Biodegradation of polyesters containing aromatic constituents

Polymers, which undergo a controlled biological degradation by micro-organisms came to remarkable interest during the last years. Composting for instance could so be established as an alternative waste management system for parts of the plastic waste. Within this group of innovative polymer, polyesters play a predominant role, due to their potentially hydrolyzable ester bonds. While aromatic polyesters such as poly(ethylene terephthalate) exhibit excellent material properties but proved to be almost resistant to microbial attack, many aliphatic polyesters turned out to be biodegradable but lack in properties, which are important for application. To combine good material properties with biodegradability, aliphatic-aromatic copolyesters have been developed as biodegradable polymers for many years. This article reviews the attempts to combine aromatic and aliphatic structures in biodegradable plastics and work, which has been done to evaluate the degradation behaviour and environmental safety of biodegradable polyesters, containing aromatic constituents.     

Biodegradation of microbial and synthetic polyesters by fungi

A variety of biodegradable polyesters have been developed in order to obtain useful biomaterials and to reduce the impact of environmental pollution caused by the large-scale accumulation of non-degradable waste plastics. Polyhydroxyalkanoates, poly(epsilon-caprolactone), poly( l-lactide), and both aliphatic and aromatic polyalkylene dicarboxylic acids are examples of biodegradable polyesters. In general, most aliphatic polyesters are readily mineralized by a number of aerobic and anaerobic microorganisms that are widely distributed in nature. However, aromatic polyesters are more resistant to microbial attack than aliphatic polyesters. The fungal biomass in soils generally exceeds the bacterial biomass and thus it is likely that fungi may play a considerable role in degrading polyesters, just as they predominantly perform the decomposition of organic matter in the soil ecosystem. However, in contrast to bacterial polyester degradation, which has been extensively investigated, the microbiological and environmental aspects of fungal degradation of polyesters are unclear. This review reports recent advances in our knowledge of the fungal degradation of microbial and synthetic polyesters and discusses the ecological importance and contribution of fungi in the biological recycling of waste polymeric materials in the biosphere.     

Lipase/esterase-catalyzed ring-opening polymerization: A green polyester synthesis technique

In the last decade, there has been increased interest in lipase/esterase-catalyzed ring-opening polymerization as an alternative to metal-based catalytic processes. This review focuses on three components in the reaction system, namely biocatalysts, reaction medium and monomers. Novel lipases or esterases are described with particular emphasis on, those derived from thermophiles, immobilized enzymes and recombinant whole-cell biocatalysts. Green solvents in enzymatic ring-opening polymerization, including water, ionic liquids, supercritical carbon dioxide and hydrofluorocarbon solvents, are also discussed. Enzymatic ring-opening polymerization is reviewed with regard to the variety of polymers obtainable, such as polyesters, polycarbonates, polyphosphates and polythioesters. Among these, enzymatic synthesis of polyesters has been most widely investigated, and is discussed for lactones with small to large ring sizes. Finally, the mechanism of enzymatic ring-opening polymerization is described, which is generally accepted as a monomer-activated mechanism. Overall, the review demonstrates that lipase/esterase-catalyzed synthesis of polymers via ring-opening polymerization provides an effective platform for conducting “green polymer chemistry”.     

Systems Metabolic Engineering Strategies for Non‐Natural Microbial Polyester Production

Plastics, used everyday, are mostly synthetic polymers derived from fossil resources, and their accumulation is becoming a serious concern worldwide. Polyhydroxyalkanoates (PHAs) are naturally produced polyesters synthesized and intracellularly accumulated by many different microorganisms. PHAs are good alternatives to petroleum‐based plastics because they possess a wide range of material properties depending on monomer types and molecular weights. In addition, PHAs are biodegradable and can be produced from renewable biomass. Thus, producing PHAs through the development of high‐performance engineered microorganisms and efficient bioprocesses gained much interest. In addition, non‐natural polyesters comprising 2‐hydroxycarboxylic acids as monomers have been produced by fermentation of metabolically engineered bacteria. For example, poly(lactic acid) and poly(lactic acid‐co‐glycolic acid), which have been chemically synthesized using the corresponding monomers either fermentatively or chemically produced, can be produced by metabolically engineered bacteria by one‐step fermentation. Recently, PHAs containing aromatic monomers could be produced by fermentation of metabolically engineered bacteria. Here, metabolic engineering strategies applied in developing microbial strains capable of producing non‐natural polyesters in a stepwise manner are reviewed. It is hoped that the detailed strategies described will be helpful for designing metabolic engineering strategies for developing diverse microbial strains capable of producing various polymers that can replace petroleum‐derived polymers.     

