polymers Review Enzymatic Synthesis of Biobased Polyesters and Polyamides Yi Jiang 1,2 and Katja Loos 1,2, * 1 Department of Polymer Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands; [email protected] 2 Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands * Correspondence: [email protected]; Tel.: +31-50-363-6867 Academic Editor: André Laschewsky Received: 16 May 2016; Accepted: 6 June 2016; Published: 25 June 2016 Abstract: Nowadays, “green” is a hot topic almost everywhere, from retailers to universities to industries; and achieving a green status has become a universal aim. However, polymers are commonly considered not to be “green”, being associated with massive energy consumption and severe pollution problems (for example, the “Plastic Soup”) as a public stereotype. To achieve green polymers, three elements should be entailed: (1) green raw materials, catalysts and solvents; (2) eco-friendly synthesis processes; and (3) sustainable polymers with a low carbon footprint, for example, (bio)degradable polymers or polymers which can be recycled or disposed with a gentle environmental impact. By utilizing biobased monomers in enzymatic polymerizations, many advantageous green aspects can be fulfilled. For example, biobased monomers and enzyme catalysts are renewable materials that are derived from biomass feedstocks; enzymatic polymerizations are clean and energy saving processes; and no toxic residuals contaminate the final products. Therefore, synthesis of renewable polymers via enzymatic polymerizations of biobased monomers provides an opportunity for achieving green polymers and a future sustainable polymer industry, which will eventually play an essential role for realizing and maintaining a biobased and sustainable society. Keywords: enzymatic polymerization; biobased polyesters; biobased polyamides; biobased monomer; lipase; renewable resources 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 lignocellulose, 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 lignocellulose. 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. Synthetic Polymers 2016 , 8 , 243; doi:10.3390/polym8070243 www.mdpi.com/journal/polymers Polymers 2016 , 8 , 243 2 of 53 polymers which include the large group known as plastics, became prominent since the early 20th 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]. Polymers 2016 , 8 , 243 2 of 52 phenol-formaldehyde resins, poly olefins, polyvinyl chloride, polystyrene, polyesters and polyamides, and so on. Synthetic polymers which in clude the large group known as plastics, became prominent since the early 20th 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 ma tter of fact, laws have been approved by the European Union to reduce the usage of environm entally abusive materials, and to trigger more efforts to find eco-friendly materials based on renewable resources [4,5]. Scheme 1. Global production of plastics from 2004 to 2014 [2]. Biobased polymers are pointed out to be the most promising alternatives [5–16], 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, bi obased polymers can be produced via three routes [8,11]: (1) pristine natural polymers, or chemical or physical modifications of natural polymers; (2) manufactured biobased poly mers 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 mo nomers 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. Consider ing the rich abundance of biomass feedstocks in nature, it is of great interest to produce biobas ed polymeric materials by chemical or physical modifications of na tural polymers, or from biobased mole cules that are converted from biomass feedstocks. Actually human beings already used the fo rmer approach long time ago during the 1800s. Many commercially important polymers are prepared via this approach, for example, vulcanized natural rubber, gun cotton (nitrocellulose), cellulose esters and cellulose ethers. However, chemical and physical modifications of natural polymers ar e often subject to the poor solubility and process difficulty of natural polymers, as well as, un wanted impurities within the network of natural polymers which are hard to remove. On the other hand, conversion of biomass feedstocks to end- products is a promising pathway for the production of high tonnage consumer polymeric products such as paper, paints, resins and foams [11]. For instance, oleochemicals can be converted from vegetable oils and fats, which are biobased building blocks for the producti on of thermoset resins and polyurethanes. However, the obtained biob Polymers 2016 , 8 , 243 3 of 53 it is nearly impossible to produce biobased polymers with identical structures as the petrol-based counterparts, due to the use of biomolecule mixtures. Besides, some unwanted structures or impurities might be inherited from the biomolecule mixtures, which might greatly influence the properties and applications of the final polymeric materials. Utilization of biobased monomers with tailored structures in polymer synthesis is the most promising approach towards biobased polymers, which can result in not only sustainable alternatives to petrol-based counterparts with similar or identical structures, but also in novel green polymers that cannot be produced from petrol-based monomers [ 5 , 8 , 9 , 14 – 16 ]. However, this is also the most expensive approach of all three as aforementioned. Benefiting from solar energy, numerous biobased monomers can be produced from yearly-based biomass feedstocks via biocatalytic or chemo-catalytic processes, which provide a great opportunity to access diverse biobased polymers [ 5 , 7 – 11 , 14 – 16 , 18 – 27 ]. Meanwhile, more and more biobased monomers are already or will become commercially available in the market due to the fast