Enzymatic Synthesis of Biobased Polyesters and Polyamides

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

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Polymers 2016, 8, 243 2 of 53 phenol-formaldehyde resins, polyolefins, polyvinyl chloride, polystyrene, polyesters and polyamides, and so on. Synthetic polymers which include the large group known as plastics, became polymersprominent which since includethe early the 20th large century; group and known plastics as plastics, are widely became used prominent as bottles, since bags, the boxes, early textile 20th century;fibers, films, and plasticsand so on. are widely used as bottles, bags, boxes, textile fibers, films, and so on. Currently,Currently, there there is is a hugea huge demand demand for for polymers. polymers. The The global global production production of plastics of plastics increased increased from 225from million 225 million tons in tons 2004 in to 3112004 million to 311 tons million in 2014 tons (Scheme in 20141 )[(Scheme2]; and 1) the [2]; global and polymer the global production polymer isproduction expected tois reachexpected 400 to million reach tons400 million in 2020 tons [3]. Thisin 2020 huge [3]. polymer This huge consumption polymer consumption leads to a massive leads to demanda massive for demand fossil resources for fossil for resources the polymer for the industry, polymer which industry, however which brings however some brings severe some problems. severe Onproblems. the one On hand, the fossilone hand, resources fossil are resources depleting are resources depleting with resources limited with storage; limited and storage; their formation and their requiresformation millions requires of years. millions There of years. is a great There concern is a great that fossilconcern resources that fossil will beresources exhausted will within be exhausted several hundredwithin several years. hundred On the other years. hand, On the hazardous other hand, waste hazardous and emissions waste areand generated emissions along are generated with the consumptionalong with the of fossilconsumption resources, of whichfossil induceresources, severe which environmental induce severe problems environmental such as global problems warming such andas global pollutions warming like smogand pollutions and haze whichlike smog are breakingand haze outwhich frequently, are breaking for instance out frequently, in China for nowadays. instance Drivenin China by nowadays. the growing Driven environmental by the growing concerns, environmental it is necessary concerns, and appealing it is necessary to develop and appealing sustainable to polymersdevelop forsustainable reducing polymers the current for dependence reducing onthe fossil current resources dependence and decreasing on fossil the resources production and of pollutants.decreasing Asthe a matterproduction of fact, of laws pollutants. have been As approveda matter byof thefact, European laws have Union been to approved reduce the by usage the ofEuropean environmentally Union to abusive reduce materials, the usage and of toenvironm trigger moreentally efforts abusive to find materials, eco-friendly and materialsto trigger based more onefforts renewable to find resources eco-friendly [4,5 ].materials based on renewable resources [4,5].

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SchemeScheme 1.1.Global Global productionproduction ofof plasticsplastics fromfrom 20042004 to to 2014 2014 [ 2[2].].

BiobasedBiobased polymerspolymers areare pointedpointed outout toto bebe thethe mostmost promisingpromising alternativesalternatives [[5–16],5–16], whichwhich areare defineddefined as as “sustainable “sustainable materials materials for for which which at at least least aa portionportion ofof thethe polymerpolymer consistsconsists ofof materialsmaterials thatthat areare producedproduced fromfrom renewablerenewable rawraw materials” materials” [17[17]].. GenerallyGenerally speaking,speaking, biobasedbiobased polymerspolymers cancan bebe producedproduced via via three three routes routes [ 8[8,11]:,11]: (1)(1) pristinepristine naturalnatural polymers,polymers, oror chemicalchemical oror physicalphysical modificationsmodifications ofof naturalnatural polymers; (2) (2) manufactured manufactured biobased biobased poly polymersmers from from a mixture a mixture of biobased of biobased molecules molecules with withsimilar similar functionalities functionalities that that are areconverted converted from from biomass biomass feedstocks; feedstocks; and and (3) (3) synthesis ofof biobasedbiobased polymerspolymers via via polymerization polymerization of of biobased biobased monomers monomers with with tailored tailored chemical chemical structures. structures. SomeSome naturalnatural polymerspolymers suchsuch asas naturalnatural rubber,rubber, cotton,cotton, starchstarch andand PHAs,PHAs, areare usefuluseful materials;materials; however,however, theythey areare limitedlimited inin variety,variety, andand theirtheir propertiesproperties andand applicationsapplications areare alsoalso limitedlimited asas theythey areare determineddetermined byby theirtheir chemical structure. Consider Consideringing the the rich rich abundance abundance of of biomass biomass feedstocks feedstocks in innature, nature, it itis isof of great great interest interest to to produce produce biobas biobaseded polymeric materials by chemicalchemical oror physicalphysical modificationsmodifications ofof naturalnatural polymers,polymers, oror fromfrom biobasedbiobased moleculesmolecules thatthat areare convertedconverted fromfrom biomassbiomass feedstocks.feedstocks. ActuallyActually humanhuman beingsbeings alreadyalready usedused thethe formerformer approachapproach longlong timetime agoago duringduring thethe 1800s.1800s. ManyMany commerciallycommercially importantimportant polymerspolymers areare preparedprepared viavia thisthis approach,approach, forfor example,example, vulcanizedvulcanized naturalnatural rubber,rubber, gungun cotton cotton (nitrocellulose), (nitrocellulose), cellulosecellulose estersesters andand cellulosecellulose ethers.ethers. However,However, chemicalchemical andand physicalphysical modificationsmodifications ofof naturalnatural polymerspolymers areare oftenoften subjectsubject toto thethe poorpoor solubilitysolubility andand processprocess difficultydifficulty of of natural natural polymers, polymers, as wellas well as, unwanted as, unwanted impurities impurities within thewithin network the ofnetwork natural of polymers natural whichpolymers are hardwhich to are remove. hard Onto remove. the other On hand, the conversionother hand, of conversion biomass feedstocks of biomass to feedstocks end-products to end- is a promisingproducts is pathway a promising for the pathway production for the of highproduction tonnage of consumer high tonnage polymeric consumer products polymeric such asproducts paper, paints,such as resins paper, and paints, foams resins [11]. Forand instance, foams [11]. oleochemicals For instance, can oleochemicals be converted fromcan be vegetable converted oils from and fats,vegetable which oils are and biobased fats, which building are blocks biobased for thebuilding production blocks of for thermoset the producti resinson andof thermoset polyurethanes. resins However,and polyurethanes. the obtained However, biobased the polymeric obtained materials biobased often polymeric possess diversematerials chemical often possess structures; diverse and

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chemical structures; and it is nearly impossible to produce biobased polymers with identical it isstructures nearly impossible as the petrol-based to produce co biobasedunterparts, polymers due to the with use identical of biomolecule structures mixtures. as the petrol-based Besides, some counterparts,unwanted duestructures to the useor impurities of biomolecule might mixtures. be inherited Besides, from some the unwantedbiomolecule structures mixtures, or impuritieswhich might mightgreatly be inherited influence from the properties the biomolecule and applic mixtures,ations which of the mightfinal polymeric greatly influence materials. the properties and applicationsUtilization of the finalof biobased polymeric monomers materials. with tailored structures in polymer synthesis is the most promisingUtilization approach of biobased towards monomers biobased withpolymers, tailored which structures can result in in polymer not only synthesis sustainable is thealternatives most promisingto petrol-based approach counterparts towards biobased with similar polymers, or iden whichtical can structures, result innot but only also sustainable in novel green alternatives polymers to petrol-basedthat cannot be counterparts produced from with petrol-based similar or identical monomers structures, [5,8,9,14–16]. but also However, in novel this green is also polymers the most thatexpensive cannot be approach produced of fromall three petrol-based as aforementioned. monomers [5,8,9,14–16]. However, this is also the most expensiveBenefiting approach from of all solar three energy, as aforementioned. numerous biobased monomers can be produced from yearly-based biomassBenefiting feedstocks from solar via energy,biocatalytic numerous or chemo-catalytic biobased monomers processes, can which be produced provide from a great yearly-based opportunity biomassto access feedstocks diverse via biobased biocatalytic polymers or chemo-catalytic [5,7–11,14–16,18–27]. processes, Meanwhile, which provide more a greatand more opportunity biobased to accessmonomers diverse are biobasedalready or polymers will become [5,7–11 commercially,14–16,18–27]. available Meanwhile, in the more market and more due biobasedto the fast monomersdevelopment are already of biotechnologies or will become and commercially their price available will be incompetitive the market duewith to that the fastof the development petrol-based of biotechnologieschemicals [26,28–34]. and their price will be competitive with that of the petrol-based chemicals [26,28–34]. EnzymaticEnzymatic polymerization polymerization is anis an emerging emerging alternative alternative approach approach for for the the production production of polymericof polymeric materials,materials, which which can can compete compete against against conventional conventional chemical chemical synthesis synthesis and and physical physical modification modification techniquestechniques [35 [35–44].–44]. Enzymatic Enzymatic polymerization polymerization also also provides provides a great a great opportunity opportunity for for accessing accessing novel novel macromoleculesmacromolecules that that are are not not accessible accessible via via conventional conventional approaches. approaches Moreover,. Moreover, with with mild mild synthetic synthetic conditionsconditions and and renewable renewable non-toxic non-toxic enzyme enzyme catalysts, catalysts, enzymatic enzymatic polymerization polymerization isis consideredconsidered as an aneffective effective way way to reducereduce thethe dependencedependence ofof fossilfossil resources resources and and to to address address the the high high material material consumptionconsumption and and pollution pollution problems problems in thein the polymer polymer industry. industry. AtAt present, present, petrol-based petrol-based monomers monomers are are still still predominately predominately used used in enzymaticin enzymatic polymerizations. polymerizations. ByBy combining combining biobased biobased monomers monomers and and enzymatic enzymatic polymerizations polymerizations in polymerin polymer synthesis, synthesis, not not only only thethe research research field field of of enzymatic enzymatic polymerization polymerization could could be be greatly greatly accelerated accelerated but but also also thethe utilizationutilization of of renewablerenewable resourcesresources willwill bebe promoted. promoted. This This will will provide provide an an essential essential contribution contribution for for achieving achieving sustainabilitysustainability for for the the polymer polymer industry, industry, which which will eventuallywill eventually play anplay important an important role for role realizing for realizing and maintainingand maintaining a sustainable a sustainable society. society.

2.2. Polyesters Polyesters PolyestersPolyesters are are polymers polymers in in which which the the monomer monomer units units are are linked linked together together by by ester ester groups. groups. ExamplesExamples of polyestersof polyesters include include some some naturally naturally occurring occurring polyesters polyesters like like cutin, cutin, shellac, shellac, and and poly poly (hydroxybutyrate)(hydroxybutyrate) (PHB), (PHB), and and many many synthetic synthetic polyesters polyesters such such as as poly(butylene poly(butylene succinate) succinate) (PBS), (PBS), poly(lacticpoly(lactic acid) acid) (PLA), (PLA), poly(ethylene poly(ethylene terephthalate) terephthalat (PET),e) (PET), polybutylene polybutylene terephthalate terephthalate (PBT) (PBT) and and poly(4-hydroxybenzoate-poly(4-hydroxybenzoate-co-6-hydroxynaphthalene-2-carboxylicco-6-hydroxynaphthalene-2-carboxylic acid) acid) (Vectran (Vectran®, Kuraray,®, Kuraray, Chiyoda-ku, Chiyoda- Tokyo,ku, Tokyo, Japan). AccordingJapan). According to the chemical to the chemical composition composition of the main of chain, the main polyesters chain, canpolyesters be classiﬁed can be as aliphatic,classified semi-aromaticas aliphatic, semi-aromatic and aromatic and polyesters aromatic (Scheme polyesters2). (Scheme 2).

SchemeScheme 2. 2.General General chemicalchemical structuresstructures ofof aliphatic, semi-aromatic semi-aromatic and and aromatic aromatic polyesters polyesters and andpolyamides. polyamides.

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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-4,41-biphenylene terephthalate) (Xydar®, Solvay, Brussels, Belgium; Ekonol®, Saint-Gobain, Courbevoie, France) and poly(6-hydroxynaphthalene-2-carboxylate-co-4-hydroxybenzoate-co-4,41-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]. Aromatic polyesters are good candidates for thermotropic main-chain polymers due to the highly rich aromatic (mesogenic) fragments, and the low inter-chain forces because of the relatively low energy of association of the ester groups. Generally speaking, polyesters can be produced via two methods: (1) step-growth polycondensation of diols and diacid/diesters, or hydroxyacids/hydroxyesters; and (2) ring-opening polymerization of cyclic monomers (lactones, cyclic diesters and cyclic ketene acetals) and cyclic oligomers. Both of these two methods have some merits and also suffer from some drawbacks. On the one hand, the building blocks for step-growth polycondensation are generally easily obtained at a relatively cheap price. However, elevated reaction temperatures (150–280 ˝C), long reaction times, high vacuum condition, heavy metal catalysts and a precise stoichiometric balance between monomers are normally required for polycondensation. In addition, side-reactions and volatilization of monomers may occur at elevated temperatures or under high vacuum [50,57]. On the other hand, removal of by-products is not required by ring-opening polymerization and, therefore, high molecular weight products can be obtained under relatively mild conditions in a matter of minutes. Besides, side reactions can be greatly suppressed during ring-opening polymerization. However, extra synthesis steps and heavy 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 (Table1)[ 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 NatureWorks (Minnetonka, MN, USA) Ingeo™, NatureWorks® Synbra (Etten-Leur, The Netherlands) BioFoam® Zhejiang Hisun Biomaterials Biological Engineering REVODE 100 and 200 series (Taizhou, Zhejiang, China) PLA up to 100 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 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™ PHAs 100 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® PTT MCC Biochem ( Chatuchak, Bangkok, Thailand) BioPBS™ PBS 50 Showa Denko K.K. (Tokyo, Japan) Bionolle™ Mitsubishi Chemical (Chiyoda-ku, Tokyo, Japan) GS Pla® PEF 100 Avantium (Geleen, The Netherlands) - Coca Cola (Atlanta, GA, USA) PlantBottle™ PET up to 30 Toyota Tsusho Corporation (Nagoya, Aichi Prefecture, Japan) GLOBIO® 37 Sorona® PTT DuPont (Wilmington, DE, USA) up to 35 Biomax® Novamont (Novara, Italy) Origo-Bi™ PBAT 30–70 BASF (Ludwigshafen, Germany) Ecoﬂex® FS Co-polyester 9–30 SK Chemicals (Seongnam-si, Gyeonggi-do, Korea) ECOZEN® Co-polyester - DuPont (Wilmington, DE, USA) Biomax® a Biosourcing (%): the percentage of carbon originating from biomass sources among the total organic carbon.

3. Polyamides Polyamides are polymers in which the monomeric units are linked together by amide bonds. Examples of polyamides include naturally occurring polyamides like proteins, and synthetic polyamides such as polycaprolactam (nylon 6 or PA 6), poly(hexamethylene adipamide) (nylon 6,6 or PA 6,6), poly(hexamethylene terephathamide) (PA 6,T), and poly(p-phenylene terephathamide) (PPTA, Kevlar®, DuPont, Wilmington, DE, USA). Similar to polyesters, polyamides can be classified to three types: aliphatic, semi-aromatic and aromatic polyamides, depending on the chemical composition of the main chain (Scheme2). Aliphatic polyamides, commercially known as nylons or nylon fibers, are highly valued semi-crystalline thermoplastics that are widely used as synthetic fibers, construction materials, food packing materials, engineering resins, and so on [69]. Currently, a variety of aliphatic polyamides are commercially manufactured, including nylon 6 (PA 6), nylon 10 (PA 10), nylon 11 (PA11) and nylon 12 (PA 12), and nylon 4,6 (PA 4,6), nylon 6,6 (PA 6,6), nylon 6,10 (PA 6,10) and nylon 6,12 (PA 6,12). Among them, nylon 6 is the largest produced aliphatic polyamide by far, with a global production of 4.2 million tons in 2010; and nylon 6,6 ranked the second largest aliphatic polyamide Polymers 2016, 8, 243 6 of 53 in the market, with a global production of 2.1 million tons. Actually, nylon 6,6 is the first example of aliphatic polyamides, which was firstly produced in the laboratory by Carothers and Hill at DuPont in 1930. After that, this polyamide was prepared by DuPont as nylon 6,6 fiber on 28 February 1935, and then produced at full-scale in July 1935. Regarding nylon 6, it was firstly developed by Schlack at IG Farbenindustrie in 1938, for the purpose of reproducing the properties of nylon 6,6 without violating the patents [70,71]. At present, 66% of nylon 6 production is used as fibers, 30% is applied as engineering thermoplastics, and the rest 10% is consumed as films. For nylon 6,6, 55% of the current production is used as fibers, and the remainder is applied as engineering thermoplastics. Other nylons like nylon 4,10, nylon 6,12, nylon 10,10, nylon 11 and nylon 12 are commonly used as high performance materials [72]. Semi-aromatic polyamides consist of both aliphatic and aromatic fragments in the polymer main chain. Especially, polyphthalamides (PPAs), a type of semi-aromatic polyamides, are defined by ASTM D5336 as “polyamides in which at least 55 mol % of the carboxylic acid portion of the repeating unit in the polymer chain is comprised by a combination of terephthalic acid (TPA) and isophthalic acid (IPA)” [73]. Compared to aliphatic polyamides, semi-aromatic polyamides are much stiffer, rendering the polyamides with higher mechanical strength and better thermal resistance. In addition, semi-aromatic polyamides possess many other merits such as high heat chemical/abrasion/corrosion resistance, good dimensional stability, superior processing characteristics and direct bonding to many elastomers. Semi-aromatic polyamides can be used as thermal engineering materials and high performance materials, which have found various applications in many areas, for example, in marine, automotive industry, oil industry, electronics, machinery, domestic appliances, medical devices, personal care, and so on. Examples of semi-aromatic polyamides are PA 6,T, poly(nonamethylene terephthalamide) (PA 9,T), and poly(decamethylene terephthalamide) (PA 10,T). They are commercially produced by many companies such as DuPont (Zytel®HTN, PA 6,T), Solvay (Amodel®, PA 6,T), EMS-GRIVORY (Grilamid®HT, PA 6,T), Mitsui (ARLEN®, PA 6,T/6,6), Kuraray (Genesta®, PA 9,T), and Evonik (VESTAMID®HTplus, PA 6,T/X or PA 10,T/X) [72,74]. Aromatic polyamides are normally referred to wholly aromatic polyamides, or aramids in which at least 85% of the amide linkages are directly attached to two aromatic groups [73,75,76]. Due to the amide linkages and the rigid aromatic structures, the stiff rod-like aromatic polyamide chains interact with each other by strong and highly directional hydrogen bonds and π-π stackings. Therefore, aromatic polyamides possess outstanding thermal and mechanical resistance, and excellent chemically inert property, but a poor solubility and processability. Aromatic polyamides are high performance materials that are used as advantageous replacement for metals or ceramics, cut-resistant, flame resistant and high-tensile strength synthetic fibers and coatings, bullet-proof body armor, protective clothing, electrical insulation materials, sealing materials, composites, and so on. Examples of aromatic polyamides are PPPTA and poly(m-phenylene isophthalamide) (PMPI). These two aramids are the most well-known commercially available aromatic polyamides, with the trademark of Kevlar® (DuPont) and Nomex® (DuPont), respectively. Besides, some aromatic polyamides display liquid crystalline properties. For example, the solid-state PPPT (Kevlar®, DuPont, Wilmington, DE, USA) is an example of main chain lyotropic liquid crystal polymers [55]. Similar to polyesters, polyamides can be generally synthesized via two methods: (1) step-growth polycondensation of diacids/diesters with diamines, or ω-amino acids/esters; and (2) ring-opening polymerization of lactams. For example, nylon 6,6 is produced by polycondensation of adipic acid and 1,6-hexanediamine, while nylon 6 is typically produced by ring-opening polymerization of ε-caprolactam. Regarding the equipment and the reaction conditions followed, the polymerization steps in polyester and polyamide synthesis are similar [57]. However, with respect to the formation of high molecular weight products, the polymerization of polyamides differs from that of polyesters to some extent. Firstly, the chemical equilibrium is favored for the amide formation but is less favored for the ester formation. Secondly, when dicarboxylic acids are used as starting materials, salts are formed in Polymers 2016, 8, 243 7 of 53 polyamide synthesis, but there is no salt formation in polyester synthesis. In this case, stoichiometric equivalence can be much more easily achieved in polyamide synthesis. Thirdly, the amide interchange reactions (transamidations) are much slower than the ester interchange reactions (transesterifications). Currently, some biobased polyamides are already commercially available, including fully biobased nylon 4,10, nylon 10,10 and nylon 11, and partially biobased nylon 6,10, nylon 10,12 and PA 10,T, and so on (Table2).

Table 2. A selected list of commercially available biobased polyamides and their manufacturers.

Biobased Polyamide Biosourcing (%) a Manufacturer Trademark Nylon 4,10 (PA 4,10) 100 DSM (Heerlen, The Netherlands) EcoPaXX® BASF (Ludwigshafen, Germany) Ultramid® S Balance EMS-GRIVORY (Domat/Ems, Switzerland) Grilamid® 2S Evonik (Essen, Germany) VESTAMID® Terra HS Nylon 6,10 (PA 6,10) 63 Solvay (Rhodia) (Brussels, Belgium) Technyl® eXten DuPont (Wilmington, DE, USA) Zytel® RS LC3030 Arkema (Colombes, France) Rilsan® S Suzhou Hipro Polymers (Suzhou, Jiangsu, China) Hiprolon® 70 EMS-GRIVORY (Domat/Ems, Switzerland) Grilamid® 1S Evonik (Essen, Germany) VESTAMID® Terra DS Nylon 10,10 (PA 100 ® 10,10) DuPont (Wilmington, DE, USA) Zytel RS LC1000 Arkema (Colombes, France) Rilsan® T Suzhou Hipro Polymers (Suzhou, Jiangsu, China) Hiprolon® 200, Hiprolon®211 Nylon 10,12 (PA Evonik (Essen, Germany) VESTAMID® Terra DD 45 10,12) Suzhou Hipro Polymers (Suzhou, Jiangsu, China) Hiprolon® 400 Arkema (Colombes, France) Rilsan® PA11 Nylon 11 (PA 11) 100 Suzhou Hipro Polymers (Suzhou, Jiangsu, China) Hiprolon® 11 EMS-GRIVORY (Domat/Ems, Switzerland) Grilamid® HT3 PA 10,T 50 Evonik (Essen, Germany) VESTAMID® HTplus M3000 Polyphthalamide >70 Arkema (Colombes, France) Rilsan® HT (PPA) Transparent 54 Arkema (Colombes, France) Rilsan® Clear G830 Rnew polyamide Co-polyamide Tailored, up to 100 Arkema (Colombes, France) Platamid® Rnew Polyamide High Bio-Content EMS-GRIVORY (Domat/Ems, Switzerland) Grilamid® TR a Biosourcing (%): the percentage of carbon originating from biomass sources among the total organic carbon.

4. Biobased Monomers for Polyester and Polyamide Synthesis Generally speaking, lactones, diacids and their ester and anhydride derivatives, diols, polyols, and hydroxyacids and their esters are good building blocks for polyester synthesis, while lactams, ω-amino acids and their esters, diacids and their derivatives, and diamines are suitable monomers for polyamide synthesis. Herein, some predominate biobased monomers for polyester and polyamide synthesis are outlined.

4.1. Biobased Lactones and Lactams Lactones and lactams are abundant moieties in naturally occurring compounds with diversified structures and varied ring sizes. Examples of naturally occurred lactones and lactams are tetronic acid, 5,6-dihydropyran-2-one, coumarin, α-alkylidene-γ-lactones and lactams, α-alkylidene-δ-lactones and lactams, β-lactam, and so on. They are widely applied in the fine and functional perfumery and in the pharmaceutical industry. However, few studies referred to the synthesis of polyesters and polyamides from naturally occurring lactones and lactams, probably due to their complicated structures, limited availability, and high price [77]. 3-Hydroxybutyrolactone (3-HBL) is a biobased platform molecules listed in “DOE TOP 10” [18]. It is a chiral compound that can be used for the synthesis of pharmaceuticals, polymers and organic solvents. However, the chemical synthesis of 3-HBL is quite difficult, with multiple steps [5,18,78]. Currently, (S)-3-HBL is commercially produced from L-malic acid via a continuous chemical synthesis process under high pressure in a fixed-bed reactor using a ruthenium-based catalyst [79,80] This process involves hazardous conditions, expensive catalysts, as well as multiple purification steps [78]. Polymers 2016, 8, 243 8 of 52 synthesisPolymers 2016 process, 8, 243 under high pressure in a fixed-bed reactor using a ruthenium-based catalyst [79,80]8 of 53 This process involves hazardous conditions, expensive catalysts, as well as multiple purification steps [78]. Recently, Prather et al. [78,81] developed a biosynthesis pathway for 3-HBL in recombinant E. Recently, Prather et al. [78,81] developed a biosynthesis pathway for 3-HBL in recombinant E. coli coli (Escherichia coli) using glycolic acid or glucose as the starting material. However, the large-scale (Escherichia coli) using glycolic acid or glucose as the starting material. However, the large-scale biological production of 3-HBL is still challenging, which requires further studies. biological production of 3-HBL is still challenging, which requires further studies. Some other lactones, such as propiolactone, γ-butyrolactone, angelilactone, γ-valerolactone, and Some other lactones, such as propiolactone, γ-butyrolactone, angelilactone, γ-valerolactone, and furan-2(5H)-one, can be derived from renewable resources [18,82]. furan-2(5H)-one, can be derived from renewable resources [18,82]. In addition, lactams can be converted from biomass feedstocks. Among them, ε-caprolactam is In addition, lactams can be converted from biomass feedstocks. Among them, ε-caprolactam is an important raw material for the synthesis of nylon 6. At present, ε-caprolactam is produced via a an important raw material for the synthesis of nylon 6. At present, ε-caprolactam is produced via a six-step chemical process using benzene and ammonia as starting materials. Recently Heeres et al. six-step chemical process using benzene and ammonia as starting materials. Recently Heeres et al. [83] [83] reported the conversion of biobased 5-(hydroxymethyl)furfural (HMF) to ε-caprolactam via four reported the conversion of biobased 5-(hydroxymethyl)furfural (HMF) to ε-caprolactam via four steps steps (Scheme 3), two steps less than the traditional approach. In addition, Bouwman et al. [84] (Scheme3), two steps less than the traditional approach. In addition, Bouwman et al. [ 84] reported reported the production of ε-caprolactam from biobased levulinic acid via a four-step process. the production of ε-caprolactam from biobased levulinic acid via a four-step process. Moreover, Moreover, synthesis of ε-caprolactam from sugar-derived lysine is developed [85,86]. It is also synthesis of ε-caprolactam from sugar-derived lysine is developed [85,86]. It is also possible to possible to produce ε-caprolactam via fermentation of sugars and the relevant industrial process is produce ε-caprolactam via fermentation of sugars and the relevant industrial process is currently currently under development [31]. under development [31].