Enzymatic synthesis and curing of biodegradable epoxide-containing polyesters from renewable resources

Epoxide-containing polyesters were enzymatically synthesized via two routes using unsaturated fatty acids as starting substrate. Lipase catalysis was used for both polycondensation and epoxidation of the unsaturated fatty acid group. One route was synthesis of aliphatic polyesters containing an unsaturated group in the side chain from divinyl sebacate, glycerol, and the unsaturated fatty acids, followed by an epoxidation of the unsaturated fatty acid moiety in the side chain of the resulting polymer. In another route, epoxidized fatty acids were prepared from the unsaturated fatty acids and hydrogen peroxide in the presence of lipase catalyst, and subsequently the epoxidized fatty acids were polymerized with divinyl sebacate and glycerol. The polymer structure was confirmed by NMR and IR, and for both routes, the high epoxidized ratio was achieved. Curing of the resulting polymers proceeded thermally, yielding transparent polymeric films with high gloss surface. Pencil scratch hardness of the present films improved, compared with that of the cured film obtained from the polyester having an unsaturated fatty acid in the side chain. The obtained film showed good biodegradability, evaluated by BOD measurement in an activated sludge.     

Metabolic engineering for the synthesis of polyesters: A 100-year journey from polyhydroxyalkanoates to non-natural microbial polyesters

As concerns increase regarding sustainable industries and environmental pollutions caused by the accumulation of non-degradable plastic wastes, bio-based polymers, particularly biodegradable plastics, have attracted considerable attention as potential candidates for solving these problems by substituting petroleum-based plastics. Among these candidates, polyhydroxyalkanoates (PHAs), natural polyesters that are synthesized and accumulated in a range of microorganisms, are considered as promising biopolymers since they have biocompatibility, biodegradability, and material properties similar to those of commodity plastics. Accordingly, substantial efforts have been made to gain a better understanding of mechanisms related to the biosynthesis and properties of PHAs and to develop natural and recombinant microorganisms that can efficiently produce PHAs comprising desired monomers with high titer and productivity for industrial applications.Recent advances in biotechnology, including those related to evolutionary engineering, synthetic biology, and systems biology, can provide efficient and effective tools and strategies that reduce time, labor, and costs to develop microbial platform strains that produce desired chemicals and materials. Adopting these technologies in a systematic manner has enabled microbial fermentative production of non-natural polyesters such as poly(lactate) [PLA], poly(lactate-co-glycolate) [PLGA], and even polyesters consisting of aromatic monomers from renewable biomass-derived carbohydrates, which can be widely used in current chemical industries.In this review, we present an overview of strain development for the production of various important natural PHAs, which will give the reader an insight into the recent advances and provide indicators for the future direction of engineering microorganisms as plastic cell factories. On the basis of our current understanding of PHA biosynthesis systems, we discuss recent advances in the approaches adopted for strain development in the production of non-natural polyesters, notably 2-hydroxycarboxylic acid-containing polymers, with particular reference to systems metabolic engineering strategies.     

Combinatorial array-based enzymatic polyester synthesis.

A combinatorial strategy for biocatalytic polymer synthesis is demonstrated. A library of polymers was synthesized in 96 deep-well plates using AA-BB polycondensations of acyl donors and acceptors. The library was based on four straight-chain diesters as acyl donors (C(3)-C(10)) with aliphatic/aromatic diols as well as more diverse structures including carbohydrates, nucleic acids, and a natural steroid diol used as acyl acceptors. The lipase from Candida antarctica was active in acetonitrile and was capable of catalyzing the polycondensation of the aforementioned monomers to polymers with M(w)'s reaching as high as 20,000 Da, including the preparation of novel sugar-containing polyesters. The combinatorial approach to biocatalytic polymer synthesis described herein serves as a foundation for polymeric materials discovery by demonstrating that polymer arrays can be produced from structurally complex monomers.     

