# Enzymatic Synthesis of Biobased Polyesters and Polyamides
## 1 Polymers: From Petrol-Based to Biobased and Beyond  
Polymers are one of the most important materials that are being exploited and developed by mankind, which play an essential and ubiquitous role in our modern life. They are large molecules or macromolecules that are composed of many small molecular fragments known as repeating units. They are in widespread use as plastics, rubbers, fibers, coatings, adhesives, foams and specialty polymers [1]. According to their origin, polymers can be classified as natural polymers or synthetic polymers. Natural polymers occur in nature via in vivo reactions, where biocatalysts, normally enzymes, are inevitably involved. Natural polymers can be found in all living organisms: plants, animals and human beings. Examples of natural polymers include lignocellulosic, starch, protein, DNA, RNA and polyhydroxyalkanoates (PHAs), just to name a few. Normally, the structures of natural polymers are well-defined, with some exceptions like lignocellulosic. Synthetic polymers are commonly produced via polymerization of petrol-based chemicals having simple structures. Chemical catalysts, especially metal catalysts, are normally used in the preparation of synthetic polymers. Because of the booming of petrochemical industry and the concomitant availability of cheap petroleum oils, as well as the well establishment and advancement of polymerization techniques, numerous synthetic polymers have been developed, for example, phenol-formaldehyde resins, polyolefins, polyvinyl chloride, polystyrene, polyesters and polyamides, and so on. Syntheticpolymers which include the large group known as plastics, became prominent since the early 20 th century; and plastics are widely used as bottles, bags, boxes, textile fibers, films, and so on. Currently, there is a huge demand for polymers. The global production of plastics increased from 225 million tons in 2004 to 311 million tons in 2014 (Scheme 1) [2]; and the global polymer production is expected to reach 400 million tons in 2020 [3]. This huge polymer consumption leads to a massive demand for fossil resources for the polymer industry, which however brings some severe problems. On the one hand, fossil resources are depleting resources with limited storage; and their formation requires millions of years. There is a great concern that fossil resources will be exhausted within several hundred years. On the other hand, hazardous waste and emissions are generated along with the consumption of fossil resources, which induce severe environmental problems such as global warming and pollutions like smog and haze which are breaking out frequently, for instance in China nowadays. Driven by the growing environmental concerns, it is necessary and appealing to develop sustainable polymers for reducing the current dependence on fossil resources and decreasing the production of pollutants. As a matter of fact, laws have been approved by the European Union to reduce the usage of environmentally abusive materials, and to trigger more efforts to find eco-friendly materials based on renewable resources [4; 5]. Biobased polymers are pointed out to be the most promising alternatives [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16], which are defined as \"sustainable materials for which at least a portion of the polymer consists of materials that are produced from renewable raw materials\" [17]. Generally speaking, biobased polymers can be produced via three routes [8; 11]: (1) pristine natural polymers, or chemical or physical modifications of natural polymers; (2) manufactured biobased polymers from a mixture of biobased molecules with similar functionalities that are converted from biomass feedstocks; and (3) synthesis of biobased polymers via polymerization of biobased monomers with tailored chemical structures. Some natural polymers such as natural rubber, cotton, starch and PHAs, are useful materials; however, they are limited in variety, and their properties and applications are also limited as they are determined by their chemical structure.