Interchange reactions

Polymerisation of a diol with a dicarboxybe acid is exemplified by the production of a polyester from ethylene glycol and terephthabc 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 resibency its structure is probably ... 

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... 

Amide interchange reactions of the type represented by reaction 3 in Table 5.4 are known to occur more slowly than direct amidation nevertheless, reactions between high and low molecular weight polyamides result in a polymer of intermediate molecular weight. The polymer is initially a block copolymer of the two starting materials, but randomization is eventually produced. 

Several side reactions or post-cuting reactions are possible. Disproportionation reactions involving terminal hydride groups have been reported (169). Excess SiH may undergo hydrolysis and further reaction between silanols can occur (170—172). Isomerization of a terminal olefin to a less reactive internal olefin has been noted (169). Viaylsilane/hydride interchange reactions have been observed (165). 

Thiol—Disulfide Interchange Reactions. The interchange between thiols and disulfides has been reviewed (50). This reaction is base-catalyzed. It involves the nucleophihc attack of a thiolate ion on a disulfide. This is shown in equations 35, 36, and 37. 

Mixed esters can be prepared from Cl Ti(OR)4 and a second alcohol in the presence of a base or by mixing a tetraalkoxide and the second alcohol in the desired proportions and flash-evaporating the mixed alcohols. Mixing of two pure tetraalkoxides of titanium also leads, via a rapid ester interchange reaction, to a mixture of ah. possible combinations of tetraalkyl titanates. 

By introducing branch points into the polymer chains, for example by incorporating about 2% of 1,2,3,-trichloropropane into the polymerisation recipe, chain extension may proceed in more than two directions and this leads to the formation of networks by chemical cross-links. However, with these structures interchange reactions occur at elevated temperatures and these cause stress relief of stressed parts and in turn a high compression set. 

In 1930 Carothers and Natta prepared a number of aliphatic polycarbonates using ester-interchange reactions. These materials had a low melting point, were easily hydrolysed and did not achieve commercial significance. 

The interchange reaction implies the removal of one atom of /I at the surface for each atom of B deposited. It therefore takes place with a minimum change in weight or dimensions of the article (A). U A and B have similar atomic weights, as in the case of iron and chromium, interchange reaction will produce little change in weight and no measurable increase in dimension, whatever the thickness of the diffusion layer. 

Molten bath deposition The interchange reaction also takes place when the coating metal halide is dissolved in a fused salt. Alternatively, deposition may be by electrolysis. Another technique uses the coating metal dissolved in molten calcium ... 

TPEs associating both rigid and soft polyester blocks have also been described. They cannot be obtained by the melt polyesterification used for polyesterether TPEs, since interchange reactions would yield random—rather than block — copolyesters. The preferred method involves the reaction of OH-terminated aliphatic and aromatic-aliphatic polyesters with chain extenders such as diisocyanates and results in copoly(ester-ester-urethane)s. 

Phenolic compounds are weaker nucleophiles and better leaving groups than aliphatic alcohols. They do not yield polyesters when reacted with carboxylic acids or alkyl carboxy lates. The synthesis of polyesters from diphenols is, therefore, generally carried out through the high-temperature carboxylic acid-aryl acetate or phenyl ester-phenol interchange reactions with efficient removal of reaction by-product (Schemes 2.10 and 2.11, respectively). 

As already discussed (Section 2.2.1.3), interchange reactions are also implicated in the formation of random copolyesters exhibiting the most probable molar mass distribution when polyester blends are melt mixed. They are also involved in the randomization of block copolyesters taking place in the melt upon heating.2,m 211... 

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. 

Since 1,4-butanediol (BD) undergoes dehydration side reaction in the presence of acid resulting in THF formation, the hydroxy-ester interchange reaction is the preferred method for the preparation of PBT. The first stage of reaction is carried out at 150-200°C and consists of a hydroxy-ester interchange between DMT and excess butanediol with elimination of methanol. In the second stage, temperature is raised to 250°C and BD excess is eliminated under vacuum. Tetraisopropoxy-and tetrabutoxytitanium are efficient catalysts for bodi stages (Scheme 2.20). 

Many random copolyesters and polyester-polycarbonates have also been prepared by ester interchange reactions in the molten state. Thus, poly(ethylene terephthalate-co -isophthalates) can be obtained by simple melt blending of PET and poly(ethylene isophthalate) (PEI) homopolyesters at 270°C. The copolymer changes gradually from a block type at the beginning of reaction to a random-type... 

The dimethyl ester of a carboxy-terminated diamide is reacted with 1,2-ethanediol in the conditions of PBT synthesis. The amide functions are unaffected while the hydroxy-ester interchange reaction proceeds to the formation of an alternating copolyesteramide (Scheme 2.60). The procedures given below are reproduced from ref. 430. (Copyright 1997 Elsevier Science Ltd, with permission of the copyright owner.)... 

This synthesis involves the hydroxy-ester interchange reaction between phenyl ester and phenol endgroups436 (Scheme 2.63) (see Section 2.3.2.2). 

As is die case for odier polycondensation reactions, internal interchange reactions are possible for ADMET, similar to diat of polyesters and polyamides.16 Interchange reactions involve a catalyst molecule on a polymer chain end reacting widi an internal double bond in another polymer chain. The result is two new polymer chains however, no change in the molecular weight distribution... 

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