polyester is a type of polymer that has the ester functional group in its main chain. there are many different polyesters. the term "polyester" as a specific material most often refers to polyethylene terephthalate (pet). polyesters include chemicals found in nature, such as in plant cuticles. synthetics made by step-growth polymerization such as polycarbonate and polybutyrate are also polyesters. natural polyesters and a few synthetic ones are biodegradable, but most synthetic polyesters are not.