# Enzymatic Synthesis of Biobased Polyesters and Polyamides
## 2 Polyesters  
Polyesters are polymers in which the monomer units are linked together by ester groups. Examples of polyesters include some naturally occurring polyesters like cutin, shellac, and poly (hydroxybutyrate) (PHB), and many synthetic polyesters such as poly(butylene succinate) (PBS), poly(lactic acid) (PLA), poly(ethylene terephthalate) (PET), polybutyrate terephthalate (PBT) and poly(4-hydroxybenzoate-co-6-hydroxynphthalene-2-carboxylic acid) (Vectran(r), Kuraray, Chiyoda-ku, Tokyo, Japan). According to the chemical composition of the main chain, polyesters can be classified as aliphatic, semi-aromatic and aromatic polyesters (Scheme 2). **Scheme 2. ** General chemical structures of aliphatic, semi-aromatic and aromatic polyesters and polyamides. Most known aliphatic polyesters could be produced as biobased polymers [45; 46], as the majority of their starting monomers can be produced from biomass feedstocks. Aliphatic polyesters are also (bio)degradable materials which can be recycled, disposed, composted or incinerated with a low environmental impact [46; 47]. Aliphatic polyesters are widely used as thermoplastics and thermoset resins, with many commodity and specialty applications. Among them, PLA is the most well-known aliphatic polyester, which can be used as fibers, food packaging materials and durable goods, with a global demand of around 360 kilo tons in 2013 [48]. PBS is another important commodity polyester which can be applied as packaging films and disposable cutler, with a global market of around 10-15 kilo tons per year [49]. In addition, aliphatic polyesters have found potential applications in biomedical and pharmaceutical fields such as in sutures, bone screws, tissue engineering scaffolds, and drug delivery systems, due to their biodegradability, biocompatibility and probable bioresorbability [50; 51; 46; 52]. Compared to aliphatic polyesters, semi-aromatic polyesters generally possess better thermal and mechanical properties, which can be used as commodity plastics and thermal engineering plastics. Examples of semi-aromatic polyesters are poly(trimethylene terephthalate) (PTT), PET, PBT, and poly(ethylene naphthalate) (PEN). Among them, PET is the most commonly used semi-aromatic polyester. It is the fourth-most-produced plastic [53], with a global supply of more than 19.8 million tons in 2012 [54]. PET has been widely used as beverage bottles, food containers, fibers and fabrics, packing films, photographic and recording tapes, engineering resins, and so on. It should be noted that PET is commonly referred by its common name, polyester, in textile and fiber applications; whereas the acronym \"PET\" or \"PET resin\" is used when applied as bottles, containers and packaging materials. Aromatic polyesters are high performance thermoplastics, with high thermal stability and chemical resistance, and excellent mechanical properties. Aromatic polyesters have found many applications in the mechanical, chemical, electronic, aviation and automobile industries [55]. However, aromatic polyesters generally possess a poor solubility even in aggressive solvents and are difficult to process, caused by their extremely rigid structures [56]. Examples of aromatic polyesters are poly(4-hydroxybenzoate-co-6-hydroxynaphthalene-2-carboxylate) (Vectra(r), Celanese, Irving, TX, USA; Vectran(r), Kuraray, Chiyoda-ku, Tokyo, Japan), poly(4-hydroxybenzoate-co-4,4\({}^{\prime}\)-biphenylene terephthalate) (Xydar(r), Solvay, Brussels, Belgium; Ekonol(r), Saint-Gobain, Courbevoie, France) and poly(6-hydroxynaphthalene-2-carboxylate-co-4-hydroxybenzoate-co-4,4\({}^{\prime}\)-biphenylene terephthalate). Besides, aromatic polyesters and some semi-aromatic copolymers such as poly(2-chlorohydroquinone terephthalate-co-1,4-cyclohexinedmethylene terephthalate) and poly(_p_-hydroxybenzoate-co-ethylene terephthalate) are liquid crystalline materials in which both liquid crystalline and polymer properties are combined.