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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
The recent explosion of interdisciplinary research has fragmented the knowledge base surrounding renewable polymers. The Chemistry of Bio-based Polymers, 2nd edition brings together, in one volume, the research and work of Professor Johannes Fink, focusing on biopolymers that can be synthesized from renewable polymers. After introducing general aspects of the field, the book's subsequent chapters examine the chemistry of biodegradable polymeric types sorted by their chemical compounds, including the synthesis of low molecular compounds. Various categories of biopolymers are detailed including vinyl-based polymers, acid and lactone polymers, ester and amide polymers, carbohydrate-related polymers and others. Procedures for the preparation of biopolymers and biodegradable nanocomposites are arranged by chemical methods and in vitro biological methods, with discussion of the issue of "plastics from bacteria." The factors influencing the degradation and biodegradation of polymers used in food packaging, exposed to various environments, are detailed at length. The book covers the medical applications of bio-based polymers, concentrating on controlled drug delivery, temporary prostheses, and scaffolds for tissue engineering. Professor Fink also addresses renewable resources for fabricating biofuels and argues for localized biorefineries, as biomass feedstocks are more efficiently handled locally.
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Table of Contents
Cover
Preface
Preface to Second Edition
Preface to First Edition
How to Use This Book
Index
Acknowledgements
1 An Overview of Methods and Standards
1.1 History of Biodegradable Plastics
1.2 Green Chemistry
1.3 Commercial Situation
1.4 Environmental Situation
1.5 Properties of Biodegradable Polymers
1.6 Special Methods of Synthesis
1.7 Biodegradability Standards
1.8 Test of the Biological Origin
References
PART I: BIO-BASED POLYMERS DEGRADATION AND CHEMISTRY
2 Vinyl-Based Polymers
2.1 Polyolefins
2.2 Poly(styrene) Elastomers
2.3 Poly(vinyl alcohol)
2.4 Poly(vinyl butyral)
References
3 Acid and Lactone Polymers
3.1 Poly(lactic acid)
3.2 Poly(glycolic acid)s
3.3 Butyrolactone-Based Vinyl Monomers
3.4 Poly(caprolactone)
References
4 Ester and Amide Polymers
4.1 Poly(ester)s
4.2 Plant Oil-Based Biopolymers
4.3 Poly(amide)s
References
5 Carbohydrate-Related Polymers
5.1 Starch
5.2 Cellulose
5.3 Cellulose Ethers
5.4 Nonionic Cellulose Ethers
5.5 Cellulose Esters
5.6 Cellulose Ether Esters
5.7 Lignin
5.8 Biodegradable Nanocomposites
5.9 Chitin
References
6 Other Polymer Types
6.1 Terpenes
6.2 Poly(urethane)s
6.3 Cationic Lipopolymers
6.4 Plastics from Bacteria
6.5 Bio-based Epoxy Resins
6.6 Phosphate-Containing Polymers
6.7 Polyketals
6.8 Biorubber
6.9 Collagen
6.10 Pyridinium Modified Polymers
6.11 Commercial Biodegradable Polymers
References
PART II: APPLICATIONS
7 Packaging, Food Applications and Foams
7.1 Packaging
7.2 Fibers and Nets
7.3 Foams
7.4 Biodegradable Adhesive Compositions
7.5 Food Applications
7.6 Other Applications
References
8 Medical Applications
8.1 Drug Delivery
8.2 Tissue Engineering
8.3 Tissue Markers
8.4 Hydrogels
8.5 Microporous Materials
8.6 Implants
8.7 Shape Memory Polymers
8.8 Stents
8.9 Thermogelling Materials
8.10 Cancer Therapy
8.11 Wound Dressings
8.12 Bioceramics
8.13 Conjugates
References
9 Personal Care and Sanitary Goods
9.1 Breathable Biodegradable Composition
9.2 Personal Hygiene Applications
9.3 Sanitary Goods
9.4 Superabsorbent Materials
References
10 Miscellaneous Applications
10.1 Flooring Materials
10.2 Abrasives and Polishing Compositions
10.3 Lubricants
10.4 Renewable Cards
10.5 Biodegradable Irrigation Pipe
10.6 Thermosensitive Material
10.7 Biodegradable Scale Inhibitors
10.8 Nanocomposites
10.9 Molded Articles from Fruit Residues
10.10 Fluorescent Biodegradable Particles
10.11 Test Cylinder Mold for Testing Concrete
10.12 Flexographic Inks
10.13 Audio Systems
10.14 Automotive Uses
10.15 Oil Well Environment
10.16 Green Hot Melt Adhesives
10.17 Mechanistic Studies
References
11 Biofuels
11.1 Xenobiotics
11.2 Biopolymers
11.3 Bioethanol
11.4 Biobutanol
11.5 Biodiesel
References
Index
Trade names
Acronyms
Chemicals
General Index
Also of Interest
Check out these other books by the author published by Scrivener Publishing
End User License Agreement
List of Illustrations
Chapter 1
Figure 1.1 Reaction of carbon dioxide with epoxides (35).