SchemeScheme 3. 3. SynthesisSynthesis of of εε-caprolactam-caprolactam from from bi biobasedobased chemicals. chemicals.

4.2.4.2. Biobased Biobased Aliphatic Aliphatic Diacids Diacids SuccinicSuccinic acid acid is is a naturally occurring dicarboxylic acid, which which is predominantly produced commerciallycommercially through through petrochemical petrochemical routes routes by by catalytic catalytic hydrogenation hydrogenation of of maleic maleic acid acid or or anhydride, anhydride, with a global production of 30–50 kilo tons per year [87,88]. [87,88]. Succinic Succinic acid acid can can be be also also produced produced by by fermentationfermentation of of carbohydrates carbohydrates or or glycerol glycerol using using en engineeredgineered bacteria bacteria or or yeast. The The current current bio-route forfor succinic succinic acid acid is is based on proprietary E. coli or yeastyeast strainsstrains [[88].88]. To To lower lower the the cost, cost, other other microorganismsmicroorganisms andand yeast yeast have have been been developed, developed, like Coryne like -typeCoryne bacteria,-type whichbacteria, shows which a signiﬁcantly shows a significantlyhigher productivity higher productivity compared to comparedE. coli.[33 to] Currently,E. coli. [33] four Currently, companies four have companies built up have commercial built up commercialfacilities for facilities the production for the ofproduction biobased succinicof biobased acid: succinic Reverdia, acid: Succinity, Reverdia, Bioamber Succinity, and Bioamber Myriant and [29]. MyriantItaconic [29]. acid is an attractive unsaturated monomer that has already been produced industrially by sugarItaconic fermentation acid is using an attractiveAspergillus unsaturated terreus early monomer in the 1960s that [ 89has,90 already]. The current been produced production industrially of itaconic byacid sugar is around fermentation 80 kilo tons using per Aspergillus year, mainly terreus in USA, early China, in the Japan1960s and[89,90]. France The [ 91current]. To reduceproduction the cost of itaconicand increase acid is the around sustainability, 80 kilo tons current per studiesyear, mainly mainly in focus USA, on China, strain Japan improvement and France of microorganisms[91]. To reduce theby mutagenesis,cost and increase development the sustainability, of more cost-effective current studies process mainly methodologies, focus on and strain the improvement use of alternative of microorganismscheap substrates by such mutagenesis, as cellulolytic development biomass [91 of]. more cost-effective process methodologies, and the use ofAdipic alternative acid cheap is one substrates the most such important as cellulolytic commodity biomass chemicals, [91]. which is mainly used for the productionAdipic ofacid nylon is one 6,6 [33the,92 most]. The important current global commodity market forchemicals, adipic acid which is around is mainly 4 million used tonsfor the per productionyear [31]. At of present, nylon 6,6 over [33,92]. 90% of The adipic current acid global is manufactured market for industrially adipic acid byis around oxidation 4 million of cyclohexanol tons per yearor KA-oil [31]. (aAt mixturepresent, of over cyclohexanol 90% of adipic and cyclohexanone) acid is manufactured using concentrated industrially nitric by oxidation acid [92– 95of]. cyclohexanolIn recent years, or twoKA-oil prospective (a mixture biosynthetic of cyclohexanol pathways and cyclohexanone) to biobased adipic using acid concentrated have been developednitric acid

Polymers 2016, 8, 243 9 of 52

[92–95].Polymers 2016 In ,recent8, 243 years, two prospective biosynthetic pathways to biobased adipic acid have 9been of 53 developed and are under commercialization evaluation at the moment [31,33]: (1) chemo-catalytic conversion of biologically derived precursors such as cis,cis-muconic acid or D-glucaric acid; and (2) directand are biological under commercialization conversion of vege evaluationtable oils atand the sugars moment using [31 ,yeast.33]: (1) chemo-catalytic conversion of biologicallyIn addition, derived suberic precursors acid, suchsebacic as cisacid,cis and-muconic dodecanedioic acid or D-glucaric acid are acid; also and(potentially) (2) direct biologicalbiobased monomersconversion which of vegetable can be oilsconverte and sugarsd from using plant yeast. oils [31,96–98]. In addition, suberic acid, sebacic acid and dodecanedioic acid are also (potentially) biobased 4.3.monomers Biobased which Aliphatic can Diols be converted and Polyols from plant oils [31,96–98].

4.3. Biobased1,3-Propanediol Aliphatic Diols(1,3-PDO) and Polyols is a commodity chemical used for the production of various polymers, with an annual global demand of around 1 million tons [99]. At present, there are two 1,3-Propanediol (1,3-PDO) is a commodity chemical used for the production of various polymers, chemical processes for the industrial production of 1,3-PDO, starting from petrol-based with an annual global demand of around 1 million tons [99]. At present, there are two chemical acrylaldehyde or ethylene oxide [100,101]. Nowadays biobased 1,3-PDO is commercially synthesized processes for the industrial production of 1,3-PDO, starting from petrol-based acrylaldehyde or via fermentation of D-glucose based on corn using a genetically engineered E. coli. [100] In addition, ethylene oxide [100,101]. Nowadays biobased 1,3-PDO is commercially synthesized via fermentation it is promising to produce 1,3-PDO from biomass-derived glycerol using a bacterial fermentation of D-glucose based on corn using a genetically engineered E. coli.[100] In addition, it is promising to process [99–103]. produce 1,3-PDO from biomass-derived glycerol using a bacterial fermentation process [99–103]. 1,4-Butanediol (1,4-BDO) is widely used as a building block for polymer synthesis, with an 1,4-Butanediol (1,4-BDO) is widely used as a building block for polymer synthesis, with an annual annual global market of over 2.5 million tons [104]. The industrial production of 1,4-BDO dominantly global market of over 2.5 million tons [104]. The industrial production of 1,4-BDO dominantly depends depends on petrol-based chemicals such as maleic anhydride, acetylene, butane, propylene and butadiene.on petrol-based Since chemicalslate 2007, Genomatica such as maleic (USA) anhydride, started to acetylene,develop a biological butane, propylene process for and the butadiene. synthesis ofSince biobased late 2007, 1,4-BDO Genomatica from sugars (USA) using started a togenetically-modified develop a biological strain process of forE. coli the bacteria synthesis [99,104–106]. of biobased E coli 1,4-BDOThis process from has sugars already using been a genetically-modiﬁedcommercialized [31]. strainAlternatively, of . biobasedbacteria [1,4-BDO99,104–106 can]. be This produced process byhas reduction already been of sugar-derived commercialized succinic [31]. Alternatively,acid and this biobasedprocess is 1,4-BDO under commercialization can be produced bypreparation reduction stageof sugar-derived [31]. succinic acid and this process is under commercialization preparation stage [31]. 1,4:3,6-Dianhydrohexitols (DAHs) (DAHs) are are sugar-derived aliphatic diols diols with rigid and chiral structures [107]. [107]. It is of great interest to synthesizesynthesize DAH-based polymers with high glass transition temperatures (Tg) and/or with special optical properties [108]. According to the chirality, DAHs have temperatures (Tg) and/or with special optical properties [108]. According to the chirality, DAHs have three possible stereoisomers: isosorbide, isosorbide, isomanid isomanidee and and isoidide (Scheme 44).). DueDue toto thethe differentdifferent positions of the hydroxyl groups, the reactivity of these isomers ar aree different, showing the following sequence: isomannide isomannide < < isosorbide isosorbide <

Scheme 4. ChemicalChemical structures of 1,4: 1,4:3,6-dianhydrohexitols3,6-dianhydrohexitols (DAHs).

Other aliphatic diols such as 2,3-butanediol, 1,6-hexanediol, 1,8-octanediol and 1,10-decanediol, Other aliphatic diols such as 2,3-butanediol, 1,6-hexanediol, 1,8-octanediol and 1,10-decanediol, are (potentially) biobased monomers [5,109,110]. are (potentially) biobased monomers [5,109,110]. Moreover, glycerol and D-sorbitol are abundant and inexpensive biobased aliphatic polyols. Moreover, glycerol and D-sorbitol are abundant and inexpensive biobased aliphatic polyols. Glycerol is obtained as a byproduct in the production of biodiesel from vegetable oils and fats [5,111], Glycerol is obtained as a byproduct in the production of biodiesel from vegetable oils and fats [5,111], while D-sorbitol is produced industrially on large scale by reduction of glucose derived from biomass while D-sorbitol is produced industrially on large scale by reduction of glucose derived from biomass feedstocks [33]. feedstocks [33]. Furthermore, sugars like glucose and sucrose, and sugar alcohols such as erythritol, xylitol and Furthermore, sugars like glucose and sucrose, and sugar alcohols such as erythritol, xylitol and sorbitol, are polyols with multi hydroxyl groups. They are naturally occurring compounds which can sorbitol, are polyols with multi hydroxyl groups. They are naturally occurring compounds which can be produced via fermentation of various sources of biomass feedstocks [112]. be produced via fermentation of various sources of biomass feedstocks [112]. 4.4. Biobased Aliphatic Diamines

Polymers 2016, 8, 243 10 of 53

4.4. Biobased Aliphatic Diamines 1,4-Butanediamine (1,4-BDA, putrescine) is naturally produced by decomposition of amino acids in living and dead organisms, which is used for the production of engineering plastics and high performance materials such as nylon 4,6, nylon 4,10 and PA 4,T. It is produced industrially via chemical synthesis approaches starting from petrol-based 1,4-dichloro-2-butene or 1,4-dihalobutane, or succinodinitrile [113,114]. It is also possible to synthesize biobased 1,4-BDA via chemical conversion of biomass-derived succinic acid [18], or via fermentation of sugars using engineered E. coli strains [113]. 1,5-Pentanediamine (1,5-PDA, cadaverine) is a naturally occurring compound which is produced by hydrolysis of protein during the tissue putrefaction of animals, the same as 1,4-BDA. 1,5-PDA can be used for the production of nylon 5,6 and nylon 5,10. The industrial production of 1,5-PDA is similar to that of 1,4-BDA, using petrol-based 1,5-dichloropentane, glutarodinitrile, or glutaraldehyde as the starting material [114]. Moreover, the biosynthesis of 1,5-PDA is well established, by decarboxylation of lysine using several microorganisms [115,116]. It is also promising to produce biobased 1,5-PDA via fermentation of sugars by metabolic engineering. Currently, biobased 1,5-PDA has been produced in industrial scale by Cathay Industrial Biotech (Shanghai, China) [29]. In addition, Ajinomoto (Tokyo, Japan) is working on the industrial production of biobased 1,5-PDA by decarbonating of lysine via an enzymatic process. 1,6-Hexanediamine (1,6-HDA) is a raw material for synthesis of nylon 6,6, nylon 6,10 and PA 6,T, which is currently produced industrially from petrol-based butadiene or propylene. Recent developments show that biobased 1,6-HDA can be produced by chemical-catalytic conversion of adipic acid [117] or 1,6-hexanediol [110] derived from carbohydrates, or by a fermentation route [31]. The commercial production of biobased 1,6-HDA is already in preparation stage [31]. 1,8-Octanediamine (1,8-ODA) can be potentially derived from biomass. It can be produced by amination of suberic acid which can be converted from plant oils [118]. 1,10-Decanediamine (1,10-DDA) can be chemically converted from sebacic acid derived from castor oils. They are interesting biobased monomers for the synthesis of fully biobased nylon 10,10 which have already been commercially available in the market [31].

4.5. Biobased Aromatic Monomers Lignin is the largest non-carbohydrate components of lignocellulosic biomass which is composed by oxidative coupling of three phenylpropane components: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol [119]. Due to the unique structures and chemical properties, lignin provides a broad opportunity for the production of a wide variety of biobased chemicals, especially biobased aromatic chemicals that so far cannot be accessible via chemical or biological modifications of other biomass feedstocks (Scheme5). However, it remains a big challenge to develop an efficient approach for the recovery of aromatic chemicals with tailored structures from lignin [120]. Currently, only vanillin can be produced via a commercial process by oxidation of lignosulfonates, a byproduct from the sulfite pulping industry [121–123]. Recently, new chemical and biotechnological approaches for the production of vanillin are studied [23,120,124–126]. Starting from vanillin, many biobased aromatic monomers for polyester synthesis can be produced, for example, vanillic acid, divanillyl diol, dimethyl divanillate, and so on [127–131]. Terephthalic acid (TPA) is industrially produced by oxidation of p-xylene. It is used mainly as a precursor for the production of aromatic polyesters and polyamides such as PET, PBT and PPAs. The current global market size of TPA is around 30 million tons per year, and is expected to increase to 60 million tons in 2020 [31]. Nowadays, several technologies to produce biobased TPA and its precursors from renewable resources have been proposed (Scheme6)[ 49,132–139]; and some companies and research institutes are active in the development of biobased TPA [31,49,138] and full biobased PET. Nevertheless, no commercial biobased TPA and fully biobased PET are current available in the market. PolymersPolymers 2016 2016, ,8 8, ,243 243 1111 of of 52 52

FDCAFDCA is is readily readily produced produced from from biomass biomass feedstocks feedstocks, ,for for example, example, by by oxidat oxidationion of of HMF HMF derived derived from from variousvarious sourcessources ofof carbohydratescarbohydrates [18,24].[18,24]. ItIt isis alsoalso possiblepossible toto produceproduce FDCAFDCA viavia aa biocatalyticbiocatalytic approachapproach startingstarting fromfrom HMFHMF (Scheme(Scheme 7)7) [141].[141]. AtAt present,present, FDCAFDCA isis industriallyindustrially producedproduced byby AvantiumPolymersAvantium2016 (The ,(The8, 243 Netherlands) Netherlands) using using an an enabling enabling chem chemicalical synthesis synthesis technology technology [14,31]; [14,31]; and and the the11 price price of 53 isis expected expected to to be be cheaper cheaper than than the the bi biobasedobased and and petrol-based petrol-based TPA TPA [31,142]. [31,142].

Scheme 5. Promising biobased chemicals derived from lignin. SchemeScheme 5. 5. PromisingPromising biobased biobased chemicals chemicals derived from lignin.

Scheme 6. Proposed routes to biobased terephthalic acid (TPA). SchemeScheme 6. 6. Proposed Proposed routes routes to to biobased biobased terephthalic terephthalic acid acid (TPA). (TPA).

Other2,5-FurandicarboxylicOther interestinginteresting biobasedbiobased acid (FDCA) furanfuran monomersmonomers is an interesting forfor polyesterpolyester biobased oror rigid polyamidepolyamide monomer, synthesissynthesis which is includeinclude considered 2,5-2,5- bis(hydroxymethyl)furaasbis(hydroxymethyl)fura the most promising substitutenn (BHMF),(BHMF), to petrol-based 2,5-bis(aminomethyl)furan,2,5-bis(aminomethyl)furan, TPA and IPA [5,14,140 ].2,5-bis(hydroxymethyl)2,5-bis(hydroxymethyl) Currently, FDCA is readily -- tetrahydrofuranproducedtetrahydrofuran from biomass and and 2,5-bis(aminomethyl)tetrahydrofuran 2,5-bis(aminomethyl)tetrahydrofuran feedstocks, for example, by oxidation (Scheme (Scheme of HMF 7) 7) derived [5,18,24,143]. [5,18,24,143]. from various sources of carbohydrates [18,24]. It is also possible to produce FDCA via a biocatalytic approach starting from HMF (Scheme7)[ 141]. At present, FDCA is industrially produced by Avantium (The Netherlands) using an enabling chemical synthesis technology [14,31]; and the price is expected to be cheaper than the biobased and petrol-based TPA [31,142]. Polymers 2016, 8, 243 12 of 53

Other interesting biobased furan monomers for polyester or polyamide synthesis include 2,5-bis(hydroxymethyl)furan (BHMF), 2,5-bis(aminomethyl)furan, 2,5-bis(hydroxymethyl) -tetrahydrofuranPolymers 2016, 8, 243 and 2,5-bis(aminomethyl)tetrahydrofuran (Scheme7)[5,18,24,143]. 12 of 52

Scheme 7. Promising biobased furan monomers for polyester or polyamide synthesis.

4.6. Other Biobased Monomers Lactic acid, acid, one one of of the the most most well-known well-known organic organic acids acids occurring occurring naturally, naturally, can be can found be found in many in manycarbohydrates, carbohydrates, for example, for example, in naturally in naturally and fe andrmented fermented food foodproducts, products, plant, plant, human human beings beings and andanimals animals [144]. [144 In]. most In most living living organisms, organisms, lactic lactic acid acid is isalso also identified identiﬁed as as a a principal metabolic intermediate. Lactic Lactic acid acid can can be be manufactured chemically or biologically in industryindustry [[144].144]. In the chemical synthesis approach, lactic acid is prepared via hydrolysis of lactonitrile, a by-product of acrylonitrile production, by concentrated hydrochloric or sulfuric acid. This process is simple, but results in a racemic mixture of D- and L-lactic acid; and the production of lactic acid depends on the acrylonitrile industryindustry in in this this case case [145 ].[145]. On the On other the hand,other lactic hand, acid lactic can beacid produced can be via produced fermentation via offermentation sugars by bacteria. of sugars This by microbial bacteria. fermentation This microbial process fermentation involves theprocess utilization involves of biomass the utilization feedstocks, of lowbiomass reaction feedstocks, temperature, low reaction low energy temperature, consumption low energy and can consumption resulted in and enantio-pure can resulted lactic in enantio- acid by selectingpure lactic an acid appropriate by selecting microbial an appropriate strain [ 145micr–147obial]. Currently,strain [145–147]. the global Currently, demand the ofglobal lactic demand acid is 350of lactic kilo tonacid per is 350 year, kilo with ton a sustainableper year, with growth a sustainable in the next growth decade; in andthe next more decade; than 90% and of more lactic than acid is90% commercially of lactic acid produced is commercially via fermentation produced of glucose via fermentation [59]. Alternatively, of glucos productione [59]. Alternatively, of lactic acid fromproduction biobased of lactic glycerol acid and from its bi derivativesobased glycerol is feasibly; and its however, derivatives this routeis feasibly; cannot however, compete this with route the fermentationcannot compete process with becausethe fermentation of the high process cost. because of the high cost. 3-Hydroxypropionic acid (3-HPA) is a valuable biobased platform building blocks listed in “DOE“DOE TOPTOP 10”10” andand revisedrevised “DOE“DOE TOPTOP 10”10” [[18,22].18,22]. It can be producedproduced viavia chemicalchemical approaches staring from from 1,3-PDO, 1,3-PDO, 3-hydroxypropionaldehyde 3-hydroxypropionaldehyde or or acrylic acrylic acid, acid, which which are are not not cost-effective cost-effective [5]. [5In]. Inrecent recent years, years, promising promising biosynthetic biosynthetic pathways pathways have have been been developed developed to to produce produce biobased biobased 3-HPA, 3-HPA, for example, via fermentation of suga sugarsrs using using genetically genetically modified modiﬁed microorganisms microorganisms [148–150]. [148–150]. Currently, Currently, the commercial production of biobased 1,3-HPA isis underunder preparationpreparation stagestage byby severalseveral companiescompanies including Perstorp, Opxbio-Dow chemical,chemical, BASF-Cargilland-Novozymes,BASF-Cargilland-Novozymes, andand MetabolixMetabolix [[29].29]. Many longlong chainchain fatty fatty acids acids and and their their derivatives derivatives can can be producedbe produced from fr renewableom renewable resources resources such assuch plant as oilsplant and oils fats and [97 ,fats151 –[97,151–153],153], and they and are th goodey are building good blocksbuilding for blocks polyester for andpolyester polyamide and synthesis.polyamide Examplessynthesis. ofExamples long chain of long fatty chain acids fatty include acids oleic include acid, oleic ricinoleic acid, ricinoleic acid, erucic acid, acid, erucic vernolic acid, acid,vernolic and acid, so on and [9, 154so on]. [9,154]. Moreover, there are many other biobased building blocks fo forr polyester or polyamide synthesis, such as ethylene glycol [[31],31], polycarboxylic acids (citric acid, tartaric acid) [[8],8], 11-amino-undecanoic acid [[31],31], and so on.

5. Lipases Lipases (triacylglycerol lipases, triacylglycerol acyl hydrolases, E.C. 3.1.1.3) are enzymes which catalyze the hydrolysis of water-insoluble triglycerides with long-chain fatty acids to di-glycerides, mono-glycerides and glycerol with release of free fatty acids in aqueous solution (Scheme 8). In

Polymers 2016, 8, 243 13 of 53 Polymers 2016, 8, 243 13 of 52

5.organic Lipases synthesis, lipases can be used to catalyze other reactions in non-aqueous media, for example, esterification, transesterification, interesterification, amidation, transamidation, aminolysis, aldol condensationLipases (triacylglycerol and Michael addition lipases, [155–159]. triacylglycerol acyl hydrolases, E.C. 3.1.1.3) are enzymes which catalyzeGenerally, the hydrolysis lipases ofpossess water-insoluble high catalytic triglycerides reactivity with in nonpolar long-chain organic fatty acids solvents to di-glycerides, with log P mono-glycerides(logarithm of partition and glycerol coefficient) with releaseof more of than free fatty1.9 [160–162]. acids in aqueous Examples solution of suitable (Scheme organic8). In solvents organic synthesis,for lipases are lipases benzene can be(2), usedtoluene to catalyze(2.5), diphenyl other ether reactions (4.05), in hydrocarbons non-aqueous like media, cyclohexane for example, (3.2) esterification,and n-hexane (3.5), transesterification, and so on [163]. interesterification, Lipases also functi amidation,on in some green transamidation, solvents such aminolysis, as ionic liquids aldol condensationand supercritical and CO Michael2 [164–168]. addition [155–159].

Scheme 8. Lipase-catalyzed hydrolysis of triglyceride.

Despite their different sources and diverse structures, all lipases possess a very similar α/β Generally, lipases possess high catalytic reactivity in nonpolar organic solvents with log P hydrolase fold (Scheme 9). The α/β hydrolase fold consists of a β-sheet core of five to eight parallel (logarithm of partition coefficient) of more than 1.9 [160–162]. Examples of suitable organic solvents strands (only the second β strand shows an antiparallel orientation to the others) connected on both for lipases are benzene (2), toluene (2.5), diphenyl ether (4.05), hydrocarbons like cyclohexane (3.2) sides by α-helices, forming a α/β/α sandwich-like shape [169–172]. Lipases and other enzymes and n-hexane (3.5), and so on [163]. Lipases also function in some green solvents such as ionic liquids including esterases, proteases, dehalogenases, epoxide hydrolases and peroxidases which exhibit and supercritical CO [164–168]. similar structural features,2 belong to the α/β hydrolase family [169,173]. Despite their different sources and diverse structures, all lipases possess a very similar α/β It is generally acknowledged that the specificity, selectivity and catalytic reactivity of an enzyme hydrolase fold (Scheme9). The α/β hydrolase fold consists of a β-sheet core of five to eight parallel depend on its active site, the region that undergoes the binding of substrate molecules and the strands (only the second β strand shows an antiparallel orientation to the others) connected on both occurrence of enzymatic reactions. The active site of an enzyme consists of amino acid residuals that α α β α sidesform temporary by -helices, bonds forming with athe /substrate/ sandwich-like (binding site) shape and [other169–172 amino]. Lipases acid residues and other that enzymes catalyze includingthe corresponding esterases, reaction proteases, of that dehalogenases, substrate (catalytic epoxide site). hydrolases As for lipases, and peroxidases the active site which is situated exhibit α β similarinside a structural pocket, which features, is located belong above to the the/ centralhydrolase β-sheet family of the [169 protein,173]. [174]. Although the active sites Itof islipases generally have acknowledged different shapes, that sizes, the specificity,depths of the selectivity pockets, and and catalytic physicochemical reactivity characteristics of an enzyme dependof their amino on its acids active [175], site, the the binding region thatsites undergoesdisplay highly the homologous binding of substrate amino acid molecules sequences and [171]; the occurrenceand the active of enzymatic site of lipases reactions. consists The of active a highly site of conserved an enzyme catalytic consists triad: of amino a nucleophilic acid residuals residue that form(serine), temporary a histidine bonds base with and a the catalytic substrate acidic (binding residue site) (aspartic and other or glutamic amino acidacid, residues usually aspartic that catalyze acid) the(Scheme corresponding 9). In addition, reaction many of lipases that substrate exhibit (catalytica lid, a surface site). loop As for that lipases, is a lipophilic the active α-helical site is situateddomain β insidein the polypeptide a pocket, which chain is and located covers above the theactive central sites [171,176].-sheet of The the proteinlid controls [174 ].the Although access of thesubstrate active sitesmolecules of lipases to the have catalytic different center shapes, of lipase. sizes, In depths the pr ofesence the pockets, of a lipid-water and physicochemical interface, the characteristics lid opens the ofactive their center amino and acids thus [175 the], active the binding site becomes sites display accessible. highly In homologousthis case, a large amino hydrophobic acid sequences surface [171 of]; andthe enzyme the active is revealed, site of lipases which consists activates of the a highlyenzyme conserved. However, catalytic without triad: the lip aid-water nucleophilic interface, residue the (serine),lid is in a a closed histidine confirmation. base and a catalytic As a consequence, acidic residue the (aspartic active center or glutamic is not accessible acid, usually and aspartic the enzyme acid) α (Schemeis inactive.9). In addition, many lipases exhibit a lid, a surface loop that is a lipophilic -helical domain in the polypeptide chain and covers the active sites [171,176]. The lid controls the access of substrate molecules to the catalytic center of lipase. In the presence of a lipid-water interface, the lid opens the active center and thus the active site becomes accessible. In this case, a large hydrophobic surface of the enzyme is revealed, which activates the enzyme. However, without the lipid-water interface, the lid is in a closed confirmation. As a consequence, the active center is not accessible and the enzyme is inactive.