Genetically engineered Pseudomonas: a factory of new bioplastics with broad applications

New bioplastics containing aromatic or mixtures of aliphatic and aromatic monomers have been obtained using genetically engineered strains of Pseudomonas putida . The mutation (–) or deletion (Δ) of some of the genes involved in the β-oxidation pathway ( fad A⁻, fad B⁻Δ fad A or Δ fad BA mutants) elicits a strong intracellular accumulation of unusual homo- or co-polymers that dramatically alter the morphology of these bacteria, as more than 90% of the cytoplasm is occupied by these macromolecules. The introduction of a blockade in the β-oxidation pathway, or in other related catabolic routes, has allowed the synthesis of polymers other than those accumulated in the wild type (with regard to both monomer size and relative percentage), the accumulation of certain intermediates that are rapidly catabolized in the wild type and the accumulation in the culture broths of end catabolites that, as in the case of phenylacetic acid, phenylbutyric acid, trans -cinnamic acid or their derivatives, have important medical or pharmaceutical applications (antitumoral, analgesic, radiopotentiators, chemopreventive or antihelmintic). Furthermore, using one of these polyesters (poly 3-hydroxy-6-phenylhexanoate), we obtained polymeric microspheres that could be used as drug vehicles.     

Biochemical properties and biotechnological applications of microbial enzymes involved in the degradation of polyester-type plastics

Application of polyester-degrading enzymes should be considered as an eco-friendly alternative to chemical recycling due to the huge plastic waste disposal nowadays. Many hydrolases from several fungi and bacteria have been discovered and successfully evaluated for their activity towards different aliphatic polyesters (PHA, PBS, PBSA, PCL, PLA), aromatic polyesters (PET, PBT, PMT) as well as their co-polyesters (PBST, PBAT, PBSTIL). This revision gives an up-to-date overview on the main biochemical features and biotechnological applications of those reported enzymes which are able to degrade polyester-based plastics, including different microbial polyester depolymerases, esterases, cutinase-like enzymes and lipases. Summarized information includes available protein sequences with the corresponding accession numbers deposited in NCBI server, 3D resolved structures, and data about optimal conditions for enzymatic activity and stability of many of these microbial enzymes that would be helpful for researchers in this topic. Although screening and identification of new native polyester hydrolases from microbial sources is undeniable according to literature, we briefly highlight the importance of the design of improved enzymes towards recalcitrant aromatic polyesters through different approaches that include site-directed mutagenesis and surface protein engineering.     

Fermentative hydrogen production from low-value substrates

Hydrogen is a promising energy source that is believed to replace the conventional energy sources e.g. fossil fuels over years. Hydrogen production methods can be divided into conventional production methods which depend mainly on fossil fuels and alternative production methods including electrolysis of water, biophotolysis and fermentation hydrogen production from organic waste materials. Compared to the conventional methods, the alternative hydrogen production methods are less energy intensive and negative-value substrates i.e. waste materials can be used to produce hydrogen. Among the alternative methods, fermentation process including dark and photo-fermentation has gained more attention because these processes are simple, waste materials can be utilized, and high hydrogen yields can be achieved. The fermentation process is affected by several parameters such as type of inoculum, pH, temperature, substrate type and concentration, hydraulic retention time, etc. In order to achieve optimum hydrogen yields and maximum substrate degradation, the operating conditions of the fermentation process must be optimized. In this review, two routes for biohydrogen production as dark and photo-fermentation are discussed. Dark/photo-fermentation technology is a new approach that can be used to increase the hydrogen yield and improve the energy recovery from organic wastes.     