Figure 1.2 Polymer waste management options (73, 74).
Figure 1.3 Ultraviolet absorbers.
Figure 1.4 Chiral Schiff base (88).
Figure 1.5 α-Cyclodextrin.
Figure 1.6 Schematic illustration of a bola-amphiphile.
Figure 1.7 Inclusion complexes (112).
Figure 1.8 Typical reaction curve.
Chapter 2
Figure 2.1 1,1-Diphenyl-2-picryl-hydrazyl.
Figure 2.2 Technical synthesis of poly(vinyl butyral).
Figure 2.3 Chelating agents.
Chapter 3
Figure 3.1 Lactides.
Figure 3.2 Preparation of a chiral ligand as catalyst precursor (11, 21)....
Figure 3.3 Hydroxy acids.
Figure 3.4 Kenaf fiber.
Figure 3.5 LAK301, 5-sulfoisophthalic acid sodium salt.
Figure 3.6 SEM micrographs of cryogenic fractures with different amounts o...
Figure 3.7 MTT reduction (50).
Figure 3.8 Silane treatment (51).
Figure 3.9 Synthesis of
α
-methylene-
γ
-valerolactone from levulin...
Figure 3.10 Amphiphilic catalyst for living polymerization of methylene la...
Figure 3.11 Chemicals from lactic acid (91).
Figure 3.12
ε
-Caprolactone.
Chapter 4
Figure 4.1 Methyl-10-undecenoate.
Figure 4.2 1,5,7-Triazabicyclo[4.4.0]dec-5-ene.
Figure 4.3 Eugenol.
Figure 4.4 Monomers.
Figure 4.5 Schematic of grafting reaction (35).
Figure 4.6 Buprofezin.
Figure 4.7 Gallic acid.
Figure 4.8 Ethyl lauroyl arginate.
Figure 4.9 Butylene adipate.
Figure 4.10 Ethylene terephthalate.
Figure 4.11 Change of the molar mass with reaction time (61).
Figure 4.12 Biodegradability of copolyesters from Butanediol (B), Succinic...
Figure 4.13 1,4,3,6-Dianhydrohexitol.
Figure 4.14 Synthesis of isosorbide from glucose.
Figure 4.15 Synthesis of bis(2-hydroxyethyl)isosorbide.
Figure 4.16 Synthesis of polyesters (68).
Figure 4.17 Glass transition temperature of isosorbide-based aliphatic pol...
Figure 4.18 Fox-Flory plot of the glass transition temperatures of poly(is...
Figure 4.19 Scanning electron micrograph of the biocomposite: (a) 10% Agar...
Figure 4.20 Dicarboxylic acids of natural origin.
Figure 4.21 Micrographs of surface-treated hemp mat fibers: (a) untreated ...
Figure 4.22 Introduction of acrylic groups in plant oils (113).
Figure 4.23 Acrylic monomers.
Figure 4.24 Linking reaction of oleic acid (118, 119).
Figure 4.25 Formation of a bromoacrylated adduct (124).
Figure 4.26 Schematic of ADMET polymerization.
Figure 4.27 Synthesis of 1,3-di-10-undecenoylglycerol and 1,2-di-10-undece...
Figure 4.28 Synthesis of monomers derived from vanillin and fatty acids (1...
Figure 4.29 Vanillin compounds.
Figure 4.30 Methyl 4-(2-hydroxyethoxy)-3-methoxybenzoate.
Figure 4.31 9,10-Epoxy-18-hydroxyoctadecanoic acid.
Figure 4.32 Diglycidyl bisphenol A.
Figure 4.33 Isophorone diisocyanate.
Figure 4.34 Star polymer compatibilizer (161);
n
is between 10 and 10,000....
Figure 4.35 Stress-strain curves (160).
Chapter 5
Figure 5.1 Methylene blue.
Figure 5.2
γ
-Cyclodextrin.
Figure 5.3
N
-Methylimidazole.
Figure 5.4 Basic structure of cellulose.
Figure 5.5 Laser scanning microscopy of samples. (top) No lithium chloride...
Figure 5.6 Stress-strain curves of the dry nanopapers (79).
Figure 5.7 Alkyl ketene dimers.