Polymers 2016, 8, 243 14 of 53 PolymersPolymers 2016 2016, 8, ,8 243, 243 1414 of of 52 52

α β SchemeScheme 9. 9. Secondary Secondary structure structure diagram diagram of of thethe the α /α/β/β hydrolasehydrolase hydrolase foldfold fold and and the the lo location locationcation of of catalytic catalytic triad triad aminoamino acid acid residues residues in in lipases. lipases. Ser: Ser: serine serine residue; residue; Asp Asp or or Glu: Glu: aspartic aspartic aspartic or or or glutamic glutamic glutamic acid acid acid residue; residue; residue; His: His: α β histidinehistidine residue; residue; helixes helixes indicate indicate α α-helixes;-helixes; arrowsarrows arrows indicateindicate indicate β β-sheets-sheets [[177]. 177[177].].

TheThe general generalgeneral catalytic catalytic catalytic mechanism mechanismmechanism of lipases ofof lipases lipases is illustrated is is illustratedillustrated in Scheme in in Scheme Scheme10, which 10, 10, involves whichwhich involves aninvolves acylation an an stepacylationacylation followed step step byfollowed followed a deacylation by by a a deacylation deacylation step [171,174 step step]. At[171, [171, the174]. acylation174]. At At the the step, acylation acylation the hydroxyl step, step, the the group hydroxyl hydroxyl of the group catalyticgroup of of serinethethe catalytic catalytic is activated serine serine by is is transferringactivated activated by by a transferring protontransferring among a a proton theproton aspartate, among among histidine,the the aspartate, aspartate, and serinehistidine, histidine, residues and and serine ofserine the catalyticresiduesresidues triad,of of the the renderingcatalytic catalytic triad, antriad, increase rendering rendering of the an an nucleophilicity increase increase of of the the ofnucleophilicity nucleophilicity the hydroxyl residueof of the the hydroxyl hydroxyl of the serine. residue residue After of of that,thethe serine. serine. the hydroxyl After After that, that, residue the the hydroxyl of hydroxyl the serine residue residue attacks of of thethe the carbonylserine serine attacks attacks group the ofthe thecarbonyl carbonyl substrate group group (carboxylic of of the the substrate substrate ester or carboxylic(carboxylic(carboxylic acid), ester ester formingor or carboxylic carboxylic the ﬁrst acid), acid), tetrahedral forming forming intermediatethe the fi firstrst tetrahedral tetrahedral with a negativeintermediate intermediate charge with with on a thea negative negative oxygen charge charge of the carbonylonon the the oxygen oxygen group. of of Thethe the carbonyl oxyanion carbonyl group. holegroup. is The formedThe oxyanion oxyanion by hydrogen hole hole is is formed bondingformed by by between hydrogen hydrogen the bonding amidebonding groups between between of the the aminoamideamide acidgroups groups residuals of of the the of amino amino the enzyme acid acid residuals andresiduals the carbonylof of th thee enzyme enzyme group oxygenand and the the of carbonyl thecarbonyl substrate. group group By oxygen theoxygen formation of of the the ofsubstrate.substrate. at least two ByBy hydrogen thethe formationformation bonds of inof at theat least oxyanionleast twotwo hyhole,hydrogendrogen the charge bondsbonds distribution inin thethe oxyanionoxyanion is stabilized hole,hole, and thethe the chargecharge state energydistributiondistribution of the is is tetrahedral stabilized stabilized and intermediateand the the state state isenergy energy reduced. of of the Thenthe tetrahedral tetrahedral the alcohol intermediate intermediate component is (Ris reduced. 1reduced.–OH) is Then releasedThen the the fromalcoholalcohol the component bondcomponent with the(R (R1 intermediate,–OH)1–OH) is is released released while from thefrom “acidic the the bond bond component” with with the the of intermediate, theintermediate, substrate remainswhile while the the covalently “acidic “acidic boundcomponent”component” to the of serineof the the substrate residuesubstrate in remains remains the acyl-enzyme covalently covalentlyintermediate. bound bound to to the the Whenserine serine theresidue residue enzyme in in the isthe attackedacyl-enzyme acyl-enzyme by a nucleophileintermediate.intermediate. (R When 2When–OH), the the the enzyme enzyme deacylation is is attacked attacked step occurs. by by a a nucleophile Thenucleophile product (R (R (a2–OH),2 new–OH), carboxylic the the deacylation deacylation ester or step step carboxylic occurs. occurs. acid)TheThe product isproduct then released, (a(a newnew carboxylic whilecarboxylic the esterenzymeester oror iscarboxyliccarboxylic regenerated. acid)acid) This isis thenthen nucleophile released,released, (R while2while–OH) thethe can enzymeenzyme be water isis (hydrolysis)regenerated.regenerated. orThis This an nucleophile alcohol nucleophile (alcoholysis). (R (R2–OH)2–OH) can can be be water water (hydrolysis) (hydrolysis) or or an an alcohol alcohol (alcoholysis). (alcoholysis).

SchemeScheme 10. 10. General General catalytic catalytic mechanism mechanism of of lipases lipases [[174]. 174[174].].

ToTo increaseincrease increase the the stability stability towards towards organic organic solvents solvents and and to to facilitate facilitate the the recycling recycling and and reusing, reusing, lipaseslipases are areare normally normallynormally used used used in in in their their their immobilized immobilized immobilized forms forms forms [178–185]. [178–185]. [178–185 The The]. immobilized The immobilized immobilized lipases lipases lipases may may show mayshow showimprovedimproved improved catalyticcatalytic catalytic activity,activity, activity, specificityspecificity speciﬁcity oror selectivity.selectivity. or selectivity. SimilarSimilar Similar toto otherother to other enzymes,enzymes, enzymes, lipaseslipases lipases cancan can bebe begenerallygenerally generally immobilized immobilized immobilized via via viathree three three strategies strategies strategies [178]: [178]: [178 (1 (1)]: )chemical (1)chemical chemical or or physical physical or physical adsorptions adsorptions adsorptions onto onto ontoan an inert inert an matrix;matrix; (2) (2) entrapment entrapment within within an an inert inert matrix; matrix; and and (3) (3) immobilized immobilized as as water-insoluble water-insoluble particles: particles: cross- cross-

Polymers 2016, 8, 243 15 of 53

Polymers 2016, 8, 243 15 of 52 inert matrix; (2) entrapment within an inert matrix; and (3) immobilized as water-insoluble particles: linkedcross-linked enzyme enzyme aggregates aggregates (CLEAs), (CLEAs), cross-linked cross-linked enzyme enzyme crystals crystals (CLECs), (CLECs), and and protein-coated protein-coated microcrystalsmicrocrystals (PCMC). (PCMC). DueDue to to the the broad broad substrate substrate specificity, speciﬁcity, high selectivity, and high thermal stability and catalytic reactivity,reactivity, Candida antarctica lipase b (CALB), (CALB), which which was was reclassified reclassiﬁed as as PseudozymaPseudozyma antarctica antarctica lipaselipase b b (PALB)(PALB) more recently [186], [186], is the most popular biocatalyst which is is extensively extensively used used in in biocatalytic biocatalytic synthesissynthesis of of small small molecules molecules and and polymers. polymers. CALB CALB is is a a globular globular protein protein that that is is composed composed of of 317 317 amino amino acidsacids (Scheme (Scheme 11),11), having having a a molecular molecular weight weight of of 33 33 kDa. kDa. Similar Similar to to other other lipases, lipases, CALB CALB possesses possesses a Ser-His-Aspa Ser-His-Asp catalytic catalytic triad triad (Ser105, (Ser105, Asp187 Asp187 and and His224) His224) in in its its active active site site and and two two oxyanion oxyanion holes holes (Thr40(Thr40 and and Gln106) Gln106) [187], [187], and and the catalytic mech mechanismanism of CALB is the same as other lipases.

(a) (b)

Scheme 11. (a) The crystal structure of Candida antarctica lipase b (PDB number: 1TCA, from Scheme 11. (a) The crystal structure of Candida antarctica lipase b (PDB number: 1TCA, from http://www.rcsb.org/); and (b) a photo of Novozym® 435 beads. http://www.rcsb.org/); and (b) a photo of Novozym® 435 beads. However, the presence of the lid structure and the interfacial activation of CALB are still under debate.However, Some literature the presence suggested of the that lid structurethe two α and-helixes the interfacial (α5 and α activation10) surrounding of CALB the are active still center under ofdebate. CALB, Some the literaturemost mobile suggested part of that the the structure, two α-helixes could (workα5 and asα the10) surroundinglid [188–191], the and active CALB center is an of interfacialCALB, the activated most mobile enzyme. part of A the recent structure, study could indica workted asthe the hydrophobicity lid [188–191], and of the CALB interface is an interfacial and the overallactivated size enzyme. of the substrate A recent determi study indicatedne the interfacial the hydrophobicity activation of of CALB the interface [190], Others and the suggested overall size that of CALBthe substrate has no determinelid covering the the interfacial entrance activation of the active of CALB site [187] [190 and], Others displays suggested no interfacial that CALB activation has no [192].lid covering In addition, the entrance CALB ofhas the a activevery limited site [187 available] and displays space noin interfacialthe pocket activation of active [site192 ].compared In addition, to otherCALB lipases has a veryand limitedthis explains available its high space selectivity in the pocket [193]. of active site compared to other lipases and this explainsCALB its shows high selectivity improved [ 193thermal]. stability and more stable performance in its immobilized form. At present,CALB shows several improved immobilized thermal CALB stability formulations and more stable are performancecommercially in itsavailable, immobilized including form. ® NovozymAt present,® several435 (N435, immobilized Novozymes CALB A/S, formulations Copenhagen, are Denmark), commercially Chirazyme available,® L including-2 (Roche NovozymMolecular ® Biochemicals,435 (N435, Novozymes Mannheim, A/S, Germany), Copenhagen, LCAHNHE Denmark), and Chirazyme LCAME (SPRINL-2 (Roche S.p.A, Molecular Milano, Biochemicals, Italy), and CalBMannheim, immo Germany),Plus™ (c-LEcta, LCAHNHE Leipzig, and Germany, LCAME (SPRINand Purolite, S.p.A, Milano,Bala Cynwyd, Italy), andPA, CalBUSA) immo [194]. Plus The™ immobilized(c-LEcta, Leipzig, CALB Germany, formulations and Purolite,are currently Bala Cynwyd,frequently PA, used USA) in industry, [194]. The for immobilized example, for CALB the synthesisformulations of pharmaceutical are currently frequently chiral intermediates used in industry,, and for for example, the production for the synthesis of other ofvalue-added pharmaceutical fine chemicalchiral intermediates, compounds. and for the production of other value-added ﬁne chemical compounds. N435N435 is is the the primary immobilized immobilized CALB CALB that that is is used used both both in in the industrial area area and academia research.research. N435 N435 functions functions as as a a hydrophobic hydrophobic biocatalyst, biocatalyst, which which consists consists of of 10 10 wt wt % % of of CALB CALB physically physically absorbedabsorbed within within 90 90 wt wt % % of of Lewatit VP VP OC 1600 bead which is a macroporous DVB-crosslinked methacrylatemethacrylate polymer polymer resin resin [162,194,195]. [162,194,195]. The The bead bead size size of of N435 N435 ranges ranges from from 0.315 0.315 to to 1.0 1.0 mm mm (>80%), (>80%), thethe effective effective size size is is around around 0.32–0.45 0.32–0.45 mm, mm, and and the the average average pore pore diameter diameter is is 15 15 nm. nm. N435 N435 can can work work at at mildmild conditions conditions and and especially, especially, can tolerate some extreme conditions such as elevated temperatures ˝ (up(up to to 150 150 °C)C) [196–198]. [196–198].

6. Enzyme-Catalyzed Synthesis of Polyesters Enzymatic polymerization is defined as “in vitro (in the test tubes) chemical synthesis of polymers via a non-biosynthetic (non-metabolic) approach using an isolated enzyme as the catalyst” [36,199].

Polymers 2016, 8, 243 16 of 53

6. Enzyme-Catalyzed Synthesis of Polyesters Enzymatic polymerization is defined as “in vitro (in the test tubes) chemical synthesis of polymers via a non-biosynthetic (non-metabolic) approach using an isolated enzyme as the catalyst” [36,199]. Due to the unique properties of enzymes, enzymatic polymerization inherits many merits such as high specificity and selectivity towards monomer substrates, clean-process, energy saving, gentle environmental footprint, nontoxic natural catalysts, and recyclable catalysts (after immobilization). With these, enzymatic polymerization provides an opportunity to achieve “green polymer chemistry”. At present, 4 enzyme classes, oxidoreductases, transferases, hydrolases and ligases, are identified to induce or catalyze polymerizations (Table3)[ 200]. Many polymers are successfully synthesized via enzymatic polymerizations, for example, vinyl polymers [38,201], polysaccharides [202–205], polyesters [42,44] and polyamides [206–208]. Among them, polyesters are the most extensively studied polymers in enzymatic polymerization; and lipases are the most efficient biocatalysts for enzymatic polymerization of polyesters [42].

Table 3. Enzymes and typical examples for their use in polymer synthesis, and typical polymers Polymers 2016, 8, 243 16 of 52 synthesized via enzymatic polymerization [200]. Due to the unique properties of enzymes, enzymatic polymerization inherits many merits such as high specificity and selectivity towards monomer substrates, clean-process, energy saving, gentle environmental footprint, nontoxic natural catalysts, and recyclable catalysts (after immobilization). Enzyme classWith these, enzymatic Reaction polymerization catalyzed provides an opportunity Typical to achieve enzymes “green polymer Typical polymers chemistry”. At present, 4 enzyme classes, oxidoreductases, transferases, hydrolases and ligases, are Polyanilines identified to induce or catalyze polymerizations (Table 3) [200]. Many polymers are successfully Polyphenol Oxidation/Reduction Peroxidase EC 1. Oxidoreductasessynthesized via enzymatic polymerizations, for example, vinyl polymers [38,201], polysaccharides Polystyrenes [202–205], polyesters AH[42,44]2 +and B ÑpolyamidesA + BH [206–208].2 AmongLaccase them, polyesters are the most extensively studied polymers in enzymatic polymerization; and lipases are the most efficient Poly(methyl biocatalysts for enzymatic polymerization of polyesters [42]. methacrylate)

Table 3. Enzymes and typical examples for their use in polymerPHA synthesis, synthase and typical polymers Polyesters Group transfer EC 2. Transferasessynthesized via enzymatic polymerization [200]. Hyaluronan synthase Hyaluronan Enzyme class A-X Reaction + catalyzed B Ñ A + B-X Typical enzymes Typical polymers PhosphorylasePolyanilines Amylose Polyphenol Oxidation/Reduction Peroxidase EC 1. Oxidoreductases Polystyrenes Polyesters AH2 + B A + BH2 Laccase Lipase Poly(methyl Hydrolysis by H2O methacrylate) Polyamides PHA synthaseCellulase Polyesters EC 3. Hydrolases A-BGroup transfer + H O Ñ AH + Cellulose EC 2. Transferases 2 Hyaluronan synthase Hyaluronan A-X + B A + B-X Hyaluronidase BOH Phosphorylase Amylose (Oligo)peptides Papain Polyesters Lipase Polyamides Glycosaminoglycan Hydrolysis by H O Cellulase EC 3. Hydrolases 2 Cellulose Hyaluronidase A-B + H2O AH + BOH (Oligo)peptides Bond formation Papain Glycosaminoglycan EC 6. Ligases Bondrequiring formation requiring triphosphate triphosphate Cyanophycin synthetase Cyanophycin EC 6. Ligases Cyanophycin synthetase Cyanophycin

Generally speaking, three polymerization modes can be proceeded for the lipase-catalyzed polyester synthesis (Scheme 12): (1) step-growth polycondensation; (2) ring-opening polymerization; Generally speaking,and (3) a combination three of ring-opening polymerization polymerization and polycondensation modes can (ring-opening be proceeded addition- for the lipase-catalyzed condensation polymerization). Among them, polycondensation and ring-opening polymerization are polyester synthesisthe most (Scheme common methods 12): used (1) for step-growth biocatalytic polyester polycondensation;synthesis. (2) ring-opening polymerization; and (3) a combination of ring-opening polymerization and polycondensation (ring-opening addition-condensation polymerization). Among them, polycondensation and ring-opening polymerization are the most common methods used for biocatalytic polyester synthesis. Four modes of elemental reactions may occur during the lipase-catalyzed polyester synthesis, inducing hydrolysis, esterification, transesterification (alcoholysis and acidolysis), and interesterification (Scheme 13). These reactions are all reversible. Therefore, to facilitate the ester formation, it is crucial to remove the remaining water and byproducts like alcohols from the reaction mixture, for example, by adding absorbing and drying agents like molecular sieves, applying reduced pressure, using azeotropic distillation conditions, and so on. The first lipase-catalyzedScheme 12. Main polymerization reaction modes of lipase-catalyzed was synthesis reported of polyesters. by Okumara et al. in 1984 [209].

They investigated the enzymatic polymerization of aliphatic diacids and diols by a lipase from Aspergillus niger NRRL 337 (Scheme 14). However, only oligoesters with Mn’s of around 1000 g/mol were obtained. Polymers 2016, 8, 243 16 of 52

Due to the unique properties of enzymes, enzymatic polymerization inherits many merits such as high specificity and selectivity towards monomer substrates, clean-process, energy saving, gentle environmental footprint, nontoxic natural catalysts, and recyclable catalysts (after immobilization). With these, enzymatic polymerization provides an opportunity to achieve “green polymer chemistry”. At present, 4 enzyme classes, oxidoreductases, transferases, hydrolases and ligases, are identified to induce or catalyze polymerizations (Table 3) [200]. Many polymers are successfully synthesized via enzymatic polymerizations, for example, vinyl polymers [38,201], polysaccharides [202–205], polyesters [42,44] and polyamides [206–208]. Among them, polyesters are the most extensively studied polymers in enzymatic polymerization; and lipases are the most efficient biocatalysts for enzymatic polymerization of polyesters [42].

Table 3. Enzymes and typical examples for their use in polymer synthesis, and typical polymers synthesized via enzymatic polymerization [200].

Enzyme class Reaction catalyzed Typical enzymes Typical polymers Polyanilines Polyphenol Oxidation/Reduction Peroxidase EC 1. Oxidoreductases Polystyrenes AH2 + B A + BH2 Laccase Poly(methyl methacrylate) PHA synthase Polyesters Group transfer EC 2. Transferases Hyaluronan synthase Hyaluronan A-X + B A + B-X Phosphorylase Amylose Polyesters Lipase Polyamides Hydrolysis by H O Cellulase EC 3. Hydrolases 2 Cellulose Hyaluronidase A-B + H2O AH + BOH (Oligo)peptides Papain Glycosaminoglycan Bond formation requiring triphosphate EC 6. Ligases Cyanophycin synthetase Cyanophycin

Polymerscondensation2016, 8, 243polymerization). Among them, polycondensation and ring-opening polymerization17 of are 53 the most common methods used for biocatalytic polyester synthesis.

Polymers 2016, 8, 243 17 of 52

Four modes of elemental reactions may occur during the lipase-catalyzed polyester synthesis, inducing hydrolysis, esterification, transesterification (alcoholysis and acidolysis), and Polymers 2016, 8, 243 17 of 52 interesterification (Scheme 13). These reactions are all reversible. Therefore, to facilitate the ester formation, it is crucial to remove the remaining water and byproducts like alcohols from the reaction Four modes of elemental reactions may occur during the lipase-catalyzed polyester synthesis, mixture, for example, by adding absorbing and drying agents like molecular sieves, applying inducing hydrolysis, esterification, transesterification (alcoholysis and acidolysis), and reduced pressure, using azeotropic distillation conditions, and so on. interesterification (Scheme 13). These reactions are all reversible. Therefore, to facilitate the ester formation, it is crucial to remove the remaining water and byproducts like alcohols from the reaction Scheme 12. Main reaction modes of lipase-catalyzed synthesis of polyesters. mixture, for example,Scheme 12.by Mainadding reaction absorbing modes ofand lipase-catalyzed drying agents synthesis like molecular of polyesters. sieves, applying reduced pressure, using azeotropic distillation conditions, and so on.

Scheme 13. Basic modes of elemental lipase-catalyzed reactions in biocatalytic polyester synthesis.

The first lipase-catalyzed polymerization was reported by Okumara et al. in 1984 [209]. They investigated the enzymatic polymerization of aliphatic diacids and diols by a lipase from Aspergillus Scheme 13. Basic modes of elemental lipase-catalyzed reactions in biocatalytic polyester synthesis. niger SchemeNRRL 337 13. Basic(Scheme modes 14). of elementalHowever, lipase-catalyzed only oligoesters reactions with in biocatalytic ’s of around polyester 1000 synthesis. g/mol were obtained. The first lipase-catalyzed polymerization was reported by Okumara et al. in 1984 [209]. They investigated the enzymatic polymerization of aliphatic diacids and diols by a lipase from Aspergillus niger NRRL 337 (Scheme 14). However, only oligoesters with ’s of around 1000 g/mol were obtained.

Scheme 14. Lipase-catalyzed polycondensation of aliphatic diacidsdiacids andand diols.diols.

The lipase-catalyzedlipase-catalyzed ring-opening ring-opening polymerization polymerizati wason ﬁrstlywas reportedfirstly report in 1993ed by in two 1993 independent by two independent groups [210,211]. Gutman et al. [210] investigated the lipase-catalyzed ring-opening groups [210,211Scheme]. Gutman 14. Lipase-catalyzed et al. [210] investigated polycondensation the lipase-catalyzed of aliphatic diacids ring-opening and diols. polymerization of polymerization of ε-caprolactone (ε-CL); and polycaprolactone (PCL) with a of up to 4400 g/mol ε-caprolactone (ε-CL); and polycaprolactone (PCL) with a Mn of up to 4400 g/mol was successfully was successfully produced in n-hexane (Scheme 15). At the same time, the enzymatic ring-opening producedThe lipase-catalyzed in n-hexane (Scheme ring-opening 15). At the polymerizati same time,on the was enzymatic firstly ring-openingreported in polymerization1993 by two polymerization of lactones was performed in bulk by Kobayashi et al. [211], using different lipases as independentof lactones was groups performed [210,211]. in bulkGutman by Kobayashiet al. [210] etinvestigated al. [211], using the lipa differentse-catalyzed lipases ring-opening as catalysts. catalysts. The enzymatic polymerization gave PCL and polyvalerolactone with ’s of up to 7700, polymerizationThe enzymatic of polymerization ε-caprolactone gave(ε-CL); PCL and and polycaprolactone polyvalerolactone (PCL) with with Ma n’s of of up to to 4400 7700, g/mol and and 1900 g/mol, respectively. was1900 successfully g/mol, respectively. produced in n-hexane (Scheme 15). At the same time, the enzymatic ring-opening polymerization of lactones was performed in bulk by Kobayashi et al. [211], using different lipases as catalysts. The enzymatic polymerization gave PCL and polyvalerolactone with ’s of up to 7700, and 1900 g/mol, respectively.

Scheme 15. Lipase-catalyzed ring-opening polymerization of ε-caprolactone.

In the late 1990s, the use of N435 in the enzymatic ring-opening polymeri zation of lactones was introduced by Gross et al. [212] Since then, N435 became the working horse in biocatalytic polyester Scheme 15. Lipase-catalyzed ring-opening polymerization of ε-caprolactone. synthesis. After these pioneer works, various combinations of monomer substrates such as diacids/diesters In the late 1990s, the use of N435 in the enzymatic ring-opening polymerization of lactones was and diols, hydroxyacids/esters, and cyclic monomers like lactones, cyclic diesters and cyclic ketene introduced by Gross et al. [212] Since then, N435 became the working horse in biocatalytic polyester synthesis. After these pioneer works, various combinations of monomer substrates such as diacids/diesters and diols, hydroxyacids/esters, and cyclic monomers like lactones, cyclic diesters and cyclic ketene

Polymers 2016, 8, 243 17 of 52

Scheme 13. Basic modes of elemental lipase-catalyzed reactions in biocatalytic polyester synthesis.

Scheme 14. Lipase-catalyzed polycondensation of aliphatic diacids and diols.

The lipase-catalyzed ring-opening polymerization was firstly reported in 1993 by two independent groups [210,211]. Gutman et al. [210] investigated the lipase-catalyzed ring-opening polymerization of ε-caprolactone (ε-CL); and polycaprolactone (PCL) with a of up to 4400 g/mol was successfully produced in n-hexane (Scheme 15). At the same time, the enzymatic ring-opening polymerization of lactones was performed in bulk by Kobayashi et al. [211], using different lipases as catalysts. The enzymatic polymerization gave PCL and polyvalerolactone with ’s of up to 7700, Polymers 2016, 8, 243 18 of 53 and 1900 g/mol, respectively.

SchemeScheme 15. 15. Lipase-catalyzedLipase-catalyzed ring-o ring-openingpening polymerization polymerization of of εε-caprolactone.-caprolactone.