development of biotechnologies and their price will be competitive with that of the petrol-based chemicals [ 26 , 28 – 34 ]. Enzymatic polymerization is an emerging alternative approach for the production of polymeric materials, which can compete against conventional chemical synthesis and physical modification techniques [ 35 – 44 ]. Enzymatic polymerization also provides a great opportunity for accessing novel macromolecules that are not accessible via conventional approaches. Moreover, with mild synthetic conditions and renewable non-toxic enzyme catalysts, enzymatic polymerization is considered as an effective way to reduce the dependence of fossil resources and to address the high material consumption and pollution problems in the polymer industry. At present, petrol-based monomers are still predominately used in enzymatic polymerizations. By combining biobased monomers and enzymatic polymerizations in polymer synthesis, not only the research field of enzymatic polymerization could be greatly accelerated but also the utilization of renewable resources will be promoted. This will provide an essential contribution for achieving sustainability for the polymer industry, which will eventually play an important role for realizing and maintaining a sustainable society. 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), polybutylene terephthalate (PBT) and poly(4-hydroxybenzoate- co -6-hydroxynaphthalene-2-carboxylic acid) (Vectran ® , 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). Polymers 2016 , 8 , 243 3 of 52 chemical structures; and it is nearly impossible to produce biobased polymers with identical structures as the petrol-based co unterparts, due to the use of biomolecule mixtures. Besides, some unwanted structures or impurities might be inhe rited from the biomolecule mixtures, which might greatly influence the properties and applic ations of the final polymeric materials. Utilization of biobased monomers with tailored structures in polymer synthesis is the most promising approach towards biobased polymers, which can result in not only sustainable alternatives to petrol-based counterparts with similar or iden tical structures, but also in novel green polymers that cannot be produced from petrol-based monomers [5,8,9,14–16]. However, this is also the most expensive approach of all three as aforementioned. Benefiting from solar energy, numerous biobased monomers can be produced from yearly-based Polymers 2016 , 8 , 243 4 of 53 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 cutlery, 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 [46,50–52]. 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 ® , Celanese, Irving, TX, USA; Vectran ® , Kuraray, Chiyoda-ku, Tokyo, Japan), poly(4-hydroxybenzoate- co -4,4 1 -biphenylene terephthalate) (Xydar ® , Solvay, Brussels, Belgium; Ekonol ® , Saint-Gobain, Courbevoie, France) and poly(6-hydroxynaphthalene-2-carboxylate- co -4-hydroxybenzoate- co -4,4 1 -biphenylene terephthalate). Besides, aromatic polyesters and some semi-aromatic copolymers such as poly(2-chlorohydroquinone terephalate- co -l,4-cyclohexylenedimethylene terephthalate) and poly( p -hydroxybenzoate- co -ethylene terephthalate) are liquid crystalline materials in which both liquid crystalline and polymer properties are combined. These liquid crystalline polyesters are generally characterized by a rod-like molecular structure, rigidness of the long axis, and strong dipoles [ 55 ]. Polymers 2016 , 8 , 243 5 of 53 metal catalysts are often required for the preparation of the starting materials, cyclic monomers and cyclic oligomers. Moreover, polyesters can be also synthesized by other methods such as polyaddition of diepoxides to diacids [ 58 ], and acyclic diene metathesis (ADMET) polymerization of diene monomers containing ester bonds in the main chain [59]. At present, some biobased polyesters are already commercially available, including fully biobased PLA, PHAs, and poly(ethylene furanoate) (PEF), partially biobased PBS, PET, PTT and poly(butylene adipate- co -terephthalate) (PBAT), and so on (Table 1) [ 34 , 49 , 60 – 68 ]. However, polymers including polyesters, polyamides and other types, are still mainly derived from petroleum oils. The production capacity of biobased polymers represented only a 2% share of the total polymer production in 2013 and will increase to 4% by 2020 [3]. Table 1. A selected list of commercially available biobased polyesters and their manufacturers. Biobased Polyester Biosourcing (%) a Manufacturer Trademark PLA up to 100 NatureWorks (Minnetonka, MN, USA) Ingeo™, NatureWorks ® Synbra (Etten-Leur, The Netherlands) BioFoam ® Zhejiang Hisun Biomaterials Biological Engineering (Taizhou, Zhejiang, China) REVODE 100 and 200 series Nantong Jiuding Biological Engineering (Rugao, Jiangsu, China) - Teijin (Chiyoda, Tokyo, Japan, Japan) BIOFRONT™ Mitsui Chemicals (Minato, Tokyo, Japan) LACEA ® Futerro (Celles, Belgium) Futerro ® Corbion Purac (Amsterdam, The Netherlands) LX175, L175, L130, L105, D070 PHAs 100 Metabolix (Cambridge, MA, USA) and ADM (Decatur, IL, USA) Mirel™ MHG (Bainbridge, GA, USA) Nodax™ Bio-on (San Giorgio di Piano, Bologna, Italy) MINERV-PHA™ Tianjin Green Biosciences (Tianjin, China) GreenBio Kaneka (Tokyo, Japan) Kaneka PHBH Tianan Biological Materials (Ningbo, Zhejiang, China) ENMAT™ PHB Industrial S/A (Serrana, Brazil) BIOCYCLE ® PBS 50 PTT MCC Biochem ( Chatuchak, Bangkok, Thailand) BioPBS™ Showa Denko K.K. (Tokyo, Japan) Bionolle™ Mitsubishi Chemical (Chiyoda-ku, Tokyo, Japan) GS Pla ® PEF 100 Avantium (Geleen, The Netherlands) - PET up to 30 Coca Cola (Atlanta, GA, USA) PlantBottle™ Toyota Tsusho Corporation (Nagoya, Aichi Prefecture, Japan) GLOBIO ® PTT 37 DuPont (Wilmington, DE, USA) Sorona