Biodegradable synthetic polymers

In addition, synthetic biodegradable polymers have been designed and synthesized to obtain the desired physical and chemical properties, and have found applications in cosmetics 6, coati

Biodegradable Synthetic Polymers

Jeong F Kim and Hai YN Thi, Department of Energy and Chemical Engineering, Incheon National University, Incheon, Republic of Korea

r 2024 Elsevier Inc All rights reserved, including those for text and data mining, AI training, and similar technologies.

Biodegradable synthetic polymers are essential elements to resolve unsustainable plastic pollution and achieve a carbon-neutral society In addition to fulfilling the required market standards of conventional polymers, biodegradable polymers must also satisfy the sustainability criteria concerning their carbon lifecycle It is important to emphasize that the carbon neutrality aspect of biodegradable polymers can be ambiguous, as they can also be chemically synthesized using fossil-based monomers Nevertheless, many advancements have been made in the past few decades with respect to polymer properties, synthesis routes, degradation protocols, and applications Importantly, as of 2023, large chemical companies have finally begun to scale up the production of biodegradable polymers, such as polylactide (PLA), polybutylene adipate terephthalate (PBAT), polybutylene succinate (PBS), and polyhydroxyalkanoates (PHA), to name a few Many difficult challenges remain to be overcome, not only in technological but also in regulatory aspects, including certification and standardization This chapter aims to provide an overview of synthetic biodegradable polymers, current challenges, and prospects.

Throughout history, biopolymers have long provided humanity with a copious source of food, clothes, and shelter (1) Since the beginning of the industrial era, fossil-based synthetic polymers have slowly replaced many of the biopolymer markets due to their cost efficiency, scalability, and durability (2) Despite the negative connotation around the concept of fossil-based synthetic polymers, such as polyolefin (PP and PE) and polyethylene terephthalate (PET), these polymers certainly played a pivotal role in the acceleration of worldwide industrialization and in fulfilling the surging demands arising from the population growth on Earth In addition, it has been shown that the use of lightweight plastics actually results in lower CO2emission to achieve the same objective (e.g., transport and packaging, etc) in comparison to non-fossil derivatives (3,4).

Nevertheless, the concerns around unsustainable plastic waste due to its non-biodegradability have called for more sustainable alternatives targeting carbon neutrality, mainly by adjusting the resource consumption to a rate that they can be restored by natural cycles of the planet (2) In comparison, the use of biodegradable polymers, both bioderived and synthetic, can partly resolve the plastic waste challenge Biomass-derived biodegradable polymers include polypeptides, cellulose, polysaccharides, chitin/chitosan, and natural rubbers (polyisoprene) (5) In addition, synthetic biodegradable polymers have been designed and synthesized to obtain the desired physical and chemical properties, and have found applications in cosmetics (6), coatings, wound dressings, enzyme immobilization, gene and drug delivery, and tissue engineering scaffolds (7) Also, the biopolymers have been applied in the energy sector with applications in electrical storage (8), synthesis of electrolyte (9), binder for cathode (10) and carbon material for anode in battery (11) As illustrated inFig 1, most of the polymers can be classified as either fossil-based or bio-based(bioderived), and as biode-gradable or nonbiodebiode-gradable Shifting from the fossil-based quadrants (second and third) to bio-based quadrants (first and

fourth) implies a carbon-neutral pathway, and progressing from the third quadrant (fossil-based and nonbiodegradable) to the first quadrant (bio-based and biodegradable) implies the most sustainable pathway.

Biodegradable polymers can be further classified (Fig 2) as either natural polymers or synthetic biodegradable polymers (12, 13) It is possible to produce synthetic biodegradable polymers using fossil-based monomeric units, while biomass-derived synthetic polymers may not be biodegradable (14) Biodegradable polymers can be further subdivided based on the main chemical linkages in their structure (15) or its origin (16) A more detailed summary of polymer structure and properties are summarized inTable 1.

Fig 1 Classification of Polymers: Biodegradable versus nonbiodegradable, and bio-based versus fossil-based.

Fig 2 Classification of biodegradable polymers Re-drawn with permission from Karande and co-workers (16) 2Biodegradable Synthetic Polymers

Table 1 Structure and properties of main synthetic biodegradable polymers.