In the late 1990s, the use of N435 in the enzymatic ring-opening polymerization of lactones was In the late 1990s, the use of N435 in the enzymatic ring-opening polymerization of lactones introduced by Gross et al. [212] Since then, N435 became the working horse in biocatalytic polyester was introduced by Gross et al. [212] Since then, N435 became the working horse in biocatalytic synthesis. polyester synthesis. After these pioneer works, various combinations of monomer substrates such as diacids/diesters PolymersAfter 2016 these, 8, 243 pioneer works, various combinations of monomer substrates such as diacids/diesters18 of 52 and diols, hydroxyacids/esters, and cyclic monomers like lactones, cyclic diesters and cyclic ketene and diols, hydroxyacids/esters, and cyclic monomers like lactones, cyclic diesters and cyclic ketene acetals,acetals, areare studiedstudied forfor thethe lipase-catalyzedlipase-catalyzed polymerization.polymerization. The The recent progress in this ﬁeldfield isis comprehensivelycomprehensively summarizedsummarized inin somesome reviewreview articlesarticles [[35,36,40–42,44,213,214].35,36,40–42,44,213,214]. ItIt shouldshould be be pointed pointed out out that that the largethe large scale productionscale production of aliphatic of aliphatic polyesters polyesters via lipase-catalyzed via lipase- polymerizationcatalyzed polymerization is feasible. is fe Asasible. reported As reported by Binns by Binns et al. et[ 215al. [215],], adipic adipic acid acid and and 1,6-HDO 1,6-HDO werewere polymerizedpolymerized byby N435 N435 at aat multi-kilogram a multi-kilogram scale, scale, using ausing two-stage a two-stage method (Schememethod 16(Scheme). The enzymatic 16). The polymerizationenzymatic polymerization yielded poly(hexamethylene yielded poly(hexamethylene adipate) with adipate) a Mw of with 16,400 a g/mol. of 16,400 They alsog/mol. claimed They thatalso theclaimed enzymatic that the production enzymatic canproduction be scaled can up be toscaled the pilotup to plantthe pilot level plant (2.0 level tons) (2.0 without tons) without undue problems.undue problems. Besides, poly(hexamethyleneBesides, poly(hexamethylene adipate) produced adipate) fromproduced the enzymatic from polymerizationthe enzymatic possessespolymerization a lower possesses acid number, a lower higher acid number, degree ofhigher crystallinity degree of and crystallinity super crystalline and super growth crystalline rate comparedgrowth rate to compared the conventional to the conventional counterparts. counterparts.

SchemeScheme 16. N435-catalyzedN435-catalyzed synthesis synthesis of poly(hexamethylene of poly(hexamethylene adipate) adipate) in large in scale, large using scale, a two-stage using a two-stagemethod. method.

Moreover,Moreover, macrolidesmacrolides catalyzedcatalyzed byby lipaseslipases showedshowed higherhigher polymerizabilitypolymerizability comparedcompared toto smallersmaller ring-sizedring-sized lactoneslactones [216[216].]. ThisThis is is probably probably because because macrolides macrolides possess possess higher higher rates rates in in the the formation formation of enzyme-activatedof enzyme-activated monomers monomers (acyl-enzyme (acyl-enzyme intermediates). intermediates). However, However, reverse tendencyreverse tendency was observed was fromobserved anionic from and anionic metal and (Zn) metal catalyzed-ring (Zn) catalyzed-ring opening polymerization. opening polymerization. AlthoughAlthough aa greatgreat numbernumber ofof aliphaticaliphatic polyesterspolyesters areare readilyreadily synthesizedsynthesized withwith highhigh molecularmolecular weightsweights viavia lipase-catalyzedlipase-catalyzed polymerization,polymerization, onlyonly limitedlimited amountamount ofof semi-aromaticsemi-aromatic andand aromaticaromatic polyesterspolyesters areare enzymaticallyenzymatically producedproduced [[217–227].217–227]. This could be mainly due toto thethe highhigh meltingmelting temperaturetemperature ((TTmm)) of semi-aromatic and aromatic polyesters and theirtheir lowlow solubilitysolubility inin thethe reactionreaction media,media, asas well well as, as, the the lack lack of reactivity of reactivity of aromatic of aromatic monomers monomers in enzymatic in enzymatic polyesteriﬁcation polyesterification [210,228]. However,[210,228]. byHowever, using cyclic by using aromatic cyclic oligomers aromatic inoligomers the lipase-catalyzed in the lipase-catalyzed polymerization, polymerization, high molecular high weightmolecular poly(alkylene weight terephthalate)s,poly(alkylene polyterephthalate)s, (alkylene isophthalate)s poly (alkylene and poly(benzenedimethanol isophthalate)s and adipate)spoly(benzenedimethanol were obtained, with adipate)sMw’s were of up obtained, to 107,000 with g/mol [’s229 of]. up to 107,000 g/mol [229].

7.7. Enzyme-Catalyzed Enzyme-Catalyzed Synthesis Synthesis of of Polyamides Polyamides Lipases,Lipases, proteasesproteases andand otherother enzymesenzymes areare capablecapable ofof catalyzingcatalyzing thethe formationformation ofof amideamide bondsbonds andand therefore,therefore, theythey areare suitablesuitable enzymesenzymes forfor thethein in vitrovitro polyamidepolyamide synthesissynthesis [[206].206]. InIn thethe followingfollowing discussiondiscussion ofof thisthis section,section, wewe focusfocus onon thethe lipase-catalyzedlipase-catalyzed polymerizationpolymerization ofof syntheticsynthetic polyamides.polyamides. SimilarSimilar toto thethe biocatalyticbiocatalytic polyesterpolyester synthesis,synthesis, thethe lipase-catalyzedlipase-catalyzed polyamidepolyamide synthesissynthesis cancan proceedproceed viavia three basic modes: (1) (1) step-growth step-growth poly polycondensationcondensation of of diacid/diesters diacid/diesters and and diamines diamines or ω-amino carboxylic acids/esters; (2) ring-opening polymerization of lactams; and (3) a hybrid of step- growth polycondensation and ring-opening polymerization. Two basic modes of elemental reactions are commonly used in the biocatalytic polyamide synthesis: directly amidation and transamidation (aminolysis) (Scheme 17).

Scheme 17. Basic modes of elemental lipase-catalyzed reactions in biocatalytic polyamide synthesis.

Polymers 2016, 8, 243 18 of 52 acetals, are studied for the lipase-catalyzed polymerization. The recent progress in this field is comprehensively summarized in some review articles [35,36,40–42,44,213,214]. It should be pointed out that the large scale production of aliphatic polyesters via lipase- catalyzed polymerization is feasible. As reported by Binns et al. [215], adipic acid and 1,6-HDO were polymerized by N435 at a multi-kilogram scale, using a two-stage method (Scheme 16). The enzymatic polymerization yielded poly(hexamethylene adipate) with a of 16,400 g/mol. They also claimed that the enzymatic production can be scaled up to the pilot plant level (2.0 tons) without undue problems. Besides, poly(hexamethylene adipate) produced from the enzymatic polymerization possesses a lower acid number, higher degree of crystallinity and super crystalline growth rate compared to the conventional counterparts.

Scheme 16. N435-catalyzed synthesis of poly(hexamethylene adipate) in large scale, using a two-stage method.

Moreover, macrolides catalyzed by lipases showed higher polymerizability compared to smaller ring-sized lactones [216]. This is probably because macrolides possess higher rates in the formation of enzyme-activated monomers (acyl-enzyme intermediates). However, reverse tendency was observed from anionic and metal (Zn) catalyzed-ring opening polymerization. Although a great number of aliphatic polyesters are readily synthesized with high molecular weights via lipase-catalyzed polymerization, only limited amount of semi-aromatic and aromatic polyesters are enzymatically produced [217–227]. This could be mainly due to the high melting temperature (Tm) of semi-aromatic and aromatic polyesters and their low solubility in the reaction media, as well as, the lack of reactivity of aromatic monomers in enzymatic polyesterification [210,228]. However, by using cyclic aromatic oligomers in the lipase-catalyzed polymerization, high molecular weight poly(alkylene terephthalate)s, poly (alkylene isophthalate)s and poly(benzenedimethanol adipate)s were obtained, with ’s of up to 107,000 g/mol [229].

7. Enzyme-Catalyzed Synthesis of Polyamides Lipases, proteases and other enzymes are capable of catalyzing the formation of amide bonds and therefore, they are suitable enzymes for the in vitro polyamide synthesis [206]. In the following discussion of this section, we focus on the lipase-catalyzed polymerization of synthetic polyamides. Polymers 2016, 8, 243 19 of 53 Similar to the biocatalytic polyester synthesis, the lipase-catalyzed polyamide synthesis can proceed via three basic modes: (1) step-growth polycondensation of diacid/diesters and diamines or orω-aminoω-amino carboxylic carboxylic acids/esters; acids/esters; (2) ring-opening (2) ring-opening poly polymerizationmerization of lactams; of lactams; and (3) and a hybrid (3) a hybrid of step- of step-growthgrowth polycondensation polycondensation and ring-opening and ring-opening polymerization. polymerization. Two basicbasic modesmodes ofof elementalelemental reactionsreactions areare commonlycommonly usedused inin thethe biocatalyticbiocatalytic polyamidepolyamide synthesis: directly amidation andand transamidationtransamidation (aminolysis)(aminolysis) (Scheme(Scheme 1717).).

Scheme 17. Basic modes of elemental lipase-catalyzed reactions in biocatalytic polyamide synthesis. PolymersScheme 2016, 8, 17. 243Basic modes of elemental lipase-catalyzed reactions in biocatalytic polyamide synthesis.19 of 52

The lipase-catalyzed polymerization of polyamides has not been well studied [[208].208]. This could be attributed mainly to twotwo reasons:reasons: (1) the high TTm of polyamides, and (2) the poor solubility of polyamides in common organic solvents. On the one hand, polyamides like nylons and TPA-based ˝ polyamides are semi-crystallinesemi-crystalline polymers whichwhich normallynormally possesspossess aa highhigh TTmm above 100 °C.C. At such elevated temperatures,temperatures, the the catalytic catalytic reactivity reactivity of lipases of lipases is significantly is significantly decreased decreased due to the due occurrence to the ofoccurrence protein denaturation of protein denaturation and deactivation. and deactivati On the otheron. hand,On the many other polyamides hand, many can polyamides be only dissolved can be inonly some dissolved aggressive in solvents some suchaggressive as formic solven acid,ts concentrated such as formic H2SO 4,acid, and trifluoroaceticconcentrated acid,H2SO in4 which, and lipasestrifluoroacetic cannot act.acid, in which lipases cannot act. Nevertheless, some some oligoamides oligoamides and and polyamides polyamides are successfully are successfully produced produced via the via lipase- the lipase-catalyzedcatalyzed polymerization polymerization [206–208]. [206 –Some208]. typical Some typical examples examples are given are below. given below. Cheng et al. [230,231] [230,231] investigated investigated the lipase-catalyzed polymerization of diamines and diesters in bulk (Scheme 1818),), whichwhich resultedresulted inin aliphaticaliphatic polyamidespolyamides withwithM w’s’s ofof aroundaround 3000–15,000 3000–15,000 g/mol.g/mol. This is is the the first first report report showing showing that that high high molecular molecular weight weight polyamides polyamides can be can produced be produced from lipase- from lipase-catalyzedcatalyzed polymerization. polymerization.

Scheme 18. Lipase-catalyzed synthesis of aliphatic polyamides.

The CALB-catalyzed CALB-catalyzed ring-ope ring-openingning polymerization polymerization of ε-caprolactam of ε-caprolactam was reported was by reported Kong et byal. Kong[232]. etThey al. [claimed232]. They that claimed the enzymatic that the ring-ope enzymaticning ring-opening polymerization polymerization gave nylon 6 gave with nylon a high 6 with of a 212,000 g/mol. high Mw of 212,000 g/mol. Aliphatic polyamides such as nylon 6,13, nylon 8,13 and nylon 12,13 were synthesized via the N435-catalyzed ring-opening addition-condensation (Scheme 19) [197]. The ’s of the resulting N435-catalyzed ring-opening addition-condensation (Scheme 19)[197]. The Mn’s of the resulting nylons were around 5600–83005600–8300 g/mol.g/mol.

Scheme 19. N435-catalyzed ring-opening addition-condensation of ethylene tridecanedioate with various diamines.

In our group, enzymatic polymerization of polyamides is one of the focused research area. For example, the enzymatic polymerization of 2-azetidinone was first studied in our laboratory (Scheme 20) [233]. A different mechanism for the enzymatic ring-opening polymerization of β-propiolactam was revealed and a catalytic cycle for the oligomerization of β-lactam that rationalizes the activation of the monomers was proposed [234]. Moreover, aliphatic oligoamides [233,235,236], semi-aromatic oligoamides [237], and poly(ester amide)s [238] are successfully prepared via lipase-catalyzed polymerization in our laboratory.

Scheme 20. Enzymatic ring-opening polymerization of 2-azetidinone.

PolymersPolymers 20162016,, 88,, 243243 1919 ofof 5252

TheThe lipase-catalyzedlipase-catalyzed polymerizationpolymerization ofof polyamidespolyamides hashas notnot beenbeen wellwell studiedstudied [208].[208]. ThisThis couldcould m bebe attributedattributed mainlymainly toto twotwo reasons:reasons: (1)(1) thethe highhigh TTm ofof polyamides,polyamides, andand (2)(2) thethe poorpoor solubilitysolubility ofof polyamidespolyamides inin commoncommon organicorganic solvents.solvents. OnOn thethe oneone hand,hand, polyamidespolyamides likelike nylonsnylons andand TPA-basedTPA-based m polyamidespolyamides areare semi-crystallinesemi-crystalline polymerspolymers whichwhich normallynormally possesspossess aa highhigh TTm aboveabove 100100 °C.°C. AtAt suchsuch elevatedelevated temperatures,temperatures, thethe catalyticcatalytic reactivityreactivity ofof lipaseslipases isis significantlysignificantly decreaseddecreased duedue toto thethe occurrenceoccurrence ofof proteinprotein denaturationdenaturation andand deactivatideactivation.on. OnOn thethe otherother hand,hand, manymany polyamidespolyamides cancan bebe 2 4 onlyonly dissolveddissolved inin somesome aggressiveaggressive solvensolventsts suchsuch asas formicformic acid,acid, concentratedconcentrated HH2SOSO4,, andand trifluoroacetictrifluoroacetic acid,acid, inin whichwhich lipaseslipases cannotcannot act.act. Nevertheless,Nevertheless, somesome oligoamidesoligoamides andand polyamidespolyamides areare successfullysuccessfully producedproduced viavia thethe lipase-lipase- catalyzedcatalyzed polymerizationpolymerization [206–208].[206–208]. SomeSome typicaltypical examplesexamples areare givengiven below.below. ChengCheng etet al.al. [230,231][230,231] investigatedinvestigated thethe lipase-catalyzedlipase-catalyzed polymerizationpolymerization ofof diaminesdiamines andand diestersdiesters in bulk (Scheme 18), which resulted in aliphatic polyamides with ’s of around 3000–15,000 g/mol. in bulk (Scheme 18), which resulted in aliphatic polyamides with ’s of around 3000–15,000 g/mol. ThisThis isis thethe firstfirst reportreport showingshowing thatthat highhigh molecularmolecular weightweight polyamidespolyamides cancan bebe producedproduced fromfrom lipase-lipase- catalyzedcatalyzed polymerization.polymerization.

SchemeScheme 18.18. Lipase-catalyzedLipase-catalyzed synthesissynthesis ofof aliphaticaliphatic polyamides.polyamides.

TheThe CALB-catalyzedCALB-catalyzed ring-opering-openingning polymerizationpolymerization ofof εε-caprolactam-caprolactam waswas reportedreported byby KongKong etet al.al. [232]. They claimed that the enzymatic ring-opening polymerization gave nylon 6 with a high of [232]. They claimed that the enzymatic ring-opening polymerization gave nylon 6 with a high of 212,000212,000 g/mol.g/mol. AliphaticAliphatic polyamidespolyamides suchsuch asas nylonnylon 6,13,6,13, nylonnylon 8,138,13 andand nylonnylon 12,1312,13 werewere synthesizedsynthesized viavia thethe N435-catalyzed ring-opening addition-condensation (Scheme 19) [197]. The ’s of the resulting PolymersN435-catalyzed2016, 8, 243 ring-opening addition-condensation (Scheme 19) [197]. The ’s of the resulting20 of 53 nylonsnylons werewere aroundaround 5600–83005600–8300 g/mol.g/mol.

SchemeScheme 19.19. N435-catalyzedN435-catalyzed ring-opening ring-opening addition-conde addition-condensationaddition-condensationnsation of of ethylene ethylene tridecanedioatetridecanedioate withwith variousvarious diamines.diamines.

In our group, enzymatic polymerization of polyamides is one of the focused research area. For In our our group, group, enzymatic enzymatic polymeri polymerizationzation of ofpolyamides polyamides is one is one of the of thefocused focused research research area. area. For example, the enzymatic polymerization of 2-azetidinone was first studied in our laboratory (Scheme example,For example, the enzymatic the enzymatic polymerization polymerization of 2-azetidinone of 2-azetidinone was first was studied ﬁrst studiedin our laboratory in our laboratory (Scheme 20) [233]. A different mechanism for the enzymatic ring-opening polymerization of β-propiolactam (Scheme20) [233]. 20A )[different233]. Amechanism different for mechanism the enzymatic for the ring-opening enzymatic polymerization ring-opening polymerizationof β-propiolactam of was revealed and a catalytic cycle for the oligomerization of β-lactam that rationalizes the activation wasβ-propiolactam revealed and was a catalytic revealed cycle and afor catalytic the oligomerization cycle for the oligomerization of β-lactam that of rationalizesβ-lactam that the rationalizes activation of the monomers was proposed [234]. Moreover, aliphatic oligoamides [233,235,236], semi-aromatic ofthe the activation monomers of thewas monomers proposed was[234]. proposed Moreover, [234 aliphatic]. Moreover, oligoami aliphaticdes [233,235,236], oligoamides semi-aromatic [233,235,236], oligoamides [237], and poly(ester amide)s [238] are successfully prepared via lipase-catalyzed oligoamidessemi-aromatic [237], oligoamides and poly(ester [237], andamide)s poly(ester [238] are amide)s successfully [238] areprepared successfully via lipase-catalyzed prepared via polymerization in our laboratory. polymerizationlipase-catalyzed in polymerization our laboratory. in our laboratory.

Scheme 20. Enzymatic ring-opening polymerization of 2-azetidinone. Polymers 2016, 8, 243 Scheme 20. Enzymatic ring-opening polymerization of 2-azetidinone. 20 of 52

8. Lipase-Catalyzed Synthesis of Sustainable Polyesters and Polyamides from Biobased 8. Lipase-Catalyzed Synthesis of Sustainable Polyesters and Polyamides from Biobased MonomersMonomers At present,present, mostmost research on enzymatic polymerizationpolymerization still focused on the use of “traditional” monomers derivedderived fromfrom fossilfossil resources.resources. Due Due to to the growing awareness of energy safetysafety andand environmentalenvironmental pollution, and increased interest for the development of novel polymeric materials, utilization of biobasedbiobased monomersmonomers inin enzymaticenzymatic polymerizationpolymerization becomes an appealing topic both inin thethe academic and industrial ﬁelds.fields. Currently, manymany (potentially) biobasedbiobased polyesters and polyamides are readilyreadily synthesizedsynthesized viavia enzymaticenzymatic polymerization.polymerization. InIn this section, the recent developments inin the ﬁeldfield ofof thethe lipase-catalyzed synthesis of biobased polyesters and synthetic polyamides are discussed inin details.details

8.1. Biobased Saturated Aliphatic Polyesters

8.1.1. Poly(lactic acid) PLA isis commonlycommonly producedproduced byby ring-openingring-opening polymerizationpolymerization of lactides or by directdirect polycondensationpolycondensation of lacticlactic acid,acid, usingusing chemicalchemical catalysts.catalysts. It is also possible to synthesizesynthesize PLA viavia lipase-catalyzedlipase-catalyzed ring-openingring-opening polymerizationpolymerization (Scheme(Scheme 2121).).

SchemeScheme 21.21.L L,,LL-,-, D,D- and D,,LL-lactide,-lactide, and and lipase-catalyzed lipase-catalyzed ring-o ring-openingpening polymerization polymerization of of lactides. lactides.

Although the resulting PLLA possessed a high (12,900 g/mol) and a good dispersity (around 1.2), the reaction yield was quite low, less than 12%. A new biocatalytic approach was developed for the efficient synthesis of high molecular weight PLLA and PDLA, starting from an O-carboxylic anhydride derived from lactic acid (L- or D-lacOCA)

(Scheme 22) [244]. The , dispersity and reaction yield of the resulting PLLA and PDLA were up to 38,400 g/mol, ≤1.4, and around 90%, respectively. In addition, the tested lipases showed slight preference to L-lacOCA over D-lacOCA. Moreover, the molecular weights of the obtained PLLA can be controllable by altering the concentration of N435 in the reaction media.

Polymers 2016, 8, 243 21 of 53

However, the direct enzymatic ring-opening polymerization of lactides generally resulted in PLA with low molecular weights or low reaction yields, indicating that the enzymatic polymerization efﬁciency was quite low. It was also found that the enzymatic polymerization of D,L-lactide resulted in higher molecular weight products compared to D,D- and L,L-lactide [239–241]. Nevertheless, after careful adjusting the reaction conditions, high molecular weight poly(D,D-lactide) (PDLA) can be synthesized from the N435-catalyzed ring-opening polymerization, with a Mn, dispersity and conversion of 12,000 g/mol, 1.1 and 60%, respectively [242]. In addition, poly(L,L-lactide) (PLLA) can be produced from the N435-catalyzed ring-opening polymerization in supercritical CO2 [243]. Although the resulting PLLA possessed a high Mw (12,900 g/mol) and a good dispersity (around 1.2), the reaction yield was quite low, less than 12%. A new biocatalytic approach was developed for the efﬁcient synthesis of high molecular weight PLLA and PDLA, starting from an O-carboxylic anhydride derived from lactic acid (L- or D-lacOCA) (Scheme 22)[244]. The Mn, dispersity and reaction yield of the resulting PLLA and PDLA were up to 38,400 g/mol, ď1.4, and around 90%, respectively. In addition, the tested lipases showed slight preference to L-lacOCA over D-lacOCA. Moreover, the molecular weights of the obtained PLLA can be controllablePolymers by altering2016, 8, 243 the concentration of N435 in the reaction media. 21 of 52

Polymers 2016, 8, 243 21 of 52

Scheme 22. Immobilized lipase Amano PS-C II from Pseudomonas sp. (lipase PS) or N435-catalyzed Scheme 22. Immobilized lipase Amano PS-C II from Pseudomonas sp. (lipase PS) or N435-catalyzed ring-opening polymerization of an O-carboxylic anhydride derived from lactic acid (L- or D-lacOCA) ring-opening[244]. polymerization of an O-carboxylic anhydride derived from lactic acid (L- or D-lacOCA)Scheme [244]. 22. Immobilized lipase Amano PS-C II from Pseudomonas sp. (lipase PS) or N435-catalyzed Byring-opening employing polymerization alcohol initiators of an O -carboxyliccontaining anhydride different derived hydroxyl from numbers lactic acid (2, (L- or4, D6,-lacOCA) 8, and 22), various[244]. linear and branched PLAs were synthesized via the Pseudomonas fluoresaens lipase-catalyzed By employingring-opening alcoholpolymerization initiators of containingL,L-, D,D-, and different D,L-lactide hydroxyl in bulk numbers[245]. The (2,enzymatic 4, 6, 8, and 22), By employing alcohol initiators containing different hydroxyl numbers (2, 4, 6, 8, and 22), various linearpolymerization and branched yielded PLAsPLAs with were synthesized’s and dispersities via of the aroundPseudomonas 1500–36,700 ﬂuoresaens g/mol, and lipase-catalyzed1.0–1.5, various linear and branched PLAs were synthesized via the Pseudomonas fluoresaens lipase-catalyzed ring-openingrespectively. polymerization of L,L-, D,D-, and D,L-lactide in bulk [245]. The enzymatic polymerization ring-openingMoreover, polymerization many PLA co -polyestersof L,L-, Dwere,D-, andsuccessfully D,L-lactide synthesized in bulk via[245]. lipase-catalyzed The enzymatic co- yielded PLAs with Mn’s and dispersities of around 1500–36,700 g/mol, and 1.0–1.5, respectively. polymerization,polymerization yieldedincluding PLAs poly(lactide- with ’sco and-trimethylene dispersities carbonate) of around [246],1500–36,700 poly(lactide- g/mol,co and-glycolide) 1.0–1.5, respectively. co Moreover,[247], and poly(lactide- manyco PLA-alkylene dicarboxylate)-polyesters [248] (Scheme were 23). successfully The corresponding synthesized‘s were via lipase-catalyzedaroundMoreover, 12,000–21,000,co-polymerization, many 2200–20,600,PLA co-polyesters and including 10,000–38,000 were successfully poly(lactide- g/mol, respectively.synthesizedco-trimethylene Besides,via lipase-catalyzed the obtained carbonate) co-co- [246], polyesterspolymerization, can be includingoptically active,poly(lactide- due to cothe-trimethylene retention of thecarbonate) chiral configuration [246], poly(lactide- of lactateco -glycolide)units after poly(lactide-co-glycolide) [247], and poly(lactide-co-alkylene dicarboxylate) [248 ] (Scheme 23). the[247], enzymatic and poly(lactide- polymerizationco-alkylene [248]. dicarboxylate) [248] (Scheme 23). The corresponding ‘s were The correspondingaround 12,000–21,000,Mw‘s were2200–20,600, around and 10,000–38,000 12,000–21,000, g/mol, 2200–20,600, respectively. Besides, and the 10,000–38,000 obtained co- g/mol, respectively.polyesters Besides, can be the optically obtained active,co- duepolyesters to the retentio cann of be the optically chiral configuration active, due of lactate to the units retention after of the chiral conﬁgurationthe enzymatic of polymerization lactate units [248]. after the enzymatic polymerization [248].

Scheme 23. Enzymatic co-polymerization with lactides [246–248].

