Polyesters ester exchange

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. 

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 ... 

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. 

Scheme 12.2 Ester exchange in polyesters through intramolecular rearrangement of two ester groups...

Transesterification (also known as ester exchange, ester interchange, or ester alcoholysis) is the most important of the esterification reactions. Polyesters of relatively low molecular weight (around 2000) prepared from aliphatic dicarboxylic acids and glycols are most commonly made by direct esterification (Section 5.2) with or without addition of an external acidic catalyst. Phosphoric acid, p-toluenesulphonic acid, and antimony pentafluoride have been used as catalysts. The preparation of PET by the direct esterification of TA with ethylene glycol was not practical until the... 

CAS 25038-59-9. (CI0II804),. A thermoplastic polyester formed from ethylene glycol by direct esterification or by catalyzed ester exchange between ethylene glycol and dimethyl terephthalate. Offered as oriented film or fiber. It melts at 265C tenacity is 2.2-4 g/denier as staple and up to 9.0 g/denier as continuous filament d 1.38. It has good electrical resistance and low moisture absorption. Resists combustion and is self-extinguishing. 

The synthesis of the unsamrated polyester is analogous to the formation of a saturated polyester, and the side reactions discussed earlier, such as ester exchange, can occur at high tempemture. The presence of the unsaturated group does mean that there are further possible side reactions ... 

Ester-exchange reactions in polyesters (and analogous amide reactions in nylons) that result in a new block copolymer. An example is the formation of a blend between polycarbonate and poly(butylene terephthalate) in which the compatibilizer is formed by a transesterification reaction at elevated temperatures (350 °C), but, when processed at lower temperatures, the systems are not compatible. 

For use in plastics, this might require chain extension with diisocyanates, bisoxazolines, carbodiimides, bis(an-hydrides), and such. One use of the oligomers would be to form polyurethanes by reaction with isocyanates. The use of acid chlorides can be avoided if the polymers are made by ester exchange or made enzymatically, with compounds such as divinyl adipate. Poly(butylene sebacate) with a molecular weight of 46,400 has been made has been made from bis(2,2,2-trifluoroethyl)sebacate and 1,4-butane diol.150 One polyester with a molecular weight of 24,000 has been made by ester exchange from isosorbide and a... 

Ester Exchange. The most practical methods for the preparation of high molecular weight polyesters involve ester exchange reactions. The simplest exchange method involves a catalyzed alcoholysis reaction between a diol and a dicarboxylic acid ester with elimination of the alcohol ... 

Whinfield and Dixon, in UK, developed polyethylene terephthalate fibers (Dacron, Terylene). This first Dacron polyester plant went into operation in 1953. Easter interchange (also known as ester exchange or alcoholysis) was once the preferred method for making polyethylene terephthalate (PET) because... 

In 1958, Eastman Kodak introduced a more hydrophobic polyester fiber under the trade name Kodel. The raw material for this polyester is dimethyl terephthalate. Reduction leads to 1,4-cyclohexylene glycol, which is used with dimethyl terephthalate in the polycondensation (ester exchange) reaction. 

However, the formation of cyclic oligomers in tire pyrolysis of several aliphatic and aromatic polyesters is well-known, ° ° and a distribution of cyclic compounds is expected if the thermal degradation proceeds through macrocyclization processes involving ester-exchange reactions. ... 

Fragmentation of polyesters via intramolecular ester exchange. (Reprinted with permission from Ref. 48 Copyright 1995 American Chemical Society)... 

Moreover, inter- and intramolecular ester exchange reactions causing rearranged polyester sequences at the pyrolysis stage were also demonstrated to take place for aliphatics and aliphatic-aromatic polyesters" using Py-MS. 

Thermoplastic aromatic polyesters need to be produced to a molecular weight of between 12,000 and 60,000 to be useful. The first stage is usually esterification (diacid and diol) or ester-exchange (diester and diol) to provide the first stage product, e.g., a bis( hydroxy alkyl) terephthalate, and potentially some linear oligomeric species. Water or alcohol (usually methanol) is evolved, and removed by fractional distillation. The decision to use the diacid or dimethyl ester in the manufacturing process is usually a matter of balancing questions... 

The P hydrogen atoms necessary for the chain scission via cyclic intermediate much cited for the preceeding classes of polyester are absent. It is therefore thought that thermal degradation will occur selectively by ester exchange to produce high yields of cyclic... 

The process by which bi-or poly-functional reactants are condensed to form polymer chains with the elimination of small molecules in each condensing step, is known as condensation polymerisation. As the polymer chains are grown incrementally, it may also be called step growth polymerisation. Esterification (direct or ester exchange), amidation, nucleophilic and electrophilic substitution are the general reactions for this polymerisation process. The formation of vegetable oil-based polyester is an example of this type of polymerisation (Fig. 1.1). 

Alcoholysis or transesterification (ester exchange) progresses under milder conditions, so that the equilibrium can be shifted more easily to the polyester side, e.g.,... 

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