A high-throughput expression and screening platform for applications-driven PETase engineering

The environmental consequences of plastic waste have impacted all kingdoms of life in terrestrial and aquatic ecosystems. However, as the burden of plastic pollution has increased, microbes have evolved to utilize anthropogenic polymers as nutrient sources. Of depolymerase enzymes, the best characterized is PETase, which hydrolyzes aromatic polyesters. PETase engineering has made impressive progress in recent years; however, further optimization of engineered PETase toward industrial application has been limited by lower throughput techniques used in protein purification and activity detection. Here, we address these deficiencies through development of a higher-throughput PETase engineering platform. Secretory expression via YebF tagging eliminates lysis and purification steps, facilitating production of large mutant libraries. Fluorescent detection of degradation products permits rapid screening of depolymerase activity in microplates as opposed to serial chromatographic methods. This approach enabled development of more stable PETase, semi-rational (SR) PETase variant containing previously unpublished mutations. SR-PETase releases 1.9-fold more degradation products and has up to 7.4-fold higher activity than wild-type PETase over 10 days at 40°C. These methods can be adapted to a variety of chemical environments, enabling screening of PETase mutants in applications-relevant conditions. Overall, this work promises to facilitate advancements in PETase engineering toward industrial depolymerization of plastic waste.     

Economic and environmental assessment of automotive plastic waste end-of-life options: Energy recovery versus chemical recycling

Most automotive plastic waste (APW) is landfilled or used in energy recovery as it is unsuitable for high-quality product mechanical recycling. Chemical recycling via pyrolysis offers a pathway toward closing the material loop by handling this heterogeneous waste and providing feedstock for producing virgin plastics. This study compares chemical recycling and energy recovery scenarios for APW regarding climate change impact and cumulative energy demand (CED), assessing potential environmental advantages. In addition, an economic assessment is conducted. In contrast to other studies, the assessments are based on pyrolysis experiments conducted with an actual waste fraction. Mass balances and product composition are reported. The experimental data is combined with literature data for up- and downstream processes for the assessment. Chemical recycling shows a lower net climate change impact (0.57 to 0.64 kg CO₂e/kg waste input) and CED (3.38 to 4.41 MJ/kg waste input) than energy recovery (climate change impact: 1.17 to 1.25 kg CO₂e/kg waste input; CED: 6.94 to 7.97 MJ/kg waste input), while energy recovery performs better economically (net processing cost of −0.05 to −0.02€/kg waste input) compared to chemical recycling (0.05 to 0.08€/kg waste input). However, chemical recycling keeps carbon in the material cycle contributing to a circular economy and reducing the dependence on fossil feedstocks. Therefore, an increasing circularity of APW through chemical recycling shows a conflict between economic and environmental objectives.     

Lipase-catalyzed synthesis of aromatic polyesters

The enzymatic synthesis of aromatic polyesters by direct polyesterification between a diacid and a diol is described. The effects of the type of substrate, type and quantities of lipase, temperature, vacuum, and reaction time on the synthesis of aromatic polyesters were studied in detail. Among three lipases investigated, only Novozym 435 worked well for aromatic polyester synthesis. Temperature and vacuum played an important role in obtaining a high molar mass of the aromatic polyesters. Furthermore, with isophthalic acid and 1,6-hexanediol as substrates, the mass average molar mass of the polyester obtained increased with an increase in the lipase quantity up to 0.375 g (11.7%, w/w of total reactor contents). The mass average molar mass of the polyester was as high as 50000 g mol⁻¹ in 168 h, with a polydispersity of PD ≈ 1.4. Received 27 January 1998/ Accepted in revised form 19 May 1998     