2,000–6,000 MPa 50–70 MPa 8–12% 55–65°C 130–180°C • Soluble in various organic solvents

• Degraded by lipases and proteases, at

• Degraded by lipases, esterases

• Significant thermal degradation4170°C

• pH4 11 and pH o 3 accelerate degradation.

Polydioxanone (PDS) (C4H6O3)n (34,35)

• Degraded by hydrolysisin physiological conditions, 37°C, and pH 7.4 is favorable.

Poly(lactic-co-glycolic) acid (PLGA) (36,37)

• High lactide content can be dissolved in chlorinated solvents; high glycolic content requiresfluorinated solvents.

• Bulk degradation in aqueous medium,

• Degraded by biological and physical agents, chemical hydrolysis, no degradation in marine and freshwater

• 25°C and pH 7 for mesophilic phase

• 55–65°C for thermophilic phase

• Degraded by enzymes (esterases, lipases, etc.) or microorganism

• Degraded by enzymes (cutinase 1)

• high hydrolysis at 50°C or lipase in aqueous media at neutral pH

• Degraded upon exposure to soil, marine sediment, and by bacteria (Alcaligenes

1,000–4,000 MPa 20–43 MPa 5–10% −15–15°C 160–182°C • Soluble in chlorinated hydrocarbons

• Degraded by bacteria (Bacillus, Pseudomonas, etc.) or fungi (Sporotrichum, Talaromyces, etc.)

Natural polymers can be extracted from abundant biomass sources such as polysaccharides, proteins, or lipids Another important route is to utilize the metabolic pathways of microorganisms to biologically synthesize monomeric building blocks for polylactide (PLA), polyglycolide (PGA), poly(lactic-co-glycolic acid) (PLGA), poly(butylene succinate) (PBA), polycaprolactone (PCL), poly(ethylene adipate) (PEA), and poly(p-dioxanone) (PDS) It is also possible to produce the polymers directly using genetically modified microorganisms such as polyhydroxyalkanoates (PHA) and polyhydroxybutyrate (PHB).

The development of synthetic biodegradable polymers has been focused on employing degradable backbones such as poly-anhydrides, polycarbonates, and polylactones (46) In addition, derivatized polymers from natural polymers (e.g., chitin, chitosan, starch, and cellulose), such as cellulose acetate and nitrocellulose, are also classified as synthetic biodegradable polymers Over the last few decades, efforts have focused on different copolymerization approaches to overcome some of the disadvantages of synthetic biodegradable polymers (47) Synthetic biodegradable polymers also offer a wide range of applications in chemical separation, water treatment, bio-separation, and chemical sensors (48–50) More recent emerging applications include orthopedic products (51), cardiac patches (52), and tissue engineering technology (53).

2Properties of Biodegradable Polymers

2.1Factors Affecting Biodegradation

A polymer can degrade in several different ways, including mechanical, chemical, thermal, and biological degradation The term biodegradation technically refers to the work of enzymes within microorganisms and the natural environment, but relevant documents and literature often deliberately include chemical degradation mechanisms such as hydrolysis and oxidation However, two mechanisms generally proceed simultaneously, and it may not be necessary to distinguish them strictly.

The biodegradability of a polymer depends on many factors, such as the molecular structure, molecular weight, and crystal-linity (54–56,57) Most biodegradable polymers consist of hydrolyzable linkages within the chain that hydrolytic enzymes in natural environments can exploit Notably, biodegradability in polymers with both hydrophobic and hydrophilic structures is generally higher than in polymers only composed of either one (5,58) Moreover, it has been reported that enzymes can better access the amorphous regions of the polymers in comparison to their crystalline counterpart (59,60).

The presence of oxygen also plays a key role during biodegradation.Fig 3summarizes the two main degradation pathways of biodegradable polymers In the presence of oxygen (aerobic condition), the polymers can be metabolized into carbon dioxide, water, humic compounds (a type of organic macromolecule), and other natural components These materials can be naturally recycled through biological processes (61–63) On the other hand, anaerobic degradation of polymers yields CH4(methane) in a relatively high fraction, and it can be used as fuel or as feedstock for petrochemical pipelines.