8.1.2. Poly(butylene succinate) Scheme 23. Enzymatic co-polymerization with lactides [246–248]. PBS is normallyScheme synthesized 23. Enzymatic via polycondensationco-polymerization of succinic with lactides acid or [succinic246–248 anhydride]. with 1,4-BDO8.1.2. Poly(butylene at elevated succinate)temperatures, using a chemical catalyst [249]. It is also promising to synthesize biobased PBS via enzymatic polymerization. ThePBS lipase-catalyzedis normally synthesized polycondensation via polycondensation of PBS was ofstudied succinic by acidGross or etsuccinic al. [250], anhydride using a two-with stage1,4-BDO method at elevated which temperatures, is similar to usingthose a used chemical for thecatalyst industrial [249]. Itproduction is also promising but at tomuch synthesize lower temperaturesbiobased PBS via(Scheme enzymatic 24). The polymerization. solvent-free enzymatic polycondensation with succinic acid gave oligomers.The lipase-catalyzed However, by replacing polycondensation succinic acid of PBSwith was diethyl studied succinate, by Gross the temperatureet al. [250], using varied a two-two- stage method which is similar to those used for the industrial production but at much lower stage method in diphenyl ether resulted in PBS with a of 38,000 g/mol and a dispersity of 1.39. temperatures (Scheme 24). The solvent-free enzymatic polycondensation with succinic acid gave oligomers. However, by replacing succinic acid with diethyl succinate, the temperature varied two-

stage method in diphenyl ether resulted in PBS with a of 38,000 g/mol and a dispersity of 1.39.

Polymers 2016, 8, 243 22 of 53

8.1.2. Poly(butylene succinate) PBS is normally synthesized via polycondensation of succinic acid or succinic anhydride with 1,4-BDO at elevated temperatures, using a chemical catalyst [249]. It is also promising to synthesize biobased PBS via enzymatic polymerization. The lipase-catalyzed polycondensation of PBS was studied by Gross et al. [250], using a two-stage method which is similar to those used for the industrial production but at much lower temperatures (Scheme 24). The solvent-free enzymatic polycondensation with succinic acid gave oligomers. However, by replacing succinic acid with diethyl succinate, the temperature varied two-stage method in diphenylPolymers 2016 ether, 8, 243 resulted in PBS with a Mw of 38,000 g/mol and a dispersity of 1.39. 22 of 52

PolymersPolymers 20162016,, 88,, 243243 2222 ofof 5252

SchemeScheme 24. Enzymatic 24. Enzymatic polycondensation polycondensation of succinicof succinic acid acid or or diethyl diethyl succinate succinate and and 1,4-butanediol, 1,4-butanediol, using using a two-stage method [250]. a two-stage method [250]. SchemeScheme 24.24. EnzymaticEnzymatic polycondensationpolycondensation ofof succinicsuccinic acacidid oror diethyldiethyl succinatesuccinate andand 1,4-butanediol,1,4-butanediol, To usingsynthesizeusing aa two-stagetwo-stage PBS methodmethod with [250].[250].higher molecular weights, another two enzymatic strategies were Todeveloped: synthesize (1) using PBS cyclic with oligomers higher molecular [251]; and (2) weights, co-polymerization anothertwo of succinic enzymatic acid and strategies 1,4-BDO were To synthesize PBS with higher molecular weights, another two enzymatic strategies were developed:with succinate (1)To usingsynthesize anhydride cyclic PBS oligomers [252].with Byhigher using [251 molecular ];cyclic and butylene (2)weigcohts,-polymerization succinateanother two oligomers enzymatic of succinic in the strategies N435-catalyzed acid andwere 1,4-BDO developed:developed: (1)(1) usingusing cycliccyclic oligomersoligomers [251];[251]; andand (2)(2) coco-polymerization-polymerization ofof succinicsuccinic acidacid andand 1,4-BDO1,4-BDO withpolymerization, succinate anhydride PBS with [252 a]. By usingof up cyclicto 130,000 butylene g/mol succinate and a dispersity oligomers of in1.6 the was N435-catalyzed obtained. withwith succinatesuccinate anhydrideanhydride [252].[252]. ByBy usingusing cycliccyclic butylenebutylene succinatesuccinate oligomersoligomers inin thethe N435-catalyzedN435-catalyzed However, under similar reaction conditions, the direct enzymatic polycondensation gave PBS with a polymerization,polymerization,polymerization, PBS with PBSPBS a withwithMw ofaa up toofof 130,000upup toto 130,000130,000 g/mol g/molg/mol and and aand dispersity aa dispersitydispersity of 1.6ofof 1.61.6 was waswas obtained. obtained.obtained. However, lower (45,000 g/mol) and a broader dispersity (3.7) (Scheme 25). On the other hand, the under similarHowever,However, reaction underunder similarsimilar conditions, reactionreaction the conditions,conditions, direct thethe enzymatic didirectrect enzymaticenzymatic polycondensation polycondensationpolycondensation gave gavegave PBS PBSPBS withwith with aa a lower enzymaticlower co-polymerization (45,000 g/mol) ofand succinic a broader acid dispersity and 1,4-BDO (3.7) with (Scheme succinate 25). Onanhydride the other resulted hand, thein PBS M lower (45,000 g/mol) and a broader dispersity (3.7) (Scheme 25). On the other hand, the w (45,000 g/mol) and a broader dispersity (3.7) (Scheme 25). On the other hand, the enzymatic withenzymaticenzymatic a of coco73,000-polymerization-polymerization g/mol and ofof a succinic succinicdispersity acidacid of andand 1.7 1,4-BDO1,4-BDO (Scheme withwith 26). succinatesuccinate However, anhydrideanhydride although resultedresulted high molecularinin PBSPBS co-polymerization of succinic acid and 1,4-BDO with succinate anhydride resulted in PBS with a Mw weightwithwith PBS aa can of of be73,00073,000 enzymatically g/molg/mol andand aa produceddispersitydispersity ofofvia 1.71.7 th (Scheme(Schemeese two 26).26). approaches, However,However, althoughalthoughan extra high highsynthesis molecularmolecular step is of 73,000 g/mol and a dispersity of 1.7 (Scheme 26). However, although high molecular weight PBS required.weightweight PBSPBS cancan bebe enzymaticallyenzymatically producedproduced viavia ththeseese twotwo approaches,approaches, anan exextratra synthesissynthesis stepstep isis can be enzymaticallyrequired.required. produced via these two approaches, an extra synthesis step is required.

SchemeSchemeScheme 25.Scheme N435-catalyzed25. 25.25.N435-catalyzed N435-catalyzedN435-catalyzed synthesis synthesis synthesissynthesis of of ofof poly(butylene poly(butylene poly(butylenepoly(butylene succinate)succinate)succinate) succinate) byby by cyclizationcyclization by cyclization cyclization withwith with subsequentsubsequent with subsequent subsequent ring-openingring-openingring-openingring-opening polymerization polymerization polymerizationpolymerization of of the ofof the thethe cyclic cyclic cycliccyclic oligomers oligomers oligomersoligomers [251].[251]. [[251].251].

SchemeScheme 26.SchemeSynthesis 26.26. SynthesisSynthesis of poly(butylene ofof poly(butylenepoly(butylene succinate) succinate)succinate) viavia N435-catalyzedN435-catalyzed coco-polymerization-polymerizationco-polymerization ofof succinicsuccinic of succinic acid and 1,4-butanediol with succinate anhydride [252]. acid andSchemeacid 1,4-butanediol and26. 1,4-butanediolSynthesis with of poly(butylene with succinate succinate anhydride anhydridesuccinate) [[252]. 252via ].N435-catalyzed co-polymerization of succinic acid and 1,4-butanediol with succinate anhydride [252]. 8.1.3.8.1.3. OtherOther BiobasedBiobased AliphaticAliphatic PolyestersPolyesters 8.1.3. OtherManyMany Biobased otherother (potential) (potential)Aliphatic Polyestersbiobasedbiobased aliphaticaliphatic polyesterspolyesters areare synthesizedsynthesized viavia lipase-catalyzedlipase-catalyzed polycondensation.polycondensation. SomeSome examplesexamples areare discusseddiscussed asas follows.follows. Many other (potential) biobased aliphatic polyesters are synthesized via lipase-catalyzed TheThe lipase-catalyzedlipase-catalyzed solvent-freesolvent-free polycondensationpolycondensation ofof aliphaticaliphatic diacidsdiacids (C2–C12)(C2–C12) andand aliphaticaliphatic polycondensation. Some examples are discussed as follows. diolsdiols (C2–C12)(C2–C12) waswas performedperformed byby KobayashiKobayashi etet al.al. [253][253] TheThe enzymaticenzymatic polymerizationpolymerization yieldedyielded The lipase-catalyzed solvent-free polycondensation of aliphatic diacids (C2–C12) and aliphatic diols (C2–C12) was performed by Kobayashi et al. [253] The enzymatic polymerization yielded

Polymers 2016, 8, 243 23 of 53

8.1.3. Other Biobased Aliphatic Polyesters Many other (potential) biobased aliphatic polyesters are synthesized via lipase-catalyzed polycondensation.Polymers 2016, 8, 243 Some examples are discussed as follows. 23 of 52 PolymersThe 2016 lipase-catalyzed, 8, 243 solvent-free polycondensation of aliphatic diacids (C2–C12) and aliphatic23 of 52

diolsvarious (C2–C12) aliphatic was polyesters performed with by Kobayashi ’s and etdispersities al. [253] The of enzymatic around 1300–14,000 polymerization g/mol, yielded and various1.1–2.3, variousaliphaticrespectively. aliphatic polyesters polyesters with Mn ’swith and dispersities’s and dispersities of around 1300–14,000of around 1300–14,000 g/mol, and g/mol, 1.1–2.3, and respectively. 1.1–2.3, respectively.Biodegradable coco-polyesters-polyesters containing containing malic malic acid acid units un wereits synthesizedwere synthesized via the N435-catalyzedvia the N435- polycondensationcatalyzedBiodegradable polycondensation ofco-polyesters adipic of acidadipic containing and acid 1,8-octanediol and malic 1,8-octanediol acid with units withL -malicwere L-malic synthesized acid, acid, a a natural natural via the occurringoccurring N435- catalyzedmonomer polycondensation (Scheme 2727))[ [254].254]. of The adipic solvent-free acid and enzymatic 1,8-octanediol polycondensation with L-malic gaveacid, poly(octamethylenea natural occurring monomeradipate-co (Scheme-malate), 27) with with [254]. Mw ’s,The’s, dispersities dispersities solvent-free and and enzymatic reaction reaction yieldspolycondensation yields of of4700–9500 4700–9500 gave g/mol, g/mol, poly(octamethylene 1.50–1.92, 1.50–1.92, and and88– adipate-88–96%,96%, respectively.co respectively.-malate), with ’s, dispersities and reaction yields of 4700–9500 g/mol, 1.50–1.92, and 88– 96%, respectively.

Scheme 27. Enzymatic synthesis of poly(octamethylene adipate-co-malate) using N435 as the SchemeSchemebiocatalyst 27. [254]. EnzymaticEnzymatic synthesis synthesis of of poly(octamethylene poly(octamethylene adipate- adipate-coco-malate)-malate) using using N435 N435 as as the the biocatalystbiocatalyst [254]. [254]. The lipase-catalyzed polymerization of aliphatic diacid ethyl esters (C2, C4 and C8) and diols The lipase-catalyzed polymerization of aliphatic diacid ethyl esters (C2, C4 and C8) and diols (C4, TheC6 and lipase-catalyzed C8) were perfor polymerizationmed in bulk of aliphaticor by using diacid β-cyclodextrin ethyl esters (C2, as the C4 andsupport C8) andarchitecture. diols (C4, (C4, C6 and C8) were performed in bulk or by using β-cyclodextrin as the support architecture. C6Various and C8)saturated were performed aliphatic poly in bulkesters or were by using produced,β-cyclodextrin with ’s as ranging the support from architecture. 5300 to 44,600 Various g/mol Various[255,256]. saturated aliphatic polyesters were produced, with ’s ranging from 5300 to 44,600 g/mol saturated aliphatic polyesters were produced, with Mw’s ranging from 5300 to 44,600 g/mol [255,256]. [255,256]. Recently, we succeeded in preparingpreparing aa seriesseries ofof (potentially)(potentially) biobasedbiobased poly(butylenepoly(butylene Recently, we succeeded in preparing a series of (potentially) biobased poly(butylene dicarboxylate)s via the N435-catalyzedN435-catalyzed polycondensation of 1,4-butanediol and diacid ethyl estersesters dicarboxylate)s via the N435-catalyzed polycondensation of 1,4-butanediol and diacid ethyl esters differing in chainchain lengthlength (C2,(C2, C3,C3, C4,C4, C6,C6, C8C8 andand C10)C10) (Scheme(Scheme 2828))[ [257].257]. HighHigh molecularmolecular weightweight differingpoly(butylene in chain dicarboxylate)s length (C2, C3, were C4, obtained, C6, C8 and with C10) (Scheme’s of up 28)to [257].94,000 High g/mol. molecular We found weight that poly(butylene dicarboxylate)s were obtained, with Mw’s of up to 94,000 g/mol. We found that poly(butylene dicarboxylate)s were obtained, with ’s of up to 94,000 g/mol. We found that increasingincreasing thethe chain chain length length of diacid of diacid ethyl esterethyl from ester C2 tofrom C4 resultedC2 to inC4 poly(butylene resulted in dicarboxylate)spoly(butylene increasing the chain length of diacid ethyl ester from C2 to C4 resulted in poly(butylene ofdicarboxylate)s signiﬁcant higher of significant molecular higher weights; molecular however, weig uponhts; furtherhowever, increasing upon furt theher chain increasing length the from chain C4 dicarboxylate)s of significant higher molecular weights; however, upon further increasing the chain tolength C10, from poly(butylene C4 to C10, dicarboxylate)s poly(butylene withdicarboxylate)s lower molecular with lower weights molecular were obtained. weights Meanwhile, were obtained. the length from C4 to C10, poly(butylene dicarboxylate)s with lower molecular weights were obtained. enzymaticMeanwhile, polymerization the enzymatic withpolymerization diethyl succinate with diet (C2)hyl gave succinate the lowest (C2) moleculargave the lowest weight molecular products. Meanwhile, the enzymatic polymerization with diethyl succinate (C2) gave the lowest molecular Thisweight suggested products. that This CALB suggested possesses that a CALB higher possesses selectivity a higher towards selectivity diacid ethyl towards esters diacid with aethyl >C2 esters chain weight products. This suggested that CALB possesses a higher selectivity towards diacid ethyl esters length;with a >C2 and chain CALB length; prefers and diethyl CALB adipate prefers (C4) diethyl over adipate the other (C4) tested over counterparts. the other tested counterparts. with a >C2 chain length; and CALB prefers diethyl adipate (C4) over the other tested counterparts.

Scheme 28.28. EnzymaticEnzymatic synthesis synthesis of of poly(butylen poly(butylenee dicarboxylate)s dicarboxylate)s via N435-catalyzedN435-catalyzed Schemepolycondensation 28. Enzymatic ofof 1,4-butanediol1,4-butanedi synthesisol andand diacidofdiacid poly(butylen ethylethyl estersesterse differingdifferingdicarboxylate)s inin chainchain lengthlengthvia [[257].257N435-catalyzed]. polycondensation of 1,4-butanediol and diacid ethyl esters differing in chain length [257]. Moreover, biobasedbiobased telechelictelechelic polyesters polyesters were were produced produced via via N435-catalyzed N435-catalyzed polycondensation polycondensation of Moreover, biobased telechelic polyesters were produced via N435-catalyzed polycondensation azelaicof azelaic acid acid and and 1,6-HDO 1,6-HDO with with different different functional functional end-cappers end-cappers in in supercritical supercritical COCO2 [[258].258]. The Mn,, 2 ofdispersity azelaic acid and and reaction 1,6-HDO yield with of thedifferent resulting functional telechelic end-cappers poly(hexamethylene in supercritical azelate)s CO2 [258]. were The around , dispersity1500–2400 andg/mol, reaction 1.73–2.18, yield andof the 78%–88%, resulting respective telechelicly. poly(hexamethylene The obtained telechelic azelate)s polyesters were around can be 1500–2400further modified g/mol, by1.73–2.18, cross-linking and 78%–88%, or by chain respective extensionly. reactions. The obtained telechelic polyesters can be further modified by cross-linking or by chain extension reactions. 8.2. Biobased Unsaturated Aliphatic Polyesters 8.2. Biobased Unsaturated Aliphatic Polyesters

Polymers 2016, 8, 243 24 of 53 dispersity and reaction yield of the resulting telechelic poly(hexamethylene azelate)s were around 1500–2400 g/mol, 1.73–2.18, and 78%–88%, respectively. The obtained telechelic polyesters can be further modiﬁed by cross-linking or by chain extension reactions.

Polymers 2016, 8, 243 24 of 52 8.2. Biobased Unsaturated Aliphatic Polyesters Currently, the synthesissynthesis ofof biobasedbiobased unsaturatedunsaturated polyesters,polyesters, especiallyespecially itaconate-baseditaconate-based unsaturated polyesters, has has not not been been well well studied. studied. This This is isbecause because the the sensitive sensitive C=C C=C bond bond can can be bedeteriorated deteriorated easily easily under under conventional conventional polymerization polymerization conditions conditions such such as elevated as elevated temperatures temperatures and andmetal metal catalysts. catalysts. However, However, this problem this problem can be can easily be easily overcome overcome by using by using enzyme enzyme catalysts catalysts in the in thepolymerization, polymerization, due dueto the to mild the mildsynthetic synthetic conditions conditions and the and high the catalytic high catalytic specificity speciﬁcity of the enzyme of the enzymecatalysts. catalysts. However, the lipase-catalyzedlipase-catalyzed direct polycondensationpolycondensation of itaconate and aliphatic diols with short chainchain lengthlength generallygenerally resultedresulted inin oligomers.oligomers. As reported byby GardossiGardossi etet al.,al., thethe solvent-freesolvent-free polyesteriﬁcationpolyesterification ofof dimethyldimethyl itaconateitaconate andand 1,4-BDO1,4-BDO catalyzedcatalyzed byby CALB gave a mixture of oligomers from dimerdimer to to pentamer pentamer [259 ,[259,260].260]. Similarly, Similarly, the N435-catalyzed the N435-catalyzed polymerization polymerization of itaconic of anhydride itaconic withanhydride aliphatic with diols aliphatic (C4–C10) diols gave (C4–C10) oligomers gave with oligomersMn’s of around with 150–390 ’s of g/mol,around although 150–390 itaconic g/mol, anhydridealthough itaconic was completely anhydride consumed was completely [261]. This consumed is because [261]. the This enzymatic is because polycondensation the enzymatic is hamperedpolycondensation by the is low hampered reactivity by of the itaconate low reactivity due to of the itaconate lower electrophilicitydue to the lower of electrophilicity the acyl carbon of (Cthes, acyl Scheme carbon 29) adjacent (Cs, Scheme to the 29) vinyl adjacent group [to259 the]. However, vinyl group the low[259]. reactivity However, of itaconate the low inreactivity enzymatic of polymerizationitaconate in enzymatic could bepolymerization overcome by could optimizing be overcome the reaction by optimizing conditions: the (1) reaction improving conditions: the mass (1) transferimproving and the the mass enzyme transfer distribution and the enzyme in the reactiondistributi mixture;on in the (2)reaction increasing mixture; the (2) enzyme increasing loading; the (3)enzyme lowering loading; the diol(3) lowering concentration; the diol and concentration; (4) choosing and more (4) appropriate choosing more diols appropriate [259]. diols [259].

Scheme 29. Enzymatic polymerization of itaconateitaconate derivatives and aliphatic diols.

Indeed, by using glycols with longer chain lengths or with a rigid structure, itaconate-based Indeed, by using glycols with longer chain lengths or with a rigid structure, itaconate-based homo-polyesters with relatively higher molecular weights were obtained from the lipase-catalyzed homo-polyesters with relatively higher molecular weights were obtained from the lipase-catalyzed polycondensation. As reported by Yousaf et al. [262], the N435-catalyzed polymerization of itaconic polycondensation. As reported by Yousaf et al. [262], the N435-catalyzed polymerization of acid with 1,4-cyclohexanedimethanol/poly(ethylene glycol) gave homo-polymers with a of 2600 itaconic acid with 1,4-cyclohexanedimethanol/poly(ethylene glycol) gave homo-polymers with a and 8600 g/mol, respectively. On the contrary, the tin(II) 2-ethylhexanoate-catalyzed Mw of 2600 and 8600 g/mol, respectively. On the contrary, the tin(II) 2-ethylhexanoate-catalyzed polycondensation with itaconic acid gelled within hours. polycondensation with itaconic acid gelled within hours. In addition, itaconate-based co-polyesters with high molecular weights can be prepared via In addition, itaconate-based co-polyesters with high molecular weights can be prepared via lipase-catalyzed co-polymerization, as discussed below. lipase-catalyzed co-polymerization, as discussed below. The N435-catalyzed co-polymerization of itaconic acid, adipic acid and 3-methyl-1,5-pentanediol The N435-catalyzed co-polymerization of itaconic acid, adipic acid and 3-methyl-1,5-pentanediol resulted in a co-polymer with a of 19,000 g/mol [262]. resulted in a co-polymer with a Mw of 19,000 g/mol [262]. Poly(12-hydroxystearate-co-butylene itaconate) with a of 30,000 g/mol was obtained from Poly(12-hydroxystearate-co-butylene itaconate) with a Mw of 30,000 g/mol was obtained from the the lipase-catalyzed directly polycondensation of methyl 12-hydroxystearate, dimethyl itaconate and lipase-catalyzed directly polycondensation of methyl 12-hydroxystearate, dimethyl itaconate and 1,4-BDO (Scheme 30) [263]. Moreover, the lipase-catalyzed ring-opening addition-condensation 1,4-BDO (Scheme 30)[263]. Moreover, the lipase-catalyzed ring-opening addition-condensation polymerization of methyl 12-hydroxystearate and cyclic butylene itaconate dimer resulted in poly(12- polymerization of methyl 12-hydroxystearate and cyclic butylene itaconate dimer resulted in hydroxystearate-co-butylene itaconate) with a significantly higher of 160,000 g/mol. poly(12-hydroxystearate-co-butylene itaconate) with a signiﬁcantly higher Mw of 160,000 g/mol. Furthermore, the NMR study indicated that the enzymatic polymerization catalyzed by different Furthermore, the NMR study indicated that the enzymatic polymerization catalyzed by different lipases lipases yielded poly(12-hydroxystearate-co-butylene itaconate) with different microstructures. As yielded poly(12-hydroxystearate-co-butylene itaconate) with different microstructures. As shown shown in Scheme 30, no ester bond was formed between the hydroxyl group of 12-hydroxystearate in Scheme 30, no ester bond was formed between the hydroxyl group of 12-hydroxystearate and and the carboxyl group of itaconate when the polymerization was catalyzed by N435. However, by the carboxyl group of itaconate when the polymerization was catalyzed by N435. However, by using immobilized Burkholderia cepacia lipase (lipase BC), an ester bond was formed between the 12- hydroxystearate and itaconate unit. Recently, we investigated the N435-catalyzed polymerization of fully biobased poly(butylene succinate-co-itaconate) (PBSI) (Scheme 31) [264,265]. We found that the enzymatic polycondensation

of succinic acid, itaconic acid, and 1,4-butanediol only yielded oligomers, with ’s of around 500– 1500 g/mol, despite different polymerization methods were used. By replacing the unactivated dicarboxylic acids with alkyl diesters, a series of PBSIs with various molar compositions and

Polymers 2016, 8, 243 25 of 52

significant higher molecular weights were obtained, with ’s of up to 28,300 g/mol. In addition, we found that: (1) the most suitable approach is azeotropic polymerization using the solvent mixture of cyclohexane and toluene, which results in PBSIs with high molecular weights and desirable chemical compositions ; (2) high molecular weight PBSIs with <30 mol % of itaconate can be prepared by using the two-stage enzymatic polymerization in diphenyl ether; and (3) the two-stage enzymatic melt polymerization gives PBSIs with controllable chemical compositions but low molecular weights. Moreover, the 13C–NMR study revealed that different microstructures are present in PBSIs obtained Polymersfrom different2016, 8, 243 polymerization methods. The formation of I-B-I-3 microstructures is crucial25 offor 53 synthesizing high molecular weight PBSIs with desired chemical compositions; and more I-B-I-3 usingmicrostructures immobilized can Burkholderiabe produced cepacia by CALBlipase in the (lipase solvent BC), mixture an ester of bondcyclohexane was formed and toluene between under the 12-hydroxystearatean azeotropic condition. and itaconate unit.

Scheme 30. Lipase-catalyzed synthesis of poly(12-hydroxystearate- co-butylene itaconate), using N435 or immobilized Burkholderia cepacia lipaselipase (lipase(lipase BC)BC) asas thethe catalyst.catalyst.