Bacterial polyesters: biosynthesis, biodegradable plastics and biotechnology

The discovery and chemical identification, in the 1920s, of the aliphatic polyester: poly(3-hydroxybutyrate), PHB, as a granular component in bacterial cells proceeded without any of the controversies which marked the recognition of macromolecules by Staudinger. Some thirty years after its discovery, PHB was recognized as the prototypical biodegradable thermoplastic to solve the waste disposal challenge. The development effort led by Imperial Chemical Industries Ltd., encouraged interdisciplinary research from genetic engineering and biotechnology to the study of enzymes involved in biosynthesis and biodegradation. From the simple PHB homopolyester discovered by Maurice Lemoigne in the mid-twenties, a family of over 100 different aliphatic polyesters of the same general structure has been discovered. Depending on bacterial species and substrates, these high molecular weight stereoregular polyesters have emerged as a new family of natural polymers ranking with nucleic acids, polyamides, polyisoprenoids, polyphenols, polyphosphates, and polysaccharides. In this historical review, the chemical, biochemical and microbial highlights are linked to personalities and locations involved with the events covering a discovery timespan of 75 years.     

Polymer electrolyte membrane fuel cells: Principles and advances

Polymer electrolyte membrane fuel cells (PEMFC) are currently under intensive development for a range of power generation application in transportation, stationary and portable power. The PEMFC can produce electricity at high fuel efficiency and high energy density. The PEMFC is typically based on Nafion^® or similar polymers and operates at low temperatures of less than 80 °C. This article reviews the design principles of the PEMFC, the advances made in their performance and considers their limitations in relation to the source and types of fuel used to generate power.     

Construction of chiral polyesters from polycondensation of multifunctional monomer containing both flexible amino acid and rigid pendant groups with aromatic diols

A number of chiral wholly aromatic polyesters (PEs) with phthalimido and flexible chiral unit in the backbone were prepared from a chiral synthesized diacid monomer, 5-(3-methyl-2-phthalimidylpentanoylamino)isophthalic acid (1), and various aromatic diols via the polyesterification reaction. The tosyl chloride/pyridine/N,N-dimethylformamide (DMF) system was used as a condensing agent. All of the these polymers having bulky phthalimido and amino acid functionalities in the side chain showed excellent solubility and readily dissolved in various solvents such as N-methyl-2-pyrrolidinone, N,N-dimethylacetamide and DMF. Since, these chiral polymers have natural amino acids in the polymer architecture, they are expected to be biodegradable and therefore may be classified under eco-friendly polymers. They had useful levels of thermal stability associated with excellent solubility. Thermogravimetric analysis (TGA) showed that the obtained PEs are rather thermally stable, 10% weight loss temperatures in excess of 317°C, and char yields at 700°C in the nitrogen atmosphere higher than 24%. The resulting polymers were obtained in good yields with inherent viscosities ranging between 0.22 and 0.56 dL/g and were characterized with FT-IR, ¹H-NMR, elemental and TGA techniques.     

Creative biological lignin conversion routes toward lignin valorization

Lignin, the largest renewable aromatic resource, is a promising alternative feedstock for the sustainable production of various chemicals, fuels, and materials. Despite this potential, lignin is characterized by heterogeneous and macromolecular structures that must be addressed. In this review, we present biological lignin conversion routes (BLCRs) that offer opportunities for overcoming these challenges, making lignin valorization feasible. Funneling heterogeneous aromatics via a ‘biological funnel’ offers a high-specificity bioconversion route for aromatic platform chemicals. The inherent aromaticity of lignin drives atom-economic functionalization routes toward aromatic natural product generation. By harnessing the ligninolytic capacities of specific microbial systems, powerful aromatic ring-opening routes can be developed to generate various value-added products. Thus, BLCRs hold the promise to make lignin valorization feasible and enable a lignocellulose-based bioeconomy.     

Requirements on bacterial polyesters as future substitute for conventional plastics for consumer goods