Figure 5.8 Monomers for lignin.
Figure 5.9 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO).
Figure 5.10 Fibrilation yield against carboxylate content of oxidized pulp...
Figure 5.11 Chitobiose oxazoline monomer (178).
Figure 5.12 Ionic liquids.
Chapter 6
Figure 6.1 Elementary terpenes.
Figure 6.2 Epoxidation of (R)-(+)-limonene (5).
Figure 6.3 Cationic polymerization of
β
-pinene (1).
Figure 6.4 Schiff-based nickel complex (10).
Figure 6.5 Synthesis of limonene dicarbonate.
Figure 6.6 Alternating copolymerization limonene oxide with carbon dioxide...
Figure 6.7 Synthesis of poly(3-methylenecyclopentene) (16).
Figure 6.8 Thiols.
Figure 6.9 Reaction of a cyclic carbonate with amine (24).
Figure 6.10 Cationic lipopolymers (27).
Figure 6.11 Polymers with phosphate linkages.
Figure 6.12 Reactants for polymer-drug conjugate (46).
Figure 6.13 Synthesis of a 5-fluorouracil bonded to poly(phosphoester) (46...
Figure 6.14 Synthesis of 2-methoxy-2-oxo-1,3,2-dioxaphospholane (45).
Figure 6.15 Synthesis of poly(
D
,
L
-lactide-
co
-ethylene methyl phosphate) (4...
Figure 6.16 Water absorption as a function of ethylene methyl phosphate co...
Figure 6.17 Acrylate and amino compounds.
Figure 6.18 Oxidative cleavage of a polyfructose (54).
Figure 6.19 Ring-opening of a cyclic ketal (54).
Chapter 7
Figure 7.1 Additive for antistatic and antifogging properties (8).
Figure 7.2 Tannic acid.
Figure 7.3 Amylose.
Figure 7.4 Hydroxytyrosol.
Figure 7.5 Fluid storage container (46).
Figure 7.6 Melamine.
Figure 7.7 Astaxanthin.
Figure 7.8 Modifiers for corn starch (81).
Chapter 8
Figure 8.1 Decrease in molecular weight of PLA over time (24).
Figure 8.2 Entecavir.
Figure 8.3
κ
-Carrageenan.
Figure 8.4 Chitosan.
Figure 8.5 Amoxicillin.
Figure 8.6 Cyanocobalamin.
Figure 8.7 Diltiazem.
Figure 8.8 Hyaluronic acid.
Figure 8.9 Paracetamol.
Figure 8.10 Nimesulide.
Figure 8.11 Release of 5-aminosalicylic acid over time (56).
Figure 8.12 Ciprofloxacin.
Figure 8.13 Curcumin.
Figure 8.14 Light microscopy image of electrospun fibers (67).
Figure 8.15 Diclofenac.
Figure 8.16 Acyclovir.
Figure 8.17 Release profiles of tetracycline hydrochloride (68).
Figure 8.18 Dexamethasone.
Figure 8.19 Dexamethasone release (75).
Figure 8.20 External surface of dual-pore scaffolds (78).
Figure 8.21 Contrast agents.
Figure 8.22 SEM micrograph of a foam (98).
Figure 8.23 Deaminotyrosyl-tyrosine ethyl ester.
Figure 8.24 S-Nitroso-
N
-acetylpenicillamine.
Figure 8.25 Mydriatic-cycloplegic agents.
Figure 8.26 Fluvastatin.
Figure 8.27 Epoxydation of dihydrocarvone.
Figure 8.28 Chondroitin 6-sulfate.
Figure 8.29 Sirolimus.
Chapter 9
Figure 9.1 (1)
N
,
N
´-(1,4-Phenylenedimethylidene) bis(ethanolamine), (2)
N
,
Figure 9.2 Polymer with imine and ester groups (3).
Figure 9.3 Acrylic monomers.
Chapter 10
Figure 10.1 Acetyl tributyl citrate.
Figure 10.2 Cocoamidopropyl betaine.
Figure 10.3 Crosslinking agents.
Figure 10.4 Monoricinolein.
Figure 10.5 Monomers.
Figure 10.6 Polymers from
γ
-propiolactone and
δ
-valerolactone (1...
Figure 10.7 2,5-Dimethyl-2,5-di(
tert
-butyl peroxy) hexane.
Figure 10.8
Sake kasu
(28).Reproduced from Wikipedia under the terms of ...
Figure 10.9 Compostable interior panel (37).
Figure 10.10 ADMET polymerization.
Chapter 11
Figure 11.1 Biorefinery operations (7).