2.2Mechanical, Thermal, and Chemical Properties

In order for biodegradable polymers to compete with nonbiodegradable polymers, they must exhibit reasonable mechanical, thermal, and chemical stability The main mechanical properties for biodegradable polymer characterization involve factors such as Young’s Modulus, tensile strength, elongation at break, and impact resistance Compared to the fossil-derived synthetic polymers, biodegradable polymers are intrinsically weaker Biodegradable polymers generally demonstrate lower modulus, elongation, or both Hence, the general practice is to compound two or more biodegradable polymers tofine-tune the mechanical properties.

Commercial fossil-derived polymers such as PE and PP typically exhibit excellent tensile strengths in the range of 20–40 MPa, Young’s Modulus of 1–3 GPa, and elongation at break of 100–500% range These polymers also exhibit good impact resistance In comparison, PLA, one of the well-known biodegradable polymers, has a tensile strength and Young’s Modulus in the range of 50–70 MPa and 2–6 GPa, respectively However, PLA has a relatively high Tg (55–651C), and hence, it is very brittle under ambient conditions with elongation of only about 8–12% On the other hand, another well-known biodegradable polymer, PBAT, exhibits highflexibility (elongation up to 500–670%) albeit weaker mechanical properties (Young’s Modulus around 20–35 MPa) These two polymers are often compounded together in various ratios tofine-tune the flexibility of the resulting polymer product.

Various research and development works have been carried out to improve the polymer impact resistance, tensile strength, and modulus without compromising biodegradability Although current polymers can be considered to be competitive, further improvements must be made to overcome challenges related to cost, processability, and waste management to render biode-gradable polymers more competitive and sustainable across a larger scale of applications.

Thermal stability is also a key factor in determining the processing feasibility of biodegradable polymers in the polymer industry Since many polymer products are thermally processed into desired morphology (i.e., melt process), a thorough understanding of the effect of temperature on polymer properties is required to optimize the polymer processability and batch-to-batch variances Expectedly, poor thermal stability at elevated temperatures can lead to random chain scission that deteriorates the polymer properties, which is a frequent issue with biodegradable polymers Generally, the thermal degradation rate of polymers is a function of temperature, polymer structure, molecular weight, and shear force Semicrystalline fossil-derived polymers such as PE and PP contain neither ether nor ester linkages, rendering them intrinsic thermal stability In contrast, biodegradable polymers like PLA, PBAT, and PBS contain thermally labile ester bonds (required for facile degradation), and hence, heat exposure during thermal processing must be minimized.

Biodegradable Synthetic Polymers5

3Synthesis Method for Biodegradable Polymers

In comparison to natural biopolymers, synthetic biodegradable polymers have the advantage that they can befine-tuned for desired and tailored characteristics to meet market demands in a sustainable route Many works on the chemical synthesis of biodegradable polymers have been reported, including modification of natural polymers, chemical polymerization, biological synthesis, and enzyme-mediated and chemo-enzymatic synthesis routes.

3.1Modification of Natural Polymeric Materials

Chemical modification of natural polymers can be a straightforward method to control the polymer properties The modification could be done by applying chemical methods of nitration, hydroxylation, sulfonation, acylation, alkylation, phosphorylation, thiolation, xanthation, quaternization, and graft copolymerization Among the chemical modification methods, chemical grafting is considered the most versatile synthesis path, leading to a wide range of molecular designs (64).

For example, natural cellulose has been chemically modified via esterification, etherification, or diacylation Through simple esterification and etherification reactions, the hydrophilicity and polarity of natural cellulose fibers can be controlled to fulfill the surface property qualities of the most common thermoplastics (65) In addition, the hydroxyl groups of starch molecules provide favorable active sites for chemical modification to introduce carboxyl, acetyl, and hydroxypropyl groups (66,67) Moreover, chemical grafting onto chitin and chitosan can be done utilizing the amino and hydroxyl groups, enabling facile synthesis of functional derivatives (46) Modified chitosan polymers via grafting polymerization have shown improved anti-bacterial prop-erties, and they have been applied as a support material in gene delivery, cell culture, and tissue engineering (68).