Recently, we investigated the N435-catalyzed polymerization of fully biobased poly(butylene succinate-co-itaconate) (PBSI) (Scheme 31)[264,265]. We found that the enzymatic polycondensation of succinic acid, itaconic acid, and 1,4-butanediol only yielded oligomers, with Mw’s of around 500–1500 g/mol, despite different polymerization methods were used. By replacing the unactivated dicarboxylic acids with alkyl diesters, a series of PBSIs with various molar compositions and signiﬁcant higher molecular weights were obtained, with Mw’s of up to 28,300 g/mol. In addition, we found that: (1) the most suitable approach is azeotropic polymerization using the solvent mixture of cyclohexane and toluene, which results in PBSIs with high molecular weights and desirable chemical compositions; (2) high molecular weight PBSIs with <30 mol % of itaconate can be prepared by using the two-stage enzymatic polymerization in diphenyl ether; and (3) the two-stage enzymatic melt polymerization gives PBSIs with controllable chemical compositions but low molecular weights. Moreover, the 13C–NMR study revealed that different microstructures are present in PBSIs obtained from different polymerization methods. The formation of I-B-I-3 microstructures is crucial for synthesizing high molecular weight PBSIs with desired chemical compositions; and more I-B-I-3 microstructures can be produced by CALB in the solvent mixture of cyclohexane and toluene under an azeotropic condition. However, by replacing diethyl succinate (C2) with the other diacid ethyl esters with relatively longerScheme chain 31. length N435-catalyzed (C3~C10), synthesis the two-stage of poly(butylene enzymatic succinate- polymerizationco-itaconate) in diphenyl and the corresponding ether resulted in seriesmicrostructures. of unsaturated aliphatic polyesters with desired molar compositions and high Mw’s of up to 57,900 g/mol (Scheme 32)[257]. The molar percentage of itaconate in the unsaturated polyesters can be tailored from 0% to 35% by adjusting the feed ratio of itaconate; and all C=C bonds were well preserved in the resulting polyesters. We found that products with relatively lower molecular weights were generally obtained from the enzymatic polymerization at a higher feed ratio of itaconate; however, with diethyl dodecanedioate having the longest chain length (C10) among the tested diacid ethyl esters, higher molecular weight products were obtained at higher feed ratios of itaconate. Moreover, the obtained itaconate-based polyesters can be thermally cross-linked or photo-cured. By adjusting the Polymers 2016, 8, 243 25 of 52 significant higher molecular weights were obtained, with ’s of up to 28,300 g/mol. In addition, we found that: (1) the most suitable approach is azeotropic polymerization using the solvent mixture of cyclohexane and toluene, which results in PBSIs with high molecular weights and desirable chemical compositions ; (2) high molecular weight PBSIs with <30 mol % of itaconate can be prepared by using the two-stage enzymatic polymerization in diphenyl ether; and (3) the two-stage enzymatic melt polymerization gives PBSIs with controllable chemical compositions but low molecular weights. Moreover, the 13C–NMR study revealed that different microstructures are present in PBSIs obtained from different polymerization methods. The formation of I-B-I-3 microstructures is crucial for synthesizing high molecular weight PBSIs with desired chemical compositions; and more I-B-I-3 microstructures can be produced by CALB in the solvent mixture of cyclohexane and toluene under an azeotropic condition.

Polymers 2016, 8, 243 26 of 53

diacidScheme chain 30. length Lipase-catalyzed and itaconate synthesis composition, of poly(12-hydroxystearate- the thermal andco mechanical-butylene itaconate), properties using of N435 the cured polyestersor immobilized can be tuned. Burkholderia cepacia lipase (lipase BC) as the catalyst.

Polymers 2016, 8, 243 26 of 52

However, by replacing diethyl succinate (C2) with the other diacid ethyl esters with relatively longer chain length (C3~C10), the two-stage enzymatic polymerization in diphenyl ether resulted in

series of unsaturated aliphatic polyesters with desired molar compositions and high ’s of up to 57,900 g/mol (Scheme 32) [257]. The molar percentage of itaconate in the unsaturated polyesters can be tailored from 0% to 35% by adjusting the feed ratio of itaconate; and all C=C bonds were well preserved in the resulting polyesters. We found that products with relatively lower molecular weights were generally obtained from the enzymatic polymerization at a higher feed ratio of itaconate; however, with diethyl dodecanedioate having the longest chain length (C10) among the tested diacid ethyl esters, higher molecular weight products were obtained at higher feed ratios of itaconate.SchemeScheme Moreover, 31. 31. N435-catalyzedN435-catalyzed the obtained synthesis itaconate-based synthesis of poly(butylene of poly(butylenepolyesters succinate- canco-itaconate) succinate-be thermallyco and-itaconate) thecross-linked corresponding and or the photo- cured.microstructures.corresponding By adjusting microstructures. the diacid chain length and itaconate composition, the thermal and mechanical properties of the cured polyesters can be tuned.

SchemeScheme 32.32. N435-catalyzed synthesis of poly(butylenepoly(butylene dicarboxylate-dicarboxylate-coco-itaconate)-itaconate) withwith diversediverse chemicalchemical structuresstructures [[257].257].

8.3.8.3. PolyestersPolyesters DerivedDerived fromfrom LongLong ChainChain FattyFatty AcidsAcids andand theirtheir DerivativesDerivatives LongLong chainchain fattyfatty acidsacids maymay containcontain oneone oror moremore C=CC=C bondsbonds withinwithin thethe backbones.backbones. TheThe C=CC=C bondsbonds cancan bebe furtherfurther modiﬁedmodified toto formform otherother functionalfunctional groupsgroups suchsuch asas epoxy,epoxy, thiol,thiol, andand hydroxylhydroxyl groupgroup [[153,266],153,266], renderingrendering fattyfatty acid-basedacid-based polyesterspolyesters withwith diversediverse functionalities.functionalities. FattyFatty acid-basedacid-based polyesterspolyesters cancan bebe usedused asas thermosetthermoset resins,resins, coatingcoating materialsmaterials andand biomaterialsbiomaterials forfor biomedicalbiomedical applications,applications, andand soso onon [[267–269].267–269]. TheThe pioneerpioneer workwork onon thethe enzymaticenzymatic polymerizationpolymerization withwith longlong chainchain fattyfatty acidsacids waswas reportedreported byby MatsumuraMatsumura et et al. [al.270 [270].]. They They investigated investigated the lipase-catalyzed the lipase-catalyzed polymerization polymerization of ricinoleic of acid/methyl ricinoleic ricinoleateacid/methyl in ricinoleate bulk (Scheme in bulk 33). (Scheme Among the33). testedAmong lipases, the tested immobilized lipases, immobilized lipase PC showed lipase thePC showed highest reactivitythe highest towards reactivity ricinoleic towards acid ricinoleic and methyl acid ricinoleate. and methyl The ricinoleate. enzymatic The polymerization enzymatic polymerization with ricinoleic with ricinoleic acid resulted in polyricinoleate with a of up to 8500 g/mol. However, by replacing acid resulted in polyricinoleate with a Mw of up to 8500 g/mol. However, by replacing ricinoleic acid ricinoleic acid with methyl ricinoleate, polyricinoleate with a much higher of up to 100,600 g/mol with methyl ricinoleate, polyricinoleate with a much higher Mw of up to 100,600 g/mol were produced. wereLater, produced. the N435-catalyzed synthesis of poly(12-hydroxydodecanoate-co-12-hydroxystearate) was studied by the same research group (Scheme 34)[271]. The Mw, dispersity and reaction yield of the resulting co-polyesters were around 92,300–118,200 g/mol, 2.8–3.3, and 83%–88%, respectively. In addition, the molar percentage of 12-hydroxydodecanoate units in the ﬁnal products can be tailored from 0% to 100% by adjusting the feed ratio.

Scheme 33. Lipase-catalyzed synthesis of polyricinoleate in bulk [270].

Later, the N435-catalyzed synthesis of poly(12-hydroxydodecanoate-co-12-hydroxystearate) was

studied by the same research group (Scheme 34) [271]. The , dispersity and reaction yield of the resulting co-polyesters were around 92,300–118,200 g/mol, 2.8–3.3, and 83%–88%, respectively. In addition, the molar percentage of 12-hydroxydodecanoate units in the final products can be tailored from 0% to 100% by adjusting the feed ratio.

Polymers 2016, 8, 243 26 of 52

However, by replacing diethyl succinate (C2) with the other diacid ethyl esters with relatively longer chain length (C3~C10), the two-stage enzymatic polymerization in diphenyl ether resulted in

series of unsaturated aliphatic polyesters with desired molar compositions and high ’s of up to 57,900 g/mol (Scheme 32) [257]. The molar percentage of itaconate in the unsaturated polyesters can be tailored from 0% to 35% by adjusting the feed ratio of itaconate; and all C=C bonds were well preserved in the resulting polyesters. We found that products with relatively lower molecular weights were generally obtained from the enzymatic polymerization at a higher feed ratio of itaconate; however, with diethyl dodecanedioate having the longest chain length (C10) among the tested diacid ethyl esters, higher molecular weight products were obtained at higher feed ratios of itaconate. Moreover, the obtained itaconate-based polyesters can be thermally cross-linked or photo- cured. By adjusting the diacid chain length and itaconate composition, the thermal and mechanical properties of the cured polyesters can be tuned.

Scheme 32. N435-catalyzed synthesis of poly(butylene dicarboxylate-co-itaconate) with diverse chemical structures [257].

8.3. Polyesters Derived from Long Chain Fatty Acids and their Derivatives Long chain fatty acids may contain one or more C=C bonds within the backbones. The C=C bonds can be further modified to form other functional groups such as epoxy, thiol, and hydroxyl group [153,266], rendering fatty acid-based polyesters with diverse functionalities. Fatty acid-based polyesters can be used as thermoset resins, coating materials and biomaterials for biomedical applications, and so on [267–269]. The pioneer work on the enzymatic polymerization with long chain fatty acids was reported by Matsumura et al. [270]. They investigated the lipase-catalyzed polymerization of ricinoleic acid/methyl ricinoleate in bulk (Scheme 33). Among the tested lipases, immobilized lipase PC showed the highest reactivity towards ricinoleic acid and methyl ricinoleate. The enzymatic polymerization

with ricinoleic acid resulted in polyricinoleate with a of up to 8500 g/mol. However, by replacing Polymersricinoleic2016, 8, 243 acid with methyl ricinoleate, polyricinoleate with a much higher of up to 100,600 g/mol 27 of 53 were produced.

SchemeScheme 33. 33.Lipase-catalyzed Lipase-catalyzed synthesissynthesis of of polyricinoleate polyricinoleate in bulk in bulk [270]. [270 ]. Polymers 2016, 8, 243 27 of 52 Later, the N435-catalyzed synthesis of poly(12-hydroxydodecanoate-co-12-hydroxystearate) was

SchemeScheme 34. 34.N435-catalyzed N435-catalyzed polycondensation polycondensation of 12- ofhydroxydodecanoic 12-hydroxydodecanoic acid and acid methyl and 12- methyl hydroxystearate [271]. 12-hydroxystearate [271]. Biobased functional polyesters can be produced via enzymtic polymerization with unsaturated Scheme 34. N435-catalyzed polycondensation of 12-hydroxydodecanoic acid and methyl 12- Biobasedor epoxidizedhydroxystearateScheme functional α 34.,ω -carboxylicN435-catalyzed polyesters [271]. fatty canpolycondensation acid be producedderivatives of 12- via[272,273].hydroxydodecanoic enzymtic The polymerization long acid chain and methylunsaturated with 12- unsaturated and or epoxidizedepoxidizedhydroxystearateα ω,ω-carboxy-carboxylic [271].fatty acid fatty derivatives acid derivatives can be synthesized [272,273]. via The chemical long [272] chain or unsaturated biocatalytic and epoxidizedapproachesωBiobased-carboxy [273]. functionalThe fatty N435-catalyzed acid polyesters derivatives can polycondensation be produced can be via synthesized enzymtic of unsaturated polymerization via chemical or epoxidized with [ unsaturated272] ω or-carboxy biocatalytic fatty oracid epoxidizedBiobased methyl esters functionalα,ω-carboxylic (C18, polyesters C20 fatty and can acidC26) be derivatives producedwith alkane- via [272,273]. enzymticα,ω-aliphatic The polymerization long diols chain (C3 withunsaturatedand unsaturatedC4) resulted and in approachesor [epoxidized273]. The α N435-catalyzed,ω-carboxylic fatty polycondensationacid derivatives [272,273]. of unsaturated The long chain or unsaturated epoxidized andω -carboxy polyestersepoxidized with ω-carboxy’s, dispersities fatty acid and derivatives reaction can yields be synthesized of up to 11,600 via chemical g/mol, [272]1.2–2.6, or biocatalyticand 49%–84%, fatty acid methylepoxidized esters ω-carboxy (C18, fatty C20 acid and derivatives C26) with can alkane-be synthesizedα,ω-aliphatic via chemical diols [272] (C3 or andbiocatalytic C4) resulted in respectivelyapproaches (Scheme [273]. The35) N435-catalyzed[272]. In addition, polycondensation the two-step of unsaturatedbiocatalytic or approach epoxidized gave ω-carboxy biobased approaches [273]. The N435-catalyzed polycondensation of unsaturated or epoxidized ω-carboxy polyestersfatty with acidM methylw’s, dispersities esters (C18,M C20 and and reaction C26) with yields alkane- ofα, upω-aliphatic to 11,600 diols g/mol, (C3 and 1.2–2.6,C4) resulted and in 49%–84%, functionalfatty acid polyesters methyl esterswith (C18,’s C20and and dispersities C26) with of alkane- aroundα,ω 25,00-aliphatic0–76,000 diols g/mol, (C3 and and C4) around resulted 2.0–3.1, in polyesters with ’s, dispersities and reaction yields of up to 11,600 g/mol, 1.2–2.6, and 49%–84%, respectivelyrespectively (Scheme (Scheme 35)[ 36)272 [273].]. In addition, the two-step biocatalytic approach gave biobased functional respectivelypolyesters with (Scheme ’s, dispersities35) [272]. In and addition, reaction thyieldse two-step of up tobiocatalytic 11,600 g/mol, approach 1.2–2.6, gaveand 49%–84%,biobased polyestersrespectively with Mw’s (Scheme and dispersities 35) [272]. In of aroundaddition, 25,000–76,000the two-step biocatalytic g/mol, andapproach around gave 2.0–3.1, biobased respectively functional polyesters with M’s and dispersities of around 25,000–76,000 g/mol, and around 2.0–3.1, (Scheme 36respectivelyfunctional)[273]. polyesters (Scheme with36) [273]. M’s and dispersities of around 25,000–76,000 g/mol, and around 2.0–3.1, respectively (Scheme 36) [273].

Scheme 35. N435-catalyzed polycondensation of unsaturated or epoxidized ω-carboxy fatty acid

methyl esters with alkane-α,ω-aliphatic diols [272]. Scheme 35.SchemeN435-catalyzed 35. N435-catalyzed polycondensation polycondensation ofof unsaturated or epoxidized or epoxidized ω-carboxyω-carboxy fatty acid fatty acid methylScheme esters 35. N435-catalyzed with alkane-α, ωpolyco-aliphaticndensation diols [272]. of unsaturated or epoxidized ω-carboxy fatty acid methyl estersmethyl with esters alkane- with alkane-α,ω-aliphaticα,ω-aliphatic diols diols [ 272[272].].

SchemeScheme 36. A 36. two-step A two-step biocatalytic biocatalytic synthesis synthesis of of biobasedbiobased functionalfunctional polyesters polyesters from from ω-carboxy ω-carboxy fatty fatty Scheme 36. A two-step biocatalytic synthesis of biobased functional polyesters from ω-carboxy fatty Schemeacids 36.acids andA two-stepanddiols diols [273]. [273]. biocatalytic synthesis of biobased functional polyesters from ω-carboxy fatty acids and diols [273]. acids and diols [273]. On Onthe theother other hand, hand, cutin cutin and and suberin suberin areare lipophilic macromoleculesmacromolecules which which are arenatural natural substancesOn the found other in hand,cell walls cutin of higherand suberin plants asare structural lipophilic components. macromolecules Cutin coverswhich allare the natural aerial substances found in cell walls of higher plants as structural components. Cutin covers all the aerial surfacessubstances of foundplants in cellthe plantwalls cuticle,of higher while plants suberi as structuraln is the main components. constituent Cutin of cork covers cells. all Their the aerial fatty surfaces of plants in the plant cuticle, while suberin is the main constituent of cork cells. Their fatty acidsurfaces derivatives, of plants insuch the asplant long cuticle, chain while ω-hydroxyalkanoic suberin is the main acids, constituent and α,ω -alkanedioicof cork cells. acids,Their fattyand acid acidderivatives, derivatives, such such as aslong long chain chain ω ω-hydroxyalkanoic-hydroxyalkanoic acids,acids, and and α ,αω,-alkanedioicω-alkanedioic acids, acids, and and

Polymers 2016, 8, 243 28 of 53

On the other hand, cutin and suberin are lipophilic macromolecules which are natural substances found in cell walls of higher plants as structural components. Cutin covers all the aerial surfaces of plants in the plant cuticle, while suberin is the main constituent of cork cells. Their fatty acid derivatives, such as long chain ω-hydroxyalkanoic acids, and α,ω-alkanedioic acids, and substituted ω-hydroxyalkanoic acids, are attractive biobased monomers for the synthesis of functional aliphatic polyestersPolymers [152,274 2016]., 8, 243 28 of 52

Iversensubstituted et al. [ 275ω-hydroxyalkanoic] did pioneer acids, work are on attractive the enzymatic biobased synthesis monomers offor suberin-based the synthesis of polyesters (Scheme 37functional). The N435-catalyzed aliphatic polyesterspolymerization [152,274]. with cis-9,10-epoxy-18-hydroxyoctadecanoic acid in toluene resultedIversen in et epoxy-functionalizedal. [275] did pioneer work polyesterson the enzymatic with synthesis the highest of suberi molecularn-based polyesters weights. The Mw Polymers 2016, 8, 243 28 of 52 and dispersity(Scheme were 37). 20,000The N435-catalyzed g/mol, and polymerization 2.2, respectively. with cis In-9,10-epoxy-18-hydroxyoctadecanoic addition, even at a much shorter acid reaction in toluene resulted in epoxy-functionalized polyesters with the highest molecular weights. The substituted ω-hydroxyalkanoic acids, are attractive biobased monomers for the synthesis of time of 3 h,and the dispersity solvent-free were 20,000 enzymatic g/mol, and polymerization 2.2, respectively. In inaddition, an open even vial at a much without shorter any reaction drying agents functional aliphatic polyesters [152,274]. gave comparabletime of 3 highh, the molecularsolvent-free weightsenzymatic products,polymerization with in aanM openw and viala without dispersity any drying of 15,000 agents g/mol and Iversen et al. [275] did pioneer work on the enzymatic synthesis of suberin-based polyesters gave comparable high molecular weights products, with a and a dispersity of 15,000 g/mol and 2.2, respectively.(Scheme 37). The N435-catalyzed polymerization with cis-9,10-epoxy-18-hydroxyoctadecanoic acid 2.2, respectively. in toluene resulted in epoxy-functionalized polyesters with the highest molecular weights. The and dispersity were 20,000 g/mol, and 2.2, respectively. In addition, even at a much shorter reaction time of 3 h, the solvent-free enzymatic polymerization in an open vial without any drying agents

gave comparable high molecular weights products, with a and a dispersity of 15,000 g/mol and 2.2, respectively.

Scheme 37.SchemeN435-catalyzed 37. N435-catalyzed polymerization polymerization of ofcis cis-9,10-epoxy-18-hydroxyoctadecanoic-9,10-epoxy-18-hydroxyoctadecanoic acid in bulk acid or in bulk or in an organic solvent. in an organic solvent. Recently, multifunctional, bio-based oligoester resins based on 9,10-epoxy-18- Scheme 37. N435-catalyzed polymerization of cis-9,10-epoxy-18-hydroxyoctadecanoic acid in bulk or hydroxyoctadecanoic acid were enzymatically synthesized by using N435 as the catalysts (Scheme Recently,inmultifunctional, an organic solvent. bio-based oligoester resins based on 9,10-epoxy-18- 38) [276,277]. The , dispersity, monomer conversion and reaction yield of the resulting oligoesters hydroxyoctadecanoic acid were enzymatically synthesized by using N435 as the catalysts were aroundRecently, 900–1100 multifunctional, g/mol, 2.3–3.1, bio-based95%–99%, anoligoesterd 82%–89%, resins respectively. based Moreover,on 9,10-epoxy-18- the functional (Scheme 38end)[hydroxyoctadecanoic 276groups,277 and]. The the epoxyM acidn, dispersity,weregroups enzymatically were monomerwell prsynthesizedeserved conversion after by using the enzymaticN435 and as reaction the polymerization. catalysts yield (Scheme of The the resulting oligoestersobtained were38) [276,277]. around oligoesters The 900–1100 can, dispersity, undergo g/mol, furthermonomer 2.3–3.1, modificati conversion 95%–99%,ons and via reac different andtion 82%–89%,yield techniques of the resulting respectively.such as oligoesters Diels-Alder Moreover, the functionalreactions, endwere groups around radical 900–1100 and polymerization the g/mol, epoxy 2.3–3.1, and groups ring-opening95%–99%, were an welld polymerization.82%–89%, preserved respectively. after Moreover, the enzymatic the functional polymerization. end groups and the epoxy groups were well preserved after the enzymatic polymerization. The The obtainedobtained oligoesters oligoesters can can undergo undergo further further modificati modiﬁcationsons via different techniques via different such as Diels-Alder techniques such as Diels-Alder reactions,reactions, radical radical polymerization polymerization and ring-opening and ring-opening polymerization. polymerization.

Scheme 38. N435-catalyzed synthesis of bio-based oligoesters based on 9,10-epoxy-18- hydroxyoctadecanoic acid. Scheme 38.SchemeN435-catalyzed 38. N435-catalyzed synthesis synthesis of bio-base bio-basedd oligoesters oligoesters based on based 9,10-epoxy-18- on 9,10-epoxy-18- hydroxyoctadecanoic acid. hydroxyoctadecanoic acid. Polymers 2016, 8, 243 29 of 53 Polymers 2016, 8, 243 29 of 52 Polymers 2016, 8, 243 29 of 52 8.4. Glycerol-Based Polyesters 8.4. Glycerol-Based Polyesters Glycerol-based aliphatic polyesters can be used as thermosets like shape memory materials; and haveGlycerol-based found potential aliphatic applications polyesters in biomedical can be used and as pharmaceutical thermosets like fields, ﬁelds, shape for memory example, materials; they can and be haveused foundas carriers potential for drug applications delivery, in sealants biomedical or coatingsco andatings pharmaceutical for tissue repair, fields, andand for agentsagents example, for antibacterial they can be usedapplications as carriers [[278].278 for]. drug delivery, sealants or coatings for tissue repair, and agents for antibacterial applicationsThe N435-catalyzedN435-catalyzed [278]. polymerization polymerization of divinylof divinyl adipate adipate and glycerol and inglycerol bulk yielded in bulk poly(glyceryl yielded The N435-catalyzed polymerization of divinyl adipate and glycerol in bulk yielded adipate)poly(glyceryl with aadipate)Mw and with dispersity a ofand up dispersity to 10,400g/mol of up andto 10,400 2.3–3.1, g/mol respectively and 2.3–3.1, (Scheme respectively 39)[279]. poly(glycerylMALDI-ToF(Scheme 39) MS adipate)[279]. analysis MALDI-ToF with suggested a MSand that analysisdispersity linear polyesterssuggested of up to with 10,400that hydroxyl linear g/mol polyesters substituentsand 2.3–3.1, with wererespectively hydroxyl mainly (Schemeproducedsubstituents 39) and were[279]. no mainly polymer MALDI-ToF produced network MS and was analysis no formed. poly mersuggested The network number that was oflinear formed. hydroxyl polyesters The groups number with per of repeatinghydroxylhydroxyl substituentsunitsgroups was per around repeating were 0.8–0.9;mainly units andproduced was the arou pendant ndand 0.8–0.9; no groups poly andmer of the thenetwork pe syntheticndant was groups poly(glycerylformed. of the The sy ntheticnumber adipate) poly(glyceryl of consisted hydroxyl of groups90%–95%adipate) per consisted of repeating secondary of units90%–95% and was 5%–10% arouof secondarynd of 0.8–0.9; primary and and hydroxyl 5%–10% the pendant groups.of primary groups hydroxyl of the sy groups.nthetic poly(glyceryl adipate) consisted of 90%–95% of secondary and 5%–10% of primary hydroxyl groups.

Scheme 39. N435-catalyzedN435-catalyzed polymerization polymerization of ofdivinyl divinyl adipate adipate and and glycerol glycerol with with or without or without 1,4- Scheme 39. N435-catalyzed polymerization of divinyl adipate and glycerol with or without 1,4- 1,4-butanediolbutanediol in bulk in bulk [279]. [279 ]. butanediol in bulk [279]. By replacing the activated divinyl adipate with unactivated adipic acid, the N435-catalyzed ByBy replacing the activated divinyl adipate with unactivated adipic acid, the N435-catalyzed enzymatic polycondensation also resulted in poly(glyceryl adipate), with a slight low of 3700 enzymatic polycondensation also resulted in poly(glyceryl adipate), with a slight low Mw of 3700 g/mol enzymaticg/mol and apolycondensation dispersity of 1.4 (Schemealso resulted 40) [280]. in poly(glyceryl adipate), with a slight low of 3700 g/moland a dispersityand a dispersity of 1.4 (Schemeof 1.4 (Scheme 40)[280 40)]. [280].

Scheme 40. N435-catalyzed polymerization of glycerol and adipic acid with or without 1,8-octanediol Scheme 40. N435-catalyzed polymerization of glycerol and adipic acid with or without 1,8-octanediol Schemein bulk [280–282]. 40. N435-catalyzed polymerization of glycerol and adipic acid with or without 1,8-octanediol in bulk [280–282]. in bulk [280–282]. Poly(glyceryl-1,18-cis-9-octadecenedioate) was successfully produced via the N435-catalyzed polycondensationPoly(glyceryl-1,18- of 1,18-cis-9-octadecenedioate)cis-9-octadecenedioic was(oleic successfully diacid) and producedglycerol in via bulk the (Scheme N435-catalyzed 41) [283]. polycondensationPoly(glyceryl-1,18- of 1,18-ciscis-9-octadecenedioate)-9-octadecenedioic (oleic was successfullydiacid) and glycerol produced in viabulk the (Scheme N435-catalyzed 41) [283]. At a molar monomer feed ratio of 1.0:1.0, the and dispersity of the obtained polyesters were up polycondensation of 1,18-cis-9-octadecenedioic (oleic diacid) and glycerol in bulk (Scheme 41)[283]. Atto 9100a molar g/mol, monomer and around feed ratio 3.3–3.4, of 1.0:1.0, respectively. the However, and dispersity the percentage of the obtained of dendritic polyesters glycerol were units up to(Den 9100 %) g/mol, was quiteand around low, around 3.3–3.4, 13%–16%. respectively. By However,increasing thethe percentage molar feed of ratio dendritic of oleic glycerol diacid units and (Den %) was quite low, around 13%–16%. By increasing the molar feed ratio of oleic diacid and

Polymers 2016, 8, 243 30 of 53

At a molar monomer feed ratio of 1.0:1.0, the Mn and dispersity of the obtained polyesters were up to 9100 g/mol, and around 3.3–3.4, respectively. However, the percentage of dendritic glycerol Polymersunits (Den 2016, %)8, 243 was quite low, around 13%–16%. By increasing the molar feed ratio of oleic diacid30 of and 52 glycerol from 1.0:1.0 to 1.0:1.5, the resulting polyesters possessed a similar M and dispersity, but a glycerol from 1.0:1.0 to 1.0:1.5, the resulting polyesters possessed a similar n and dispersity, but a signiﬁcant higher Den % (~31%). In contrast, gelation was observed in the polymerization catalyzed significant higher Den % (~31%). In contrast, gelation was observed in the polymerization catalyzed by dibutyltin oxide. by dibutyltin oxide.