Figure 11.2 Degradation progress in sterilized and inoculated soil by fung...
Figure 11.3 Degradation progress in sterilized and inoculated soil by fung...
Figure 11.4 Glucose and cellobiose.
Figure 11.5 Optimization of the concentration of PEG (51).
Figure 11.6 Saccharification of lichenan (2).
Figure 11.7 Bromelain.
Guide
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Scrivener Publishing
100 Cummings Center, Suite 541J
Beverly, MA 01915-6106
Publishers at Scrivener
Martin Scrivener ([email protected])
Phillip Carmical ([email protected])
The Chemistry of Bio-based Polymers
Second Edition
Johannes Karl Fink
Montanuniversität Leoben, Austria
This edition first published 2014 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA © 2020 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.
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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-119-68131-1
Cover image: Pixabay.ComCover design by Russell Richardson
Preface
Preface to Second Edition
The first edition of this text appeared in 2014. In the second edition, the most important literature which appeared after this date has been included. So the text is now updated up to 2019.
I want to thank the publisher, Martin Scrivener, for his interest in publishing the second edition. In addition, my thanks go to Jean Markovic, who made the final copyedit with utmost care.
J. K. F.Leoben, September 9, 2019
Preface to First Edition
This book focuses on the chemistry of renewable polymers as well as low molecular compounds that can be synthesized from renewable polymers. As is well-documented, this issue has literally exploded in the literature because of growing awareness that conventional resources based on petroleum are limited.
After an introductory section to the general aspects of the field, the first part of the book deals with the chemistry of biodegradable polymeric types in five comprehensive chapters. The second part of the book deals with the applications (packaging and food, medical) of biodegradable polymers as well as the synthesis of low molecular compounds, including bio-based fuels.
The text focuses on the literature of the past decade. Beyond education, this book will serve the needs of industry engineers and specialists who have only a passing contact with the plastics and composites industries but need to know more.
How to Use This Book
Utmost care has been taken to present reliable data. Because of the vast variety of material presented here, however, the text cannot be complete in all aspects, and it is recommended that the reader study the original literature for more complete information.
The reader should be aware that mostly US patents have been cited where available, but not the corresponding equivalent patents in other countries. For this reason, the author cannot assume responsibility for the completeness, validity or consequences of the use of the material presented herein. Every attempt has been made to identify trademarks; however, there were some that the author was unable to locate.
Index
There are four indices: an index of trademarks, an index of acronyms, an index of chemicals, and a general index.
In the index of chemicals, compounds that occur extensively, e.g., “acetone,” are not included at every occurrence, but rather when they appear in an important context.
Acknowledgements
I amindebted to our university librarians, Dr. Christian Hasenhüttl, Dr. Johann Delanoy, Franz Jurek, Margit Keshmiri, Dolores Knabl, Friedrich Scheer, Christian Slamenik, Renate Tschabuschnig, and Elisabeth Groß for support in literature acquisition. I also want to express my gratitude to all the scientists who have carefully published their results concerning the topics dealt with herein. This book could not have been otherwise compiled.
Last, but not least, Iwant to thank the publisher,Martin Scrivener, for his abiding interest and help in the preparation of the text.
Johannes Fink
Leoben, 10th October 2013
1An Overview of Methods and Standards
Biocomposites are considered the next-generation materials as they can be made using natural/green ingredients to offer sustainability, eco-efficiency, and green chemistry (1–3). Nowadays, biocomposites are being utilized by numerous sectors, which include automobile, biomedical, energy, toys, sports, and others.
An effort has been made to provide a comprehensive assessment of the available green composites and their commonly used in order to make materials capable of meeting present and future demands. Various types of natural fibers have been investigated with polymer matrixes for the production of composite materials that are on par with the synthetic fiber composite. Also, the requirements for green composites in various applications from the viewpoint of variability of fibers available and their processing techniques have been detailed (4).
1.1 History of Biodegradable Plastics
In the late 1980s, biodegradable plastics came into use. However, these came to be misapplied in a number of situations. The misapplication of inappropriate or incompletely developed technology led to products which often did not meet performance claims and expectations. The so-called first generation technologies often lacked one or more of the following issues (5):
Rate or extent of biodegradation, primarily due to limitations of starch incorporation,
Necessary physical properties and related characteristics
An economical means to effectively and efficiently manufacture starch-based blends,
Intermediate product compatibility with conventional plastics product conversion processes, and
Lower limits on film thickness caused by the use of non-gelatinized starch materials.
The synthesis, processing, and technology of renewable polymers has been reviewed (6–27). Furthermore, the state-of-the-art for food packaging applications has been reviewed (28–32). Using biomass for the production of new polymers can have both economic and environmental benefits (33).