Bacterial polyesters such as polyhydroxybutyrate (PHB) or polyhydroxyalkanoates (PHAs) have to pass the following requirements to be accepted on a large scale: (i) they have to fulfil an urgent market need: (ii) they require that new and efficient composting systems are installed in urban areas; (iii) they have to complete with the present plastics as far as quality and processing performance are concerned; (iv) they have to meet the requirement for the registration as food packages; and (v) they have to meet competitive price limits. (i) Some 30% of the plastics in the municipal waste originates from goods which are less than 1 year in use and tend to be heavily soiled by food and feed residues. This part is difficult and expensive to dispose of. Biodegradable alternatives could replace a large part of it. The waste could be diverted from landfills and incineration to composting sites near the end user. The savings in costs and frustrations are the source of a pressing demand for biopolymers, especially for producing goods which do not demand longevity and which are likely to end up soiled with organic matters. (ii) Composting infrastructures exist in rural areas. In urban areas new systems for collecting and composting ‘garden and kitchen wastes’ are being installed for reducing landfill problems, especially in Austria, Denmark, Germany and the Netherlands. These installations give biopolymers a competitive edge in the disposal discussion. (iii) Bacterial polyesters meet various quality and processing performances. They are water-resistant, and goods made of the polyesters are water-tight. The material can be processed by injection and by blow moulding. However, the esters are not flexible enough for forming films or foils. They also tend to become brittle and to lose their vapour barrier properties. It is expected that these limits will be overcome by improving blend formulations. (iv) Bacterial polyesters are not yet allowed for use as food package material. Since the esters represent a novel product, the procedure for the registration poses serious, but no insoluble problems. They require long and costly tests. There are no indications that bacterial polyesters would not attain the requirements. However, they are not expected to serve as food packages in the near future. (v) The present prices for bacterial polyesters are far too high to be accepted on a large scale by the processing and packaging industry. Costs are high mainly because of the raw material prices and to the small-scale production units. They can be lowered to accepted levels by investing in larger units in countries where inexpensive raw materials are available. Thus they will be able to meet the price limits. Since bacterial polyesters increasingly meet the requirements for the penetration of a mass market and since more and more consumers accept composting as an environmentally sound way of recycling organic materials, the polyesters are expected to penetrate a significant part of the short-lived and contaminated plastic products markets by the turn of the century.     

Genetics and Biochemistry of Polyhydroxyalkanoate Granule Self-assembly: The Key Role of Polyester Synthases

PHAs (polyhydroxyalkanoates = biopolyester) composed of hydroxy fatty acids represent a rather complex class of storage polymers synthesized by various bacteria and archaea and are deposited as water-insoluble cytoplasmic nano-sized inclusions. These spherical particles are composed of a polyester core surrounded by phospholipids and proteins. The key enzymes of polyester biosynthesis and polyester particle formation are the polyester synthases, which catalyze the formation of polyesters. Various metabolic routes have been identified and established in bacteria to provide substrate for polyester synthases. Although not essential for particle formation, non-covalently attached proteins, the so-called phasins, can be found at the particle surface and are considered as structural proteins. Protein engineering of polyester synthases and phasins was used to shed light into the topology of these granule attached proteins. Biopolyesters and the respective micro-/nano-structures are currently considered as biocompatible and biodegradable biomaterials with numerous potential applications particularly in the medical field. Received 12 October 2005; Revisions requested 1 November 2005; Revisions received 25 November 2005; Accepted 25 November 2005     

Dopant‐Free,Donor–Acceptor‐Type Polymeric Hole‐Transporting Materials for the Perovskite Solar Cells with Power Conversion Efficiencies over 20%

The rich molecular design of electron donor (D)–acceptor (A) polymers offers many valuable clues to obtain high‐efficiency hole‐transporting materials (HTMs) for use in perovskite solar cells (PVSCs). The fused aromatic or heteroaromatic units can increase the conjugation of the polymer backbone to facilitate electron delocalization, which increases the rigidity of adjacent units to prevent rotational disorder and lower the reorganization energy, leading to improved carrier mobility and optimized film morphology. In this work, fused‐ring ladder‐type indacenodithiophene and indacenodithieno[3,2‐b]thiophene are used as D units, benzodithiophene‐4,8‐dione as the A unit, and thienothiophene as a π‐bridge to form the D–A polymers PBDTT and PBTTT, respectively. Both polymers exhibit favorable properties as HTMs including suitable energy levels, high hole mobility, and excellent film quality. Both dopant‐free HTMs endow n‐i‐p PVSCs with promising performance and stability. A maximum power conversion efficiency of 20.28% is achieved for PBDTT‐based devices, which is among the highest values reported to date.