Polyesters are another form of condensation polymer. Typically, synthesis of polyester is obtained when a dicarboxylic acid and a diol are reacted. The ester product has a free (unreacted) carboxyl group at one end and a free alcohol group at the other after the first reaction. Subsequent esterification utilizing step-growth polymerization results in the formation of polyester. Polyethylene terephthalate (PET), the most significant polyester, is produced from the reaction of 1,4-benzenedicarboxylic (terephthalic acid) and 1,2-ethanediol (ethylene glycol) monomers.
Synthesis of Polyester
Polyesters can be produced by a variety of processes, the most significant of which are the reaction of acids and alcohols, alcoholysis and/or acidolysis of low-molecular-weight esters, and alcoholysis of acyl chlorides. Polyesters are typically produced by a polycondensation reaction. Polyesters are polymers made by a condensation reaction taking place between monomers in which the linkage between the molecules occurs through the formation of ester groups. Several chemical processes used in the synthesis of polyester are discussed here.
Polyesters are typically produced by a polycondensation reaction. The general reaction equation for a diol and a diacid is:
(n+1) R(OH)2 + n R´(COOH)2 → HO[ROOCR´COO]nROH + 2n H2O
Polyesters can be produced by reacting dicarboxylic acids with diols, such as propanedioic acid and ethane-1,2-diol. Terylene is the most prevalent polyester fiber. Terylene is produced by combining benzene-1,4-dicarboxylic acid and ethane-1,2-diol. The necessary conditions include a catalyst such as antimony(III) oxide at a temperature of around 280 degrees Celsius.
The esterification of azeotropes is a traditional condensation technique. By azeotropic distillation, the water produced by the reaction of alcohol and a carboxylic acid is continuously eliminated. When the melting temperatures of the monomers are low enough, a polyester can be generated by direct esterification while vacuuming away the reaction water.
Transesterification is the process by which an alcohol-terminated oligomer and an ester-terminated oligomer condense to create an ester bond, with the loss of an alcohol. R and R’ are the oligomer chains, whereas R” is a sacrifice unit such as a methyl group.
In acylation, the acid begins as an acyl chloride, therefore the polycondensation occurs with hydrochloric acid (HCl) emission rather than water.
The reaction between diacyl chlorides and alcohols or phenolic compounds has been extensively utilized to the production of polyester. High-temperature solution condensation, amine-catalyzed, and interfacial reactions are examples of conceivable types of reactions that occur at lower temperatures than equilibrium techniques. The employment of activating chemicals is also considered a non-equilibrium strategy. This reaction is sometimes referred to as a “non-equilibrium” polyesterification.
The reaction of a diacyl chloride with a dialcohol at high temperatures (100 to > 300 °C) produces polyester and hydrogen chloride. Without a catalyst, the reaction proceeds quickly at these relatively high temperatures:
At extremely mild circumstances, aliphatic polyesters can be built from lactones via anionic, cationic, metalorganic, or enzyme-based catalysis. Recent research has demonstrated that a number of catalytic techniques for the copolymerization of epoxides with cyclic anhydrides produce a vast array of functionalized polyesters, both saturated and unsaturated. On an industrial scale, ring-opening polymerization of lactones and lactides is also utilized.
PLA, or poly(lactic acid), is another kind of polyester. Nevertheless, PLA is produced with only a single monomer, lactic acid. Its chemical designation is 2-hydroxypropanoic acid. Each molecule of the monomer comprises carboxylic acid and alcohol groups.
As a biodegradable alternative to petroleum-based plastics, polyester PLA, whose source material is plant starch such as maize, is now in use. When discarded objects containing polyesters are buried at landfill sites, the ester linkages can be hydrolyzed under acidic conditions to break down the polymer chains. Alcohol and carboxylic acids are the byproducts of polyesters’ acid hydrolysis.
Important Points to Remember
- Polyesters may be manufactured from a vast array of chemicals by a variety of techniques, such as step-growth polycondensation, ring-opening polymerization, and polyaddition processes.
- Direct esterification of a dicarboxylic acid with a diol or transesterification of dialkyl esters produces the biggest volume of polyesters (PET, PBT, PEN).
- If both moieties are difunctional, the condensation product is a linear polymer, and if at least one of the moieties is tri- or tetra-functional, the polymer is a thermoset with crosslinking.
- S. Watanabe, T. Miho, T. Fujii, Ring-opening polymerization of cyclic ε-caprolactone, US Patent 4,379,915 (1983)
- J. Scheirs and T. E. Long, Modern Polyesters: Chemistry and Technology of Polyesters and Copolyesters, John Wiley & Sons, Chichester, West Sussex 2003
- Anthony J. East, Preparation of aromatic Polyesters by Direct Self condensation of Aromatic Hydroxy Acids, US Patent 4,393,191 (1993)
- Paal J. Flory, Kent, and Frederick S. Leutne, Method of Preparing Linear Polyesters, US Patent 2,589,688 (1952)