Biomass-derived monomers can be classified into four major categories according to their natural resource origins (34):
Oxygen-rich monomers including carboxylic acids, e.g., lactic acid succinic acid, itaconic acid, and levulinic acid, but also ethers, such as furan,
Hydrocarbon-rich monomers including vegetable oils, fatty acids, terpenes, terpenoids and resin acids,
Hydrocarbon monomers, i.e., bio-olefins, and
Non-hydrocarbon monomers such as carbon dioxide.
Carbon dioxide is an interesting synthetic feedstock, which can be copolymerized with heterocycles such as epoxides, aziridines, and episulfides. In 1969, Inoue reported the zinc catalyzed sequential copolymerization of carbon dioxide and epoxides as a new route to poly(carbonate)s (9, 35). The reaction is shown in Figure 1.1.
Figure 1.1 Reaction of carbon dioxide with epoxides (35).
Plants produce a wide range of biopolymers for purposes such as maintenance of structural integrity, carbon storage, and defense against pathogens as well as desiccation. Several of these natural polymers can be used by humans as food and materials, and increasingly as an energy carrier. Plant biopolymers can also be used as materials in certain bulk applications such as plastics and elastomers (36).
Lignin, suberin, vegetable oils, tannins, natural monomers like terpenes, and monomers derived from sugars are typically natural precursors for bio-based industrial polymers. Glycerol and ethanol also play a potential role as future precursors to monomers (37).
1.2 Green Chemistry
The principles and concepts of green chemistry are the subjects of several monographs (38–47). Recent progress in enzyme-driven green syntheses of industrially important molecules has been summarized (48). Studies in biotechnological production of pharmaceuticals, flavors, fragrance and cosmetics, fine chemicals, as well as polymeric materials (49) have been documented. Biocatalysis is a transformational technology uniquely suited to delivering green chemistry solutions for safer, efficient, and more cost-effective chemical synthesis.
The different catalytic processes for the conversion of terpenes, triglycerides and carbohydrates to valuable chemicals and polymers have been reviewed (50).
A basic task of green chemistry is to design chemical products and processes that use and produce less hazardous materials. The term hazardous covers several aspects, including toxicity, flammability, explosion potential and environmental persistence (51).
The synthesis of maleic anhydride illuminates a possibility of multiple pathways. Maleic anhydride can be synthesized both from benzene and from butene by oxidation. In the first route, a lot of carbon dioxide is formed as an undesirable byproduct. Thus, the first route is addressed as atom uneconomic. In Table 1.1, some uneconomic and economic reaction types in organic chemistry are opposed.
There were in total 12 basic principles in green chemistry (52–55). These principles are summarized in Table 1.2.
Table 1.1 Atom uneconomic and economic reaction types.
Economic
Uneconomic
Rearrangement reaction
Substitution reaction
Addition reaction
Elimination reaction
Diels-Alder reaction
Wittig reaction
Claisen reaction
Grignard reaction
Table 1.2 Basic principles of green chemistry (53).
Principle
Ensure that all material and energy inputs and outputs are as inherently nonhazardous as possible.
Better prevent waste than cleanup.
Minimize energy consumption and materials.
Maximize efficiency of mass, energy, space, and time.
Products, processes, and systems should be
output pulled
rather than
input pushed
.
Embedded entropy and complexity must be viewed as an investment when making design choices on recycle, reuse, or beneficial disposition.
The design goal should be targeted durability.
Unnecessary capacity or capability is not desirable.
Material diversity in multicomponent products should be minimized.
Development of products, processes, and systems must consider energy and materials flows.
The design should consider a commercial
afterlife
.
Material and energy inputs should be renewable.
Recently, the above-mentioned concept was extended (56). The special volume on green and sustainable chemistry and engineering has fourteen papers that were considered relevant to the present day issues and discussion, such as adequate use of raw materials and efficient energy, besides considering renewable sources for materials and energy; and changing economical canons towards circular economy. Businesses, governments and societies are facing a number of challenges along the pathway to sustainability for the well-being of future generations. Chemicals are ubiquitous in everyday activities. Their widespread presence provides benefits to societies’ well-being, but can have some deleterious effects. To counteract such effects, green engineering and sustainable assessment in industrial processes have been gathering momentum in the last thirty years. Green chemistry, green engineering, eco-efficiency, and sustainability are becoming a necessity for assessing and managing products and processes in the chemical industry. Fourteen articles have been discussed, related to sustainable resource and energy use (five articles), circular economy (one article), cleaner production and sustainable process assessment (five articles), and innