Polyester
Acylation

In acylation, the acid begins as an acyl chloride, and thus the polycondensation proceeds with emission of hydrochloric acid (HCl) instead of water.

The reaction between diacyl chlorides and alcohols or phenolic compounds has been widely applied to polyester synthesis and has been subject of numerous reviews and book chapters. The reaction is carried out at lower temperatures than the equilibrium methods; possible types are the high-temperature solution condensation, amine catalysed and interfacial reactions. In addition, the use of activating agents is counted as non-equilibrium method. The equilibrium constants for the acyl chloride-based condensation yielding yielding arylates and polyarylates are very high indeed and are reported to be 4.3 × 10 and 4.7 × 10, respectively. This reaction is thus often referred to as a 'non-equilibrium' polyesterification. Even though the acyl chloride based synthesis is also subject of reports in the patent literature, it is unlikely that the reaction is utilized on the production scale. The method is limited by the acid dichlorides' high cost, its sensitivity to hydrolysis and the occurrence of side reactions.

The high temperature reaction (100 to > 300 °C) of an diacyl chloride with an dialcohol yields the polyester and hydrogen chloride. Under these relatively high temperatures the reaction proceeds rapidly without a catalyst:

The conversion of the reaction can be followed by titration of the evolved hydrogen chloride. A wide variety of solvents has been described including chlorinated benzenes (e.g. dichlorobenzene), chlorinated naphthalenes or diphenyls, as well as non-chlorinated aromatics like terphenyls, benzophenones or dibenzylbenzenes. The reaction was also applied successfully to the preparation of highly crystalline and poorly soluble polymers which require high temperatures to be kept in solution (at least until a sufficiently high molecular weight was achieved).

In an interfacial acyl chloride-based reaction, the alcohol (generally in fact a phenol) is dissolved in the form of an alkoxide in an aqueous sodium hydroxide solution, the acyl chloride in an organic solvent immiscible with water such as dichloromethane, chlorobenzene or hexane, the reaction occurs at the interface under high-speed agitation near room temperature.

The procedure is used for the production of polyarylates (polyesters based on bisphenols), polyamides, polycarbonates, poly(thiocarbonate)s, and others. Since the molecular weight of the product obtained by a high-temperature synthesis can be seriously limited by side reactions, this problem is circumvented by the mild temperatures of interfacial polycondensation. The procedure is applied to the commercial production of bisphenol-A-based polyarylates like Unitika's U-Polymer. Water could be in some cases replaced by an immiscible organic solvent (e. g. in the adiponitrile / carbon tetrachloride system). The procedure is of little use in the production of polyesters based on aliphatic diols which have higher p K values than phenols and therefore do not form alcoholate ions in aqueous solutions. The base catalysed reaction of an acyl chloride with an alcohol may also be carried out in one phase using tertiary amines (e. g. triethylamine, Et N) or pyridine as acid acceptors:

While acyl chloride-based polyesterifications proceed only very slowly at room temperature without a catalyst, the amine accelerates the reaction in several possible ways, although the mechanism is not fully understood. However, it is known that tertiary amines can cause side-reactions such as the formation of ketenes and ketene dimers.
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