Scheme 41. Chemical and enzymatic synthesis of poly(glyceryl-1,18-cis-9-octadecenedioate) [283]. Scheme 41. Chemical and enzymatic synthesis of poly(glyceryl-1,18-cis-9-octadecenedioate) [283]. Moreover, several glycerol-based co-polyesters were successfully produced via lipase-catalyzed polycondensation.Moreover, several glycerol-based co-polyesters were successfully produced via lipase- catalyzedThe enzymatic polycondensation. co-polymerization of divinyl adipate, glycerol and 1,4-BDO gave poly(glyceryl adipate-Theco enzymatic-butylene adipate)co-polymerization (Scheme 39) of [279]. divinyl The adipate, hydroxyl glycerol number and of 1,4-BDO the obtained gave poly(glycerylco-polyesters canadipate- be wellco-butylene controlled adipate) by adjusting (Scheme the 39amount)[279]. of The 1,4-BDO hydroxyl in the number reaction of themixture. obtained co-polyesters can bePoly(octamethylene well controlled by adjustingadipate-co the-glyceryl amount adipate) of 1,4-BDO were in successfully the reaction mixture.produced via the N435- catalyzedPoly(octamethylene co-polymerization adipate- of glycerol,co-glyceryl adipic acid adipate) and 1,8-octanediol were successfully in bulk (Scheme produced 40) [280–282], via the N435-catalyzed co-polymerization of13 glycerol, adipic acid and 1,8-octanediol in bulk with ’s of up to 75,600 g/mol. The C–NMR study indicated that the obtained polyesters were 13 highly(Scheme branched 40) [280 but–282 had], with few Minterchainn’s of up cross-links; to 75,600 g/mol.and the ThedegreeC–NMR of branching study and indicated molecular that weightsthe obtained can be polyesters controlled were by altering highly branched the reaction but time had fewand interchainmolar feed cross-links; ratio of monomers and the degree[282]. In of addition,branching due and to molecular the regio-selectivity weights can of be N435, controlled the enzymatic by altering polymerization the reaction time gave and linear molar polyesters feed ratio at shortof monomers reaction [times282]. In(≤18 addition, h) but yielded due to the highly regio-selectivity branched polyesters of N435, at the a enzymaticlong reaction polymerization time (42 h). Moreover,gave linear with polyesters respect atto shortesterifications, reaction times N435 (shď18owed h) but 77% yielded to 82% highlyof the regio-selectivity branched polyesters towards at a thelong primary reaction hydroxyl time (42 groups h). Moreover, of glycerol with and respect this towas esteriﬁcations, independent N435of the showedamount 77%of glycerol to 82% in of the the reactionregio-selectivity media. towards the primary hydroxyl groups of glycerol and this was independent of the amountThe ofenzymatic glycerol in co the-polymerization reaction media. of divinyl esters, glycerol and ω-fatty acids were also investigated,The enzymatic using lipasesco-polymerization as biocatalysts of (Schem divinyle 42) esters, [284,285]. glycerol Among and ωthe-fatty tested acids lipases, were N435 also showedinvestigated, the highest using catalytic lipases asactivity. biocatalysts The enzy (Schemematic polymerization 42)[284,285]. yiel Amongded biodegradable the tested lipases, cross- N435 showed the highest catalytic activity. The enzymatic polymerization yielded biodegradable linkable polyesters with ’s of up to 8500 g/mol. The obtained polyesters were thermally cross- linked,cross-linkable which resulted polyesters in transparent with Mn’s ofpolymeric up to 8500 films g/mol. with high-gloss The obtained surfaces. polyesters were thermally cross-linked,Later, the which same resultedresearch ingroup transparent reported polymeric the synthesis ﬁlms of with epoxide-containing, high-gloss surfaces. glycerol-based co- polyesters via N435-catalzyed polymerization via two routes (Scheme 43) [286]. The first route yielded corresponding polyesters with ’s, dispersities and epoxidation ratios of around 3300–7900 g/mol, 1.3–1.6, and 76%–96%, respectively; and the second route gave relatively higher values of , dispersity and epoxidation ratio, which were around 4200–6500 g/mol, 1.9–2.1, and 88%–94%, respectively.

Polymers 2016, 8, 243 31 of 53 Polymers 2016, 8, 243 31 of 52

Polymers 2016, 8, 243 31 of 52

Scheme 42. N435-catalyzed co-polymerization of divinyl esters, glycerol andand ω-fatty acids [[284,285].284,285].

Later, the same research group reported the synthesis of epoxide-containing,R glycerol-based Route 1 O O O OH co-polyesters via N435-catalzyed polymerization via two routes (SchemeO 43)[286]. The ﬁrst O X O (CH2)8 O X HO OH O route yielded corresponding polyesters with Mn’s, dispersities and epoxidationO O ratios of around R OH (CH2)8 o n 3300–7900 g/mol, 1.3–1.6, and 76%–96%,N435, 60 C, 72 respectively; h, 2700 Pa and the secondO route gave relatively higher values of M , dispersity and epoxidation ratio, which were around 4200–6500 g/mol, 1.9–2.1, and n Route 2 H2O2 Toluene 88%–94%,Scheme respectively. 42. N435-catalyzed co-polymerization of divinyl esters, glycerolN435 andR.T., ω-fatty 24 h acids [284,285]. H2O2 Toluene X: H2CCH N435 R.T., 24 h R' R O O O OH Route 1 O X OO (CH )O O X HO OH OH O O 2 8 O O O X O (CH ) O X HO OH O R' O OH 2 8 (CH2)8 o O O n R OH N435, Toluene, 60 C, 72 h, 2700 Pa (CH2)8 O n N435, 60 oC, 72 h, 2700 Pa O O

Route 2 H2O2 Toluene N435 OR.T., 24 hO H2O2 Toluene X: H2CCH R= R' = N435 R.T., 24 h O R' O O O O OH O X O (CH ) O X HO OH O O 2 8 O O Scheme 43.R' N435-catalyzedOH synthesis of epoxide-containing polyesters(CH2)8 from polycondensation of o n divinyl sebacate, glycerolN435, and Toluene,vegetable 60 oil-basedC, 72 h, 2700 fatty Pa acids [286]. O O Recently, polymeric triglyceride analogs, poly(oleic diacid-co-glycerol-co-linoleic acid)s, were O O prepared via the N435-catalyzed polycondensation (Scheme 44) [287]. By varying the molar feed ratio R= R' = O of oleic diacid, glycerol and crude linoleic acid from 1.0:1.0:1.0 to 1.0:1.0:1.33, theO and dispersity of the obtained products decreased from 12,300 to 6300 g/mol, and 6.3 to 1.7, respectively, whereas the degree of tri-substituted units increased from 18% to 100%. In addition, when the molar feed ratio SchemeScheme 43. 43. N435-catalyzedN435-catalyzed synthesis synthesis of of epoxide-containi epoxide-containingng polyesters polyesters from from polycondensation polycondensation of of of oleic diacid, glycerol and crude linoleic acid was 1.0:1.0:0.67, all monomers were converted to divinyldivinyl sebacate, sebacate, glycerol glycerol and and vege vegetabletable oil-based oil-based fatty fatty acids acids [286]. [286]. polymers after 8 h reaction; and the and degree of tri-substituted units of the resulting co- polyestersRecently, reached polymeric 9500 g/mol, triglyceride and 64%, analogs, respectively. poly(oleic diacid-co-glycerol-co-linoleic acid)s, were Recently, polymeric triglyceride analogs, poly(oleic diacid-co-glycerol-co-linoleic acid)s, were prepared via the N435-catalyzed polycondensation (Scheme 44) [287]. By varying the molar feed ratio prepared via the N435-catalyzed polycondensation (Scheme 44)[287]. By varying the molar feed ratio of oleic diacid, glycerol and crude linoleic acid from 1.0:1.0:1.0 to 1.0:1.0:1.33, the and dispersity of oleic diacid, glycerol and crude linoleic acid from 1.0:1.0:1.0 to 1.0:1.0:1.33, the M and dispersity of of the obtained products decreased from 12,300 to 6300 g/mol, and 6.3 to 1.7, respectively,n whereas the obtained products decreased from 12,300 to 6300 g/mol, and 6.3 to 1.7, respectively, whereas the the degree of tri-substituted units increased from 18% to 100%. In addition, when the molar feed ratio degree of tri-substituted units increased from 18% to 100%. In addition, when the molar feed ratio of of oleic diacid, glycerol and crude linoleic acid was 1.0:1.0:0.67, all monomers were converted to oleic diacid, glycerol and crude linoleic acid was 1.0:1.0:0.67, all monomers were converted to polymers polymers after 8 h reaction; and the and degree of tri-substituted units of the resulting co- after 8 h reaction; and the M and degree of tri-substituted units of the resulting co-polyesters reached polyesters reached 9500 g/mol,n and 64%, respectively. 9500 g/mol, and 64%, respectively.

Scheme 44. N435-catalyzed synthesis of poly(oleic diacid-co-glycerol-co-linoleic acid) in bulk [287].

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Scheme 42. N435-catalyzed co-polymerization of divinyl esters, glycerol and ω-fatty acids [284,285].

R Route 1 O O O OH O O X O (CH2)8 O X HO OH O O O R OH (CH ) 2 8 n N435, 60 oC, 72 h, 2700 Pa O

Route 2 H2O2 Toluene N435 R.T., 24 h H2O2 Toluene X: H2CCH N435 R.T., 24 h R' O O O OH O X O (CH ) O X HO OH O O 2 8 O O R' OH (CH2)8 o n N435, Toluene, 60 C, 72 h, 2700 Pa O O

O O R= R' = O O

Scheme 43. N435-catalyzed synthesis of epoxide-containing polyesters from polycondensation of divinyl sebacate, glycerol and vegetable oil-based fatty acids [286].

Recently, polymeric triglyceride analogs, poly(oleic diacid-co-glycerol-co-linoleic acid)s, were prepared via the N435-catalyzed polycondensation (Scheme 44) [287]. By varying the molar feed ratio of oleic diacid, glycerol and crude linoleic acid from 1.0:1.0:1.0 to 1.0:1.0:1.33, the and dispersity of the obtained products decreased from 12,300 to 6300 g/mol, and 6.3 to 1.7, respectively, whereas the degree of tri-substituted units increased from 18% to 100%. In addition, when the molar feed ratio of oleic diacid, glycerol and crude linoleic acid was 1.0:1.0:0.67, all monomers were converted to Polymerspolymers2016 after, 8, 243 8 h reaction; and the and degree of tri-substituted units of the resulting32 of co 53- polyesters reached 9500 g/mol, and 64%, respectively.

Scheme 44. N435-catalyzed synthesis of poly(oleic diacid-co-glycerol-co-linoleic acid) in bulk [287]. PolymersScheme 2016, 8, 44.243N435-catalyzed synthesis of poly(oleic diacid-co-glycerol-co-linoleic acid) in bulk [287].32 of 52

8.5. 8.5. SweetSweet PolyestersPolyesters DerivedDerived fromfrom CarbohydratesCarbohydrates

8.5.1.8.5.1. SugarSugar andand SugarSugar Alcohol-BasedAlcohol-Based PolyestersPolyesters SugarsSugars andand sugarsugar alcoholsalcohols cancan bebe usedused asas starting materials forfor the production ofof biobased linearlinear andand branched functional functional polyesters polyesters which which have have various various potential potential applications, applications, for forexample, example, they they can canbe used be used as ascoating coating materials, materials, biodegradable biodegradable and and bior bioresorbableesorbable polymers, polymers, and and optically activeactive polymerspolymers [[112].112]. ItIt isis difficultdifficult toto synthesizesynthesize polyesterspolyesters withwith sugarsugar andand sugarsugar alcoholalcohol unitsunits viavia conventionalconventional techniques,techniques, asas tedious tedious protection protection and and de-protection de-protect stepsion steps are normally are normally required, required, to prevent to prevent the gelation the duringgelation the during polymerization. the polymerization. However, theseHowever, multiple-functional these multiple-functional monomers can monomers be directly can polymerized be directly viapolymerized enzymatic via polymerization, enzymatic polymerization, due to the highly due regio-selectivityto the highly regio-selectivity of the enzymes. of the enzymes. TheThe lipase-catalyzedlipase-catalyzed polymerizationpolymerization with D-sorbitol was firstfirst reportedreported byby KobayashiKobayashi etet al.al. (Scheme 45)[288]. Sorbitol-based polyesters with M ’s of around 3000–12,000 g/mol were produced (Scheme 45) [288]. Sorbitol-based polyesters with n’s of around 3000–12,000 g/mol were produced 1 13 inin moderatemoderate yieldsyields (around(around 40%–85%).40%–85%). Moreover,Moreover, 1H–H– andand 13C–NMR study indicated that the primary hydroxylhydroxyl groupsgroups ofof DD-sorbitol-sorbitol werewere exclusivelyexclusively esterifiedesterified duringduring thethe enzymaticenzymatic polymerization. However,However, toto compensatecompensate thethe lowlow catalyticcatalytic reactivityreactivity ofof N435N435 inin thethe polar aprotic solvent acetonitrile, activatedactivated divinyldivinyl sebacatesebacate andand highhigh concentrationconcentration ofof N435N435 (76(76 wtwt %)%) werewere used.used.

SchemeScheme 45.45. N435-catalyzedN435-catalyzed polymerizationpolymerization ofof DD-sorbitol-sorbitol withwith divinyldivinyl sebacatesebacate [[288].288].

In addition, divinyl sebacate and other activated monomers including bis(2,2,2- In addition, divinyl sebacate and other activated monomers including trifluoroethyl)malonate, bis(2,2,2-trifluoroethyl)glutarate and divinyl adipate, were enzymatically bis(2,2,2-trifluoroethyl)malonate, bis(2,2,2-trifluoroethyl)glutarate and divinyl adipate, were polymerized with disaccharides (sucrose, trehalose, and lactose), D-sorbitol and D-mannitol in enzymatically polymerized with disaccharides (sucrose, trehalose, and lactose), D-sorbitol and acetonitrile.[289] It was found that N435 was able to differentiate the five tested carbohydrates, as the D-mannitol in acetonitrile. [289] It was found that N435 was able to differentiate the five tested molecular weights of the resulting polyesters showed the following array: disaccharide-based carbohydrates, as the molecular weights of the resulting polyesters showed the following array: polyesters < mannitol-based polyesters < sorbitol-based polyesters. disaccharide-based polyesters < mannitol-based polyesters < sorbitol-based polyesters. Unactivated diacid monomers were also used as starting materials for the enzymatic synthesis Unactivated diacid monomers were also used as starting materials for the enzymatic synthesis of sorbitol-based polyesters. Gross et al. [280,281] reported the N435-catalyzed polycondensation of of sorbitol-based polyesters. Gross et al. [280,281] reported the N435-catalyzed polycondensation D-sorbitol, 1,8-octanediol and adipic acid in bulk (Scheme 46). Poly(sorbityl adipate) and

poly(octamethylene adipate-co-sorbityl adipate)s were produced, with ’s and dispersities of around 7000–20,300 g/mol, and 1.6–3.3, respectively. In addition, the molar percentage of sorbityl units in the co-polyesters can be tunable from 0% to 100%. Moreover, the NMR analysis revealed that N435 showed highly regio-selectivity (≥85% ± 5%) towards the primary hydroxyl groups of D-sorbitol at 1- and 6-positions.[280] In the same laboratory, the enzymatic polycondensation of adipic acid and 1,8-octanediol with

different sugar alcohols was investigated (Scheme 46) [290]. Sweet co-polyesters with ’s ranging from 11,000 and 73,000 g/mol were obtained; and no correlation was observed between the reactivity of the tested sugar diols and their chain length. However, N435 showed the highest reactivity towards D- mannitol; and the co-polyester containing D-mannitol units possessed the highest degree of branching.

Polymers 2016, 8, 243 33 of 53

of D-sorbitol, 1,8-octanediol and adipic acid in bulk (Scheme 46). Poly(sorbityl adipate) and poly(octamethylene adipate-co-sorbityl adipate)s were produced, with Mn’s and dispersities of around 7000–20,300 g/mol, and 1.6–3.3, respectively. In addition, the molar percentage of sorbityl units in the co-polyesters can be tunable from 0% to 100%. Moreover, the NMR analysis revealed that N435 showed highly regio-selectivity (ě85% ˘ 5%) towards the primary hydroxyl groups of D-sorbitol at 1- andPolymers 6-positions 2016, 8, 243 [280 ]. 33 of 52

Polymers 2016, 8, 243 33 of 52

SchemeScheme 46. N435-catalyzed46. N435-catalyzedco co-polymerization-polymerization of of adipic adipic acid acid and and 1,8-octanediol 1,8-octanediol with with different different sugar sugar diolsdiols in bulk in bulk [280 [280,281,290].,281,290].

Recently, sorbitol-based, hydroxy-functional polyesters were successfully synthesized via the In the same laboratory, the enzymatic polycondensation of adipic acid and 1,8-octanediol with N435-catalyzed co-polymerization (Scheme 47) [291]. The of the resulting co-polyesters was different sugar alcohols was investigated (Scheme 46)[290]. Sweet co-polyesters with Mw’s ranging successfully controlled at around 4000–8000 g/mol, by tuning the reaction time, enzyme fromconcentration 11,000 and 73,000 and reaction g/mol stoichiometry. were obtained; However, and no only correlation maximum was 53% observed of the added between D-sorbitol the reactivitywas of theincorporatedScheme tested 46. sugar N435-catalyzed into diolsthe final and products, co their-polymerization chaineven though length. of itsadipic feed However, acidratio and was N435 1, quite8-octanediol showedlow (≤5 molwith the %). different highest Besides, sugar reactivity the obtained polyesters displayed suitable properties for being used as solvent-borne coating resins. towardsdiolsD in-mannitol; bulk [280,281,290]. and the co-polyester containing D-mannitol units possessed the highest degree of branching. Recently, sorbitol-based,sorbitol-based, hydroxy-functionalhydroxy-functional polyesters were successfullysuccessfully synthesized via thethe

N435-catalyzed co-polymerization (Scheme 4747))[ [291].291]. The The Mn of the resulting coco-polyesters-polyesters waswas successfully controlledcontrolled at aroundat arou 4000–8000nd 4000–8000 g/mol, g/mol, by tuning by thetuning reaction the time, reaction enzyme time, concentration enzyme andconcentration reaction stoichiometry. and reaction stoichiometry. However, only However, maximum only 53% maximum of the added 53%D of-sorbitol the added was D incorporated-sorbitol was intoincorporated the ﬁnal products,into the final even products, though even its feed though ratio its was feed quite ratio low was (ď quite5 mol low %). (≤ Besides,5 mol %). the Besides, obtained the polyesters displayed suitable properties for being used as solvent-borne coating resins. obtainedScheme polyesters 47. N435-catalyzed displayed suitable co-polymerization properties of forD-sorbitol, being usedalkane- asα, ωsolvent-borne-aliphatic linear coatingdiols and resins. diacids or diesters in bulk.

8.5.2.isosorbide Polyesters polyesters Based on withRigid Sugar ’s of Derivatives up to 40,000 g/mol were produced via the enzymatic polymerization by azeotropic distillation; and significantly higher molecular weight products can be DAHs (isosorbide, isomannide and isoidide) and the diacetalized monomers 2,3:4,5-di-O- obtained by decreasing the concentration of reactants in the reaction media. Meanwhile, it was found methylene-galactaric acid (Glux diacid) and 2,4:3,5-di-O-methylene-D-glucitol (Glux diol), are rigid that the suitable solvents for the enzymatic polymerization were cyclohexane, cyclohexane/benzene compounds which are derived from sugars. They are good candidates for polyester synthesis, rendering polyesters with high values of Tg and better thermal stability. Isosorbide was enzymatically polymerized with various aliphatic diacid ethyl esters in the presence of N435 (Scheme 48), as reported by Catalani et al. [292]. The solvent-free enzymatic

polymerization gave low molecular weight poly(isosorbide adipate) ( ≤ 3800 g/mol), as the hydroxyl groups of isosorbide can be condensed by N435. However, high molecular weight

isosorbide polyesters with ’s of up to 40,000 g/mol were produced via the enzymatic polymerization by azeotropic distillation; and significantly higher molecular weight products can be obtained by decreasing the concentration of reactants in the reaction media. Meanwhile, it was found that the suitable solvents for the enzymatic polymerization were cyclohexane, cyclohexane/benzene

Polymers 2016, 8, 243 34 of 53

8.5.2. Polyesters Based on Rigid Sugar Derivatives DAHs (isosorbide, isomannide and isoidide) and the diacetalized monomers 2,3:4,5-di-O- methylene-galactaric acid (Glux diacid) and 2,4:3,5-di-O-methylene-D-glucitol (Glux diol), are rigid compounds which are derived from sugars. They are good candidates for polyester synthesis, rendering polyesters with high values of Tg and better thermal stability. Isosorbide was enzymatically polymerized with various aliphatic diacid ethyl esters in the presence of N435 (Scheme 48), as reported by Catalani et al. [292]. The solvent-free enzymatic polymerization gave low molecular weight poly(isosorbide adipate) (Mw ď 3800 g/mol), as the hydroxyl groups of isosorbide can be condensed by N435. However, high molecular weight isosorbide polyesters with Mw’s of up to 40,000 g/mol were produced via the enzymatic polymerization by azeotropic distillation; and signiﬁcantly higher molecular weight products can be obtained by Polymers 2016, 8, 243 34 of 52 decreasing the concentration of reactants in the reaction media. Meanwhile, it was found that the(6:1, suitablev/v) and solventscyclohexane/toluene for the enzymatic (6:1, v/v polymerization). Furthermore, werethe enzymatic cyclohexane, azeotropic cyclohexane/benzene polymerization (6:1, v/v) and cyclohexane/toluene (6:1, v/v). Furthermore, the enzymatic azeotropic polymerization in in cyclohexane/toluene gave isosorbide polyesters with a higher when the chain length of the cyclohexane/toluenetested aliphatic diacid gave ethyl isosorbide esters increased polyesters from with C4 ato higher C6. However,Mw when by the further chain increasing length of thethe testedchain aliphaticlength from diacid C6 ethyl to C8, esters C10 increased and 12, from isosorbide C4 to C6. polyesters However, with by further lowerincreasing molecular the weights chainlength were fromproduced. C6 to C8, C10 and 12, isosorbide polyesters with lower molecular weights were produced.

Scheme 48. N435-catalyzed synthesis of isosorbide polyesters [292]. Scheme 48. N435-catalyzed synthesis of isosorbide polyesters [292].

The same research group also investigated the N435-catalyzed azeotropic polycondensation of isosorbide/isomannideThe same research and group diethyl also adipate investigated with diff theerent N435-catalyzed fractions of azeotropicunsaturated polycondensation diesters (Scheme of49) isosorbide/isomannide[293]. No homo-polyesters and and diethyl co-polyesters adipate withcontaining different itaconate fractions units of were unsaturated obtained diestersvia the (Schemeenzymatic 49 )[polymerization.293]. No homo-polyesters However, and highco-polyesters molecular containing weight itaconateisosorbide/isomanide-based units were obtained viaunsaturated the enzymatic co-polyesters polymerization. were produced However, from high the molecularenzymatic weightazeotropic isosorbide/isomanide-based polymerization with the unsaturated co-polyesters were produced from the enzymatic azeotropic polymerization with the other other tested unsaturated diesters, with ’s of up to 15,900 g/mol. Moreover, Michael additions of testedwater to unsaturated C=C bonds diesters, occurred with duringMw the’s of enzymati up to 15,900c polymerization; g/mol. Moreover, and the Michael corresponding additions polymers of water tocan C=C undergo bonds occurredadditional during reactions the enzymatic through polymerization;hydroxyl pendant and groups. the corresponding Besides, the polymers enzymatic can undergopolymerization additional involving reactions isosorbide through hydroxyl gave pendantmuch higher groups. molecular Besides, the weights enzymatic products, polymerization which involvingsuggested isosorbide that N435 gaveprefers much isosorbide higher molecular over isomanni weightsde. products, This is however which suggested in contract that with N435 the prefers result isosorbidereported by over Boeriu isomannide. et al. [294]. This They is howeverinvestigated in contract the N435-catal with theyzed result polymerization reported by Boeriu of succinic et al. [ 294acid]. Theywith investigatedisomannide, theisosorbide N435-catalyzed or isoidide polymerization in toluene/tert of succinic-butanol; acid and with found isomannide, that N435 isosorbide showed orpreference isoidide for in toluene/isomannidetert -butanol;over isosorbide and found and over that N435isoidide. showed They preferenceattributed this for isomannideto the preference over isosorbideof N435 for and the overendo-hydroxyl isoidide. groups, They attributed which is thisdue toto thethe preferencefact that the of transition N435 for state the endo of esters-hydroxyl with groups,exo-hydroxyl which groups is due does to the not fact form that all the the transition required state hydrogen of esters bonds with forex o-hydroxylcatalysis. groups does not form all the required hydrogen bonds for catalysis.

Scheme 49. N435-catalyzed azeotropic polycondensation of isosorbide/isomannide, diethyl adipate and fractions of different unsaturated diesters.

Polymers 2016, 8, 243 34 of 52

(6:1, v/v) and cyclohexane/toluene (6:1, v/v). Furthermore, the enzymatic azeotropic polymerization in cyclohexane/toluene gave isosorbide polyesters with a higher when the chain length of the tested aliphatic diacid ethyl esters increased from C4 to C6. However, by further increasing the chain length from C6 to C8, C10 and 12, isosorbide polyesters with lower molecular weights were produced.

