a small amount of acid or base is added in catalytic amounts to promote the ester exchange. 

Ar = 1 — naphthyl and l-CH -l-naphthyl). When At was 2-CH C H the product was di-2-tolylbismuthinic acid [124066-74-6] rather than the ester. The reaction was unsuccessful when At was l-ClCgH, 3-CF3C3H4, or 2-thienyl. The methyl esters underwent ester exchange when recrystallized from ethyl or isopropyl alcohols. Methyl diphenylbismuthinate [124066-62-2] was readily hydrolyzed in water to diphenylbismuthinic acid [124066-70-2],... 

By reaction of polyhydroxy compounds with a carbonic acid derivative, a series of related polymers may be produced with carbonate (—0 C0 0—) linkages, the polymers being referred to as polycarbonates. Carbonic acid, C0(0H)2, itself does not exist in the free state but by means of ester exchange Figure 20.1) (1) and phosgenation techniques (II) it is possible to produce useful products. 

Ester exchange of dihydroxy compounds with diesters of carbonic acid and monofunctional aromatic or aliphatic hydroxy compounds. 

Ester exchange of bis-alkyl or bis-aryl carbonates of dihydroxy compounds with themselves or with other dihydroxy compound. 

The equation for the ester-exchange reaction (1) is shown in Figure 20.3. 

As shown in the introductory section to this chapter, polyesters may be produced in many ways, one of which is the method of ester exchange. This has been of importance in the manufacture of linear polyesters for a number of reasons which include the following ... 

In the case of ester exchange for the manufacture of poly(ethylene terephthalate), a low molecular weight diester, known as the monomer , is first prepared by reacting 1 mol of dimethyl terephthalate with about 2.1-2.2 mol ethylene glycol at about 150°C in the presence of catalysts such as antimony trioxide and cobaltous acetate ... 

In recent years, however, the ester exchange reaction has been decreasingly used, with manufacturers now usually preferring to use direct acid glycol condensation with purified terephthalic acid. 

In the 1960s the Carborundum company introduced the polymer Ekonol P-3000. This was the polymer of p-hydroxybenzoic acid (I) Figure 25.22), in practice produced by the self-ester exchange of its phenyl ester to prevent decarboxylation. A blend with PTFE, Ekonol T-4000, was also produced. 

In the second step a second molecule of sodium malonic ester exchanges its sodium with the substituted malonic ester and a second molecule of NaBi is then removed. 

A benzyl carbonate was prepared in 83% yield from the sodium alkoxide of glycerol and benzyl chloroformate (20°, 24 h). It was also prepared by a lipase-catalyzed ester exchange with allyl benzyl carbonate. It is cleaved by hydrogenolysis (H2/Pd-C, EtOH, 20°, 2 h, 2 atm, 76% yield) and electrolytic reduction (—2.7 V, R4N X, DMF, 70% yield). A benzyl carbonate was used to protect the hydroxyl group in lactic acid during a peptide synthesis." ... 

Rolgamidine (14) is a dihydropyrrole derivative which has antidiarrheal activity It can be synthesized by alkylation of trans 2,5-dimethyl-3 pyrroline (12) with methyl bromoacefate to give 13 An amide-ester exchange reaction with guanidine hydrochloride completes the synthesis of rolgamidine (14) [3]... 

Transesterifications, also termed ester exchange or ester interchange reactions, include hydroxy-ester, carboxy-ester, and ester-ester reactions. Hydroxy-ester reaction is the most important one and is used for many aromatic-aliphatic and wholly aromatic polyester syntheses. Carboxy-ester interchange is restricted to the synthesis of wholly aromatic polyesters while the ester-ester route is rarely used for polyester preparation due to slow kinetics. All these reactions may take place in comparable experimental conditions and can be catalyzed by similar classes of compounds. 

It is not necessary to isolate acid (40) (38) can be made directly from ester (42) by ester exchange. 

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