3.2Chemical Synthesis

The biodegradable polymers can be chemically synthesized (or polymerized), with or without a catalyst, in conventional chemical reactors Some of the widely employed methods for biodegradable polymers are ring-opening polymerization (ROP), free-radical polymerization (FRP), and condensation polymerization.

The ROP method is a powerful technique to produce biodegradable aliphatic polyesters from cyclic esters or lactones (69), this a form of chain-growth polymerization in which the terminal end of a polymer chain acts as a reactive center to form a longer polymer chain by sequential addition of cyclic monomers through a ring-opening mechanism (46) The propagating center could

Fig 3 Two different biodegradation mechanisms depending on the presence of oxygen Reprinted with permission from Ref (7) 6Biodegradable Synthetic Polymers

be either a radical, anionic, or cationic compound This versatile method has been employed to produce many biopolymers, such as PLA (70), PLGA (71), and PCL (72).

The FRP method is another versatile technique to produce polymers using vinyl monomers, following the well-known stages of initiation, propagation, and termination This method has been widely employed to synthesize polystyrene (PS) (73,74), poly (vinyl chloride) (PVC) (75), polyethylene (LDPE) (76), poly(methyl methacrylate) (PMMA) (77), and polyacrylonitrile (PAN) (78) in the polymer industry Recently, the FRP technique was used in combination with ROP to synthesize PLA-g-succinamic acid (79) Another example is the combination of radical ROP (rROP) and reversible addition-fragmentation chain-transfer (RAFT)-living radical polymerization for the synthesis of poly(vinyl alcohol) (PVA) (80,81).

The condensation polymerization method has also been widely employed in the synthesis of biodegradable polymers, especially for those derived from renewable resources The condensation reaction is an equilibrium step-wise addition reaction, often resulting in ester linkages and producing water molecules as a byproduct For instance, the synthesis of polyethylene furanoate (PEF), a biodegradable polyester, proceeds by a condensation reaction between furan-2,5-dicarboxylic acid (FDCA) and ethylene glycol Biodegradable polymers such as PLA, PHA, PBAT, and PBS can also be synthesized through condensation reactions (82, 32, 83, 84) Although the condensation polymerization method offers a versatile approach for synthesizing various biodegradable polymers, challenges such as removing byproducts and controlling inadvertent side reactions necessitate careful optimization.

3.3Microbiological Synthesis

Microbiological synthesis of biodegradable biopolymers involves utilizing the metabolic capabilities of microorganisms to pro-duce sustainable and environmental-friendly polymers Microorganisms, such as bacteria and yeast, can be genetically engineered to tailor the metabolic pathway to produce specific polymers.

One of the notable products is PHA, which was first identified in Bacillus Megaterium back in 1926 (85) Micro-organisms are fed with carbon substrate from renewable feedstocks, which are sugars or plant-based materials These substrates get metabolized and converted into precursor molecules, such as 3-hydroxyacyl-CoA (86) Then, enzymatic polymerization reactions occur within the microbial cells to produce PHA (86) Currently, more than 92 bacterial groups, including Escherichia coli, Cupriavidus Necator, and Saccharomyces cerevisiae (yeast), have been genetically modified to produce PHA and PHB (87,88–90,91–93).

Microbiological production technology plays an important role in the biopolymerfield, as it utilizes renewable bioresources and reduces the reliance on petrochemical feedstocks Moreover, the versatility of this technique allows the production of various biopo-lymers with tailored properties, paving a path for different applications, from packaging and medical devices to agricultural products However, this promising technique also faces difficult challenges, including low mass yield and difficult downstream separation Researchers are striving to improve strain engineering to enable facile modification of microorganisms to enhance their biopolymer production capabilities Moreover, Various synthetic biology tools have been developed to design and culture microorganisms with customized metabolic pathways for efficient synthesis (94).

Interesting works have been reported to utilize diverse and renewable carbon feedstocks for fermentation, aiming to reduce reliance on traditional sources of sugar and improve the sustainability of the process (95) Continuous efforts have been made to optimize fermentation conditions with temperature, pH, and nutrient supply to enhance microbial growth and biopolymer synthesis (96, 97) Additionally, current research works have focused on applying mixed-culture fermentation (98, 99) and production scale-up (100).