Scheme 48. N435-catalyzed synthesis of isosorbide polyesters [292].

The same research group also investigated the N435-catalyzed azeotropic polycondensation of isosorbide/isomannide and diethyl adipate with different fractions of unsaturated diesters (Scheme 49) [293]. No homo-polyesters and co-polyesters containing itaconate units were obtained via the enzymatic polymerization. However, high molecular weight isosorbide/isomanide-based unsaturated co-polyesters were produced from the enzymatic azeotropic polymerization with the other tested unsaturated diesters, with ’s of up to 15,900 g/mol. Moreover, Michael additions of water to C=C bonds occurred during the enzymatic polymerization; and the corresponding polymers can undergo additional reactions through hydroxyl pendant groups. Besides, the enzymatic polymerization involving isosorbide gave much higher molecular weights products, which suggested that N435 prefers isosorbide over isomannide. This is however in contract with the result reported by Boeriu et al. [294]. They investigated the N435-catalyzed polymerization of succinic acid with isomannide, isosorbide or isoidide in toluene/tert-butanol; and found that N435 showed preference for isomannide over isosorbide and over isoidide. They attributed this to the preference of N435 for the endo-hydroxyl groups, which is due to the fact that the transition state of esters with Polymers 2016, 8, 243 35 of 53 exo-hydroxyl groups does not form all the required hydrogen bonds for catalysis.

Scheme 49.49. N435-catalyzedN435-catalyzed azeotropicazeotropic polycondensationpolycondensation ofof isosorbide/isomannide,isosorbide/isomannide, diethyl adipate and fractions ofof differentdifferent unsaturatedunsaturated diesters.diesters. Polymers 2016, 8, 243 35 of 52 A series of Glux diacid- and Glux diol-based polyesters were synthesized via the N435-catalyzed polycondensationA series of Glux of diacid- diethyl and sebacate Glux diol-based and 1,4-BDO polyesters with Glux were diacid/Glux synthesized diolvia the (Scheme N435-catalyzed 50)[295]. polycondensation of diethyl sebacate and 1,4-BDO with Glux diacid/Glux diol (Scheme 50) [295]. The The Mw, reaction yield and intrinsic viscosity of the resulting polyesters were around 10,000 g/mol, 30%–70%,, reaction and yield 0.3–0.44 and intrinsic dL/g, respectively. viscosity of Inthe addition, resulting the polyesters molecular were weights around and 10,000 reaction g/mol, yields 30%– of 70%,the obtained and 0.3–0.44 Glux dL/g, diacid-based respectively. polyesters In addition, were lower the molecular than those weights of the and synthetic reaction Glux yields diol-based of the obtainedpolyesters; Glux and diacid-based these two parameters polyesters decreased were lowe monotonicallyr than those with of the increasing synthetic the Glux Glux diol-based content in polyesters;both Glux diacid-and these and two Glux parameters diol-based decreased polyesters. mono Moreover,tonically nowith polyester increasing was the obtained Glux content from the in bothenzymatic Glux diacid- polymerization and Glux of diol-based Glux diacid polyesters. and 1,4-BDO. Moreover, This can no bepolyester explained was by obtained the bulky from bicyclic the enzymaticstructure next polymerization to the carboxylate of Glux groups, diacid which and 1,4-BDO. hinders the This access can ofbe this explained group toby the the active bulky site bicyclic of the structureenzyme CALB. next to Furthermore, the carboxylate all thegroups, synthetic which polyesters hinders possessthe access random of this microstructures. group to the active site of the enzyme CALB. Furthermore, all the synthetic polyesters possess random microstructures.

SchemeScheme 50. 50. N435-catalyzedN435-catalyzed synthesis synthesis of of polyesters polyesters cont containingaining glucitylene or glucarate units [295]. [295].

8.6. Biobased Polyamides At present, studies related to the enzymatic synthesis of biobased synthetic polyamides are scarcer. A few potentially biobased aliphatic polyamides, such as nylon 4,10, nylon 6,10, and nylon 8,10, can be synthesized via lipase-catalyzed polymerization. However, the molecular weights of the obtained polyamides were quite low. Landfester et al. [197] studied the N435-catalyzed polycondensation of diethyl sebacate and 1,8- octanediamine (Scheme 51). The enzymatic polymerization gave nylon 8,10 with ’s of around 2000–5000 g/mol.

Scheme 51. N435-catalyzed synthesis of nylon 8,10 from polycondensation of diethyl sebacate and 1,8-octanediamine [197].

In our group, oligomers including nylon 4,10, nylon 6,10, and nylon 8,10 were produced via the lipase-catalyzed polymerization of diethyl sebacate with different diamines, with a of up to 16 (Scheme 52) [235].

Polymers 2016, 8, 243 35 of 52

A series of Glux diacid- and Glux diol-based polyesters were synthesized via the N435-catalyzed polycondensation of diethyl sebacate and 1,4-BDO with Glux diacid/Glux diol (Scheme 50) [295]. The

, reaction yield and intrinsic viscosity of the resulting polyesters were around 10,000 g/mol, 30%– 70%, and 0.3–0.44 dL/g, respectively. In addition, the molecular weights and reaction yields of the obtained Glux diacid-based polyesters were lower than those of the synthetic Glux diol-based polyesters; and these two parameters decreased monotonically with increasing the Glux content in both Glux diacid- and Glux diol-based polyesters. Moreover, no polyester was obtained from the enzymatic polymerization of Glux diacid and 1,4-BDO. This can be explained by the bulky bicyclic structure next to the carboxylate groups, which hinders the access of this group to the active site of the enzyme CALB. Furthermore, all the synthetic polyesters possess random microstructures.

Polymers 2016, 8, 243 36 of 53 Scheme 50. N435-catalyzed synthesis of polyesters containing glucitylene or glucarate units [295].

8.6. Biobased Polyamides At present,present, studiesstudies related related to to the the enzymatic enzymatic synthesis synthesis of biobased of biobased synthetic synthetic polyamides polyamides are scarcer. are Ascarcer. few potentially A few potentially biobased biobased aliphatic aliphatic polyamides, poly suchamides, as nylonsuch as 4,10, nylon nylon 4,10, 6,10, nylon and 6,10, nylon and 8,10, nylon can 8,10,be synthesized can be synthesized via lipase-catalyzed via lipase-catalyzed polymerization. polymerization. However, However, the molecular the molecular weights of weights the obtained of the polyamidesobtained polyamides were quite were low. quite low. Landfester et et al. al. [197] [197 ]studied studied the the N435-catalyzed N435-catalyzed polycond polycondensationensation of diethyl of diethyl sebacate sebacate and and1,8- 1,8-octanediamineoctanediamine (Scheme (Scheme 51). 51 The). The enzymatic enzymatic polymerization polymerization gave gave nylon nylon 8,10 8,10 with with Mn’s’s of of around 2000–5000 g/mol.

Scheme 51. N435-catalyzed synthesis of nylon 8,10 from polycondensation of diethyl sebacate and 1,8-octanediamine [[197].197].

In our group, oligomers including nylon 4,10, nylon 6,10, and nylon 8,10 were produced via the In our group, oligomers including nylon 4,10, nylon 6,10, and nylon 8,10 were produced via the lipase-catalyzed polymerization of diethyl sebacate with different diamines, with a of up to lipase-catalyzed polymerization of diethyl sebacate with different diamines, with a DPmax of up to 16 16 (Scheme 52) [235]. (SchemePolymers 2016 52,)[ 8, 235243 ]. 36 of 52

O O O O O Lipases H H Diethyl sebacate N N Method 1. One-stage, toluene m n o O H N NH Molecular sieves, 70 C, N2,96h 2 m 2 Oligoamides Method 2. Two-stage, diphenyl ether, molecular sieves Diamine (a) 70 oC, 20 h, 500 mmHg; m = 2, 4, 6 (b) 70 oC, 24 h, 100 mmHg

Scheme 52. Lipase-catalyzed polycondensation of diethyl sebacate and diamines.

8.7. Biobased Furan Polyesters and Furan Polyamides Furan polymers polymers are are not not new new polymers. polymers. In the In thelate late1970s, 1970s, poly(hexamethylene poly(hexamethylene furanoate), furanoate), a furan a polyester,furan polyester, was synthesized was synthesized by Moore by Moore and and Kelly Kelly [296,297]; [296,297 ];and and various various furan furan polyesters polyesters were successfully prepared by Ballauff etet al. [298], [298], Gandini et al. al. [299], [299], and and Okada Okada et et al. al. [300] [300] since since 1990s. 1990s. In recentrecent years,years, thethe research research on on FDCA-based FDCA-based polyesters polyesters and and polyamides polyamides is booming, is booming, due due to the to fastthe fastdevelopment development of biobased of biobased FDCA FDCA and and the broadthe broad potential potential applications applications of FDCA-based of FDCA-based polymers polymers [16]. The[16]. FDCA-basedThe FDCA-based polymers polymers are promising are promising sustainable sustainable aromatic polymer aromatic alternatives, polymer alternatives, and FDCA-based and polymersFDCA-based possess polymers similar possess or even similar better or properties even bette thanr properties their petrol-base than their counterparts. petrol-base counterparts. For example, Forrecent example, studies recent suggested studies that suggested poly(ethylene that poly furanoate)(ethylene (PEF) furanoate) possesses (PEF) better possesses barrier better properties barrier properties compared to PET: PEF shows surprisingly large reductions in CO2 permeability (19×), O2 compared to PET: PEF shows surprisingly large reductions in CO2 permeability (19ˆ), O2 permeability (11permeabilityˆ) and diffusivity (11×) and (31 diffusivityˆ)[301,302 (31×)]. [301,302]. At present, FDCA-based polyesters and polyamidespolyamides are predominately synthesized via melt polycondensation atat elevated elevated temperatures temperatures of of around around 200 200˝C. °C. However, However, decarboxylation decarboxylation of FDCA of FDCA takes placetakes atplace around at around 195 ˝C and195 other°C and side-reactions other side-reactions may occur may at such occur elevated at such temperatures elevated temperatures [16,303–305], which[16,303–305], may lead which to themay discoloration lead to the discoloration of the resulting of the polymers resulting and polymers the formation and the offormation low molecular of low weightmolecular products. weight However, products. these However, drawbacks these coulddraw bebacks circumvented could be circumvented by using enzyme by using catalysts. enzyme catalysts. The N435-catalyzed polymerization with dimethyl 2,5-furandicarboxylate (dimethyl FDCA)/BHMF/5-hydroxymethyl-2-furancarboxylic acid (HMFA) was reported by Habeych N. [306] and Boeriu et al. [307], using a one-stage method in the mixture of toluene and tert-butanol. However, only a mixture of linear and cyclic furan oligomers were produced (Scheme 53). Recently, we studied the N435-catalyzed polymerization of BHMF and various diacid ethyl esters, using the two-stage, three step method (Scheme 54) [308]. BHMF-based polyesters with low molecular weights were produced, with ’s of around 1800–2900 g/mol. The polymerization kinetic study and MALDI-ToF MS analysis revealed that ether end groups were formed during the enzymatic polymerization, which led to the low molecular weights. FDCA-based furanic-aliphatic polyesters were successfully produced via the enzymatic polymerization of dimethyl FDCA with various aliphatic diols, using a two-stage method in diphenyl ether at 80–140 °C (Scheme 55) [309]. The obtained polyesters reached a very high of up to 100,000 g/mol, which is normally hard to achieve by enzymatic polymerization. For the first time we demonstrated that enzymatic polymerizations are capable of producing high molecular weight FDCA-based polyesters, which have been primarily synthesized via step-growth polymerization using organometallic catalysts at elevated temperatures around 150–280 °C. Moreover, we found that CALB prefers alkane-α,ω-aliphatic linear diols of > 3 carbons. Furthermore, the FDCA-based furanic- aliphatic polyesters possess similar crystalline and thermal properties compared to their petrol-based counterparts, semi-aromatic polyesters.

Polymers 2016, 8, 243 37 of 53

The N435-catalyzed polymerization with dimethyl 2,5-furandicarboxylate (dimethyl FDCA)/ BHMF/5-hydroxymethyl-2-furancarboxylic acid (HMFA) was reported by Habeych N. [306] and Boeriu et al. [307], using a one-stage method in the mixture of toluene and tert-butanol. However, only Polymersa mixture 2016 of, 8, linear 243 and cyclic furan oligomers were produced (Scheme 53). 37 of 52

Polymers 2016, 8, 243 37 of 52

Scheme 53. N435-catalyzedN435-catalyzed polymerizations polymerizations with furanfuran monomers:monomers: bis(hydroxymethyl)furan (BHMF),(BHMF), 5-hydroxymethyl-2-furancarboxylic 5-hydroxymethyl-2-furancarboxylic acid acid (H (HMFA)MFA) and dimethyl 2,5-Furandicarboxylic acid (FDCA)(FDCA) [306,307]. [306,307].

Recently, we studied the N435-catalyzed polymerization of BHMF and various diacid ethyl esters, using the two-stage, three step method (Scheme 54)[308]. BHMF-based polyesters with low molecular weightsScheme were 53. produced, N435-catalyzed with M wpolymerizations’s of around 1800–2900 with furan g/mol. monomers: The polymerization bis(hydroxymethyl)furan kinetic study and MALDI-ToF(BHMF), 5-hydroxymethyl-2-furancarboxylic MS analysis revealed that etheracid (H endMFA) groups and dimethyl were formed 2,5-Furandicarboxylic during the enzymatic acid polymerization,(FDCA) [306,307]. which led to the low molecular weights.

Scheme 54. N435-catalyzed polycondensation of BHMF and diacid ethyl esters via a two-stage, three step method in diphenyl ether at 80 °C [308].

Scheme 54. N435-catalyzed polycondensation of BHMF and diacid ethyl esters via a two-stage, three step method in diphenyl ether at 80 ˝°CC[ [308].308].

FDCA-based furanic-aliphatic polyesters were successfully produced via the enzymatic polymerization of dimethyl FDCA with various aliphatic diols, using a two-stage method in diphenyl ˝ ether at 80–140 C (Scheme 55)[309]. The obtained polyesters reached a very high Mw of up to 100,000Scheme g/mol, 55. N435-catalyzed which is normally polycondensation hard to achieve of dimethyl by enzymatic FDCA and polymerization. aliphatic diol via For a two-stage the ﬁrst time we demonstratedmethod in diphenyl that enzymaticether [309]. polymerizations are capable of producing high molecular weight FDCA-based polyesters, which have been primarily synthesized via step-growth polymerization usingFurthermore, organometallic high catalysts molecular at weight elevated FDCA-based temperatures furanic-aliphatic around 150–280 polyamides˝C. Moreover, were weproduced found fromthat CALBthe enzymatic prefers alkane-polycondensationα,ω-aliphatic of dimethyl linear diols FDCA of > and 3 carbons. 1,8-ODA, Furthermore, using a one-stage the FDCA-based method or afuranic-aliphatic temperature-varied polyesters two-stage possess method similar (Scheme crystalline 56) and[310]. thermal The FDCA-based properties compared furanic-aliphatic to their polyamidespetrol-based can counterparts, be used as semi-aromatic a promising sustainabl polyesters.e alternatives to petrol-based polyphthalamides Scheme 55. N435-catalyzed polycondensation of dimethyl FDCA and aliphatic diol via a two-stage (semi-aromatic polyamides) and be applied as thermoplastic engineering polymers and high method in diphenyl ether [309]. performance materials. The enzymatic polymerization resulted in poly(octamethylene furanamide) (PA Furthermore,8,F) with a very high highmolecular of weight up to FDCA-based 54,000 g/mol. furanic-aliphatic This is the first polyamides time that were FDCA-based produced polyamidesfrom the enzymatic are successfully polycondensation produced of via dimethyl enzymatic FDCA polymerization; and 1,8-ODA, and using the a molecular one-stage weightsmethod ofor thea temperature-varied obtained PA 8,F are two-stage much higher method than those(Scheme produced 56) [310]. via melt-polycondensation, The FDCA-based furanic-aliphatic the primarily synthesispolyamides approach can be usedfor semi-aromatic as a promising polyamides, sustainabl eat alternatives elevated temperatures to petrol-based usually polyphthalamides above 200 °C. (semi-aromatic polyamides) and be applied as thermoplastic engineering polymers and high performance materials. The enzymatic polymerization resulted in poly(octamethylene furanamide)

(PA 8,F) with a very high of up to 54,000 g/mol. This is the first time that FDCA-based polyamides are successfully produced via enzymatic polymerization; and the molecular weights of the obtained PA 8,F are much higher than those produced via melt-polycondensation, the primarily synthesis approach for semi-aromatic polyamides, at elevated temperatures usually above 200 °C.

Polymers 2016, 8, 243 37 of 52

Scheme 53. N435-catalyzed polymerizations with furan monomers: bis(hydroxymethyl)furan (BHMF), 5-hydroxymethyl-2-furancarboxylic acid (HMFA) and dimethyl 2,5-Furandicarboxylic acid (FDCA) [306,307].

Scheme 54. N435-catalyzed polycondensation of BHMF and diacid ethyl esters via a two-stage, three Polymers 2016, 8, 243 38 of 53 step method in diphenyl ether at 80 °C [308].

SchemeScheme 55. N435-catalyzed polycondensation polycondensation of of dimethyl dimethyl FDCA FDCA and and aliphatic aliphatic diol diol via via a a two-stage methodmethod in in diphenyl diphenyl ether ether [309]. [309].

Furthermore,Furthermore, high high molecular molecular weight weight FDCA-based FDCA-based furanic-aliphatic furanic-aliphatic polyamides polyamides were were produced produced fromfrom the the enzymatic enzymatic polycondensation polycondensation of of dimethyl dimethyl FDCA FDCA and and 1,8-ODA, 1,8-ODA, using using a one-stage a one-stage method method or aor temperature-varied a temperature-varied two-stage two-stage method method (Scheme (Scheme 56) 56 )[[310].310]. The The FDCA-based FDCA-based furanic-aliphatic polyamidespolyamides can can be be used as a promising sustainablesustainable alternatives to petrol-based polyphthalamides (semi-aromatic(semi-aromatic polyamides) polyamides) and and be be applied applied as as thermoplastic engineering engineering polymers polymers and and high high performanceperformance materials. materials. The The enzyma enzymatictic polymerization polymerization resulted resulted in in poly(octamethylene poly(octamethylene furanamide) furanamide) (PA 8,F) with a very high of up to 54,000 g/mol. This is the first time that FDCA-based (PA 8,F) with a very high Mw of up to 54,000 g/mol. This is the ﬁrst time that FDCA-based polyamides polyamides are successfully produced via enzymatic polymerization; and the molecular weights of arePolymers successfully 2016, 8, 243produced via enzymatic polymerization; and the molecular weights of the obtained38 of 52 thePA obtained 8,F are much PA 8,F higher are much than higher those producedthan thosevia produced melt-polycondensation, via melt-polycondensation, the primarily the primarily synthesis synthesis approach for semi-aromatic polyamides, at elevated temperatures usually˝ above 200 °C. approachMoreover, for the semi-aromatic obtained PA polyamides,8,F possesses at a elevatedsimilar T temperaturesg and similar usuallycrystal structures, above 200 aC. comparable Moreover, theTd, obtained PA 8,F possesses a similar T and similar crystal structures, a comparable T , but a lower T , but a lower Tm, compared to its petrol-basedg counterpart, poly(octamethylene terephthalamide)d (PAm compared8,T). to its petrol-based counterpart, poly(octamethylene terephthalamide) (PA 8,T).

Scheme 56. N435-catalyzedN435-catalyzed polycondensation polycondensation of ofdimeth dimethylyl FDCA FDCA and and1,8-octanediamine 1,8-octanediamine via a via one- a one-stagestage method method in toluene in toluene and anda temperature a temperature varied varied two-stage two-stage method method in diphenyl in diphenyl ether ether [310]. [310 ].

9. Conclusions Conclusions and and Outlook Outlook Enzymatic polymerizationpolymerization is is proven proven to to be be a a powerful powerful and and versatile versatile approach approach for for the the production production of biobasedof biobased polyesters polyesters and and polyamides polyamides with differentwith different chemical chemical compositions compositions (aliphatic (aliphatic and semi-aromatic and semi- polymers),aromatic polymers), varied architectures varied architectures (linear, branched(linear, branched and hyperbranched and hyperbranched polymers), polymers), and diverse and functionalitiesdiverse functionalities (pendant (pendant hydroxyl hydroxyl groups, groups, carbon–carbon carbon–carbon double double bonds, bonds, epoxy groups,epoxy groups, and so and on). Amongso on). theAmong enzymes the enzymes studied forstudied biobased for polyesterbiobased polyester and polyamide and polyamide synthesis, synthesis, CALB, especially CALB, itsespecially immobilized its immobilized form N435, form shows N435, broad shows monomer broad monomer adaptability, adaptability, stable and stable excellent and excellent catalytic performance,catalytic performance, and great and tolerance great oftolerance various of conditions. various conditions. Moreover, Moreover, with the mild with synthetic the mild conditions, synthetic non-toxicconditions, and non-toxic renewable and enzyme renewable catalysts, enzyme and catalyst sustainables, and startingsustainable materials, starting synthesis materials, of synthesis biobased polymersof biobased via polymers enzymatic via polymerizations enzymatic polymerizations provides an opportunity provides an for opportunity achieving greenfor achieving polymers green and apolymers future sustainable and a future polymer sustainable industry, polymer which industry, will eventually which will play eventually an essential play role an foressential realizing role and for maintainingrealizing and a maintaining green and sustainable a green and society. sustainable society. However, thisthis approachapproach alsoalso possessespossesses somesome limitationslimitations andand disadvantages:disadvantages: (1) the atom efficiency is low when ester derivatives rather than acids are used; (2) non-ecofriendly solvents including diphenyl ether and toluene are commonly used; (3) long polymerization times are required for achieving high molecular weights; (4) high reaction temperatures at around 100–140 °C were applied for enzymatic synthesis of polymers having a high Tm and low solubility; and the catalytic reactivity of enzymes decreases significantly at such elevated temperatures; (5) the price of enzyme catalysts is still quite high; (6) enzymatic polymerizations involving monomers with short chain length like 1,3-propanediol, monomers with secondary hydroxyl groups such as isosorbide and 2,3-butanediol, and polyols, generally result in low molecular weight products; (7) the purify and price of biobased monomers remain a concern; (8) last but certainly not least, only limited variety of biobased monomers are currently commercially available. Therefore, more efforts are required to address these problems. For example, acids can be used to improve the atom efficiency, green solvents such as ionic liquids and supercritical CO2 can be employed as the reaction media, more robust and thermal stable enzymes should be developed for enzymatic polymerizations, improved and optimal processes should be explored for the production of diverse biobased monomers with high purity and low price, and so on. Although numerous polyesters and polyamides are readily produced by using free lipases and immobilized lipases as the catalysts, the explanations for the different polymerization results are not clear yet. This could be an interest topic for the future research. Noteworthy is that many experimental results reveal that lipase-catalyzed polymerizations involving structurally similar monomers afford polymers with different compositions and varied molecular weights. This could be attributed to the synergistic effect caused by many reasons, for example, the specificity and selectivity of lipases towards different monomers, the physical

Polymers 2016, 8, 243 39 of 53

(1) the atom efﬁciency is low when ester derivatives rather than acids are used; (2) non-ecofriendly solvents including diphenyl ether and toluene are commonly used; (3) long polymerization times are required for achieving high molecular weights; (4) high reaction temperatures at around 100–140 ˝C were applied for enzymatic synthesis of polymers having a high Tm and low solubility; and the catalytic reactivity of enzymes decreases signiﬁcantly at such elevated temperatures; (5) the price of enzyme catalysts is still quite high; (6) enzymatic polymerizations involving monomers with short chain length like 1,3-propanediol, monomers with secondary hydroxyl groups such as isosorbide and 2,3-butanediol, and polyols, generally result in low molecular weight products; (7) the purify and price of biobased monomers remain a concern; (8) last but certainly not least, only limited variety of biobased monomers are currently commercially available.

Therefore, more efforts are required to address these problems. For example, acids can be used to improve the atom efficiency, green solvents such as ionic liquids and supercritical CO2 can be employed as the reaction media, more robust and thermal stable enzymes should be developed for enzymatic polymerizations, improved and optimal processes should be explored for the production of diverse biobased monomers with high purity and low price, and so on. Although numerous polyesters and polyamides are readily produced by using free lipases and immobilized lipases as the catalysts, the explanations for the different polymerization results are not clear yet. This could be an interest topic for the future research. Noteworthy is that many experimental results reveal that lipase-catalyzed polymerizations involving structurally similar monomers afford polymers with different compositions and varied molecular weights. This could be attributed to the synergistic effect caused by many reasons, for example, the specificity and selectivity of lipases towards different monomers, the physical properties of the starting materials (purity, melting temperature, and miscibility and solubility in the reaction media), the physical properties of the resulting intermediates and the final products (glass transition temperature, melting temperature, crystallization ability, and miscibility and solubility in the reaction media), the enzymatic polymerization conditions, and so on. However, such synergistic effect has not been fully understood yet, which requires systematic studies in the future. Besides, it would be of great interest to employ computer simulations to study the specificity and selectivity of lipases for the monomers in the enzymatic polymerization, as well as, the enzymatic polymerization mechanism. At present, enzymatic polymerizations have already been poised for use in commercial process to prepare polymers targeted for cosmetic and medical applications. However, polymers including biobased polymers are still predominately produced via conventional approaches. Due to the fast development of biotechnologies and enzymatic polymerization techniques, and the increased realization of the great benefits that enzymatic polymerizations and biobased monomers have to offer, there will be more highly value-added specialty biobased polymers produced commercially via biocatalytic approach in the near future. However, for the production of biobased commodity polymers, engineering plastics and high performance polymers, the commercial enzymatic process is promising but still has a long way to go, considering the high efficiency and low cost of the current pathways to the petrol-based counterparts.

Acknowledgments: This research forms part of the research programme of the Dutch Polymer Institute (DPI), project #727c polymers go even greener. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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