5.14 Polyesters

If a compound has a molecular structure that includes both a carboxylic acid and a hydroxy group, then it can esterify with itself, as we've already seen for forming lactones. But it can also esterify into a chain of units and form a polymer. Lactic acid HOCH(CH₃)CO₂H has a structure that doesn't lend itself to forming a lactone - it would be unstable since the ring formed would have only three members, causing the bond angles to be forced far away from ideal, therefore the bonds would be strained - but its molecules can esterify with neighboring molecules.

PLA.

Polylactic acid (PLA) is a thermoplastic biopolymer made from corn, that is slowly compostable, and is finding much use among environmentally friendlier plastic cups and utensils as well as in 3D printing where it can be formed much more easily and with fewer defects than the ABS that was used in the past. The name is a misnomer since it is not a polyacid, but we can just think of it as poly-(lactic acid).

Polyesters can also be made from a diacid (a compound with two -CO₂H groups) and a glycol. Polyethylene terephthalate (PET) is a thermoplastic polyester of terephthalic acid and ethylene glycol. It is used to make water bottles, and also finds use in 3D printing, having almost as much strength as ABS, better resistance to heat and humidity than PLA, and similar ease of use to PLA.

Terephthalic acid, ethylene glycol, and PET.

Polyesters that are melted and spun into thin filaments can then be made into fabric and yarn. Some kinds of microfiber cloth are made from polyesters.

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