3.4Enzyme-Mediated and Chemo-Enzymatic Synthesis

Enzyme-mediated synthesis is considered an emerging technique utilizing the high specificity of enzymes Biodegradable polymers with amide, saccharide, and ester backbones have been synthesized via this route (46,101,102) The polymerized products can be purified with relative ease due to the high specificity of enzymes, which can also be recycled Because the catalytic reaction conditions using enzymes are relatively mild (ambient conditions), the processing cost can potentially be reduced (101) This approach is considered environmentally friendly as it avoids using traditional chemical catalysts and can proceed under ambient reaction conditions (103) Although this method can synthesize polymers with well-defined structures, the availability of suitable enzymes with high activity is often the bottleneck.

While enzyme-mediated synthesis completely relies on enzymatic catalysis for the polymerization reaction, chemo-enzymatic technique integrates chemical and chemo-enzymatic steps in a complementary strategy Chemo-chemo-enzymatic synthesis combines the flexibility of chemical synthesis and high regio- and stereoselectivity of enzyme-catalyzed reactions The enzymes perform under mild or slightly basic biological conditions, minimizing unwanted reactions such as decom-position and isomerization Chemo-enzymatic synthesis can fine-tune the molecular weight of the polymer with high specificity and stereoselectivity (46,104) Therefore, it can be an attractive strategy for producing high molecular-weight biodegradable polymers (105) (Fig 4).

Biodegradable Synthetic Polymers7

4Applications of Biodegradable Polymers

4.1Biomedical and Clinical Application

Synthetic biodegradable polymers have found versatile and diverse biomedical applications due to their tailorable designs (111) Their biodegradability provides the significant advantage of being able to be broken down and removed after they have served their function Synthetic biodegradable polymers such as materials based on lactic, glycolic acid, poly(dioxanone), poly(trimethylene carbonate) copolymers, and poly(e-caprolactone) have been used for over 50 years for surgical sutures, wound dressings, tissue regeneration, enzyme immobilization, controlled drug delivery and gene delivery, tissue engineering scaffolds, cryopreservation, nanotechnology, medical implants, and devices (112,113) For these applications, the polymers must adapt to specific requirements: being nontoxic, capable of maintaining good mechanical integrity until degraded, and capable of controlled rates of degradation Their products of degradation should also cause low or negligible toxicity in terms of both local tissue and systemic immune responses.

4.2Membranes and Filter Technology

The membrane andfilter market is one of the most promising markets for biodegradable polymers, as most single-use filters are currently non-biodegradable If a membrane fabricated from biodegradable synthetic polymers can offer competitive performance and stability for a specific period, it could be widely adopted by end-users Such a membrane offers numerous advantages and applications ranging from pharmaceutics to agriculture and packaging (114,115) Importantly, the biodegradable polymers have been employed in the variousfields of separation technology such as organic solvent nanofiltration (OSN) (108,116,117) or organic solvent ultrafiltration (118) with cellulose, lignocellulose, and its composite membranes More specifically, PHA mem-branes have been applied pervaporative methanol separation (119), lignin membranes in pharmaceutical separation (120), cellulose-chitosan blend membranes for oil and gas separation (121), and cellulose and PLA membranes for air and gasfiltration (122, 123) Recently, thinfilm composite (TFC) membranes with high selectivity and permeability have been reported using biodegradable polymers exhibiting solvent resistance (124–126), further confirming the extensive use of biodegradable polymers in membrane technology.

4.3Film and Packaging

Increased environmental concerns about sustainable packaging have led to the great demand for biopolymer packaging throughout the world Polymerfilms are extensively used for packaging in many sectors Biodegradable films fabricated from starch of corn, potato, or cellulose-based polymers were originally designed to replace PE products They offer better properties than nondegradable polymers, such as allowing controlled respiration of vegetables, forming a good barrier, maintaining struc-tural integrity, and preventing or reducing microbial spoilage.

Materials from biodegradable polymers are designed to decompose quickly and safely in the environment, thereby reducing the impact of plastic waste on land and water sources Additionally, using biodegradable polymers in agriculture could offer addi-tional advantages, such as improved soil quality and enhanced crop yields (127) Promising applications include mulchfilms, seed coatings, superabsorbent polymers, and agrochemical delivery systems (127,128).

Surprisingly, the application of biodegradable synthetic polymers in textilefields is relatively scarce, except for the PLA works in medical textiles (129) or chitosan-based textiles (130) The reason behind this limited application may come from the challenges such as difficulties in fabricating bio-based recyclable fibers with comparative properties to those of commercial fibers In addition, achieving the circular economy also requires the timely degradation of biofibers with advanced properties upon exposure to natural soil and marine environments (131) However, there have been strategies implemented to fabricate biotextile materials in a more sustainable approach, such as the use of fungal mycelium to produce biofbers that can replace conventional leather (132) as well as microbial-based nanocellulose biotextiles (133), suggesting promising future development The commercialization of biodegradable synthesis polymers in packaging and other sectors has been fueled by improved biotechnological production processes, showing that textile applications are increasingly conceivable; therefore, considering the market size of the textile industry, this is a promising application that biodegradable polymer researchers should consider.

5Current Challenges

Even though biodegradable polymers are promising candidates for a variety of applications, there are still many challenges that need to be overcome before they can be adopted as widely as conventional fossil-derived polymers, some of which are not in the technological aspects but in the regulatory challenges First and foremost, the reliability and properties of the synthetic biodegradable polymers must be improved with respect to mechanical, thermal, and chemical stability However, it should be noted that this challenge can now be overcome by compounding different biodegradable polymers in optimal ratios to obtain the desired

8Biodegradable Synthetic Polymers

properties Albeit current literature still lacks detailed thermodynamic data for compounded composite polymers, there is sufficient confidence within the industry now that biodegradable polymers can be as effective as conventional fossil-derived polymers.

Perhaps more serious and difficult challenges, in fact, are interconnected with complex regulatory provisions and unstandar-dized certification protocols between countries Every biodegradable polymer has its own optimal biodegradation conditions, such as the environment (soil, compost, water, or seawater) and conditions (temperature, humidity, and microbiomes) Moreover, there are many doubts and concerns over the toxicity of degrading compounds depending on the starting polymers Every country and government is proposing different regulations and certifications based on their own set of data, increasing the regulatory risks for chemical companies to invest in this growing industry Nevertheless, there have been meaningful strides in recent years with respect to testing standardization and certifications, particularly in the EU regions Needless to say, it is crucial to encourage as many countries as possible to participate when setting international standards and sustainability targets to be effective (otherwise, biodegradable products would not be cost-competitive).

Lastly, it is important to re-emphasize that the use of biodegradable polymers does not necessarily guarantee sustainability nor carbon neutrality It is important to carry out a thorough, unbiased lifecycle assessment (LCA) starting from the production stage all the way to degradation to ensure that the employment of biodegradable polymers can be considered green.

6Conclusions and Prospects

Biodegradable synthetic polymer technology, assuming that natural raw materials can be cost-effectively utilized, offers a closed-loop sustainable solution to mediate the intensifying plastic pollution and alleviate the waste management challenges (46) Although it is unlikely that biodegradable polymers could completely replace conventional polymers, it can be foreseen that biodegradable synthetic polymers can potentially revolutionize the polymer industry in many sectors With increasingly higher social awareness of environmental pollution, favorable government regulations, and rising consumer demand for sustainable products, the biode-gradable synthetic polymer market is likely to grow very fast and acquire a considerable portion of the polymer industry sector.

This work was supported by Nano Material Technology Development Program (2021M3H4A1A04092885) and by the Petroleum Replacement Chemical Technology Development Program (2022M3J5A1062900) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT.

Fig 4 Applications of biodegradable polymers: (A) biomedicalfields with gene delivery using polymeric nanoparticles (106); (B, C) tissue engineering and regeneration (107); (D) sustainable membrane technology (108); (E) agricultural plastic mulchfilms (109); (F) sustainable textile and clothes (110) All citedfigures were reprinted with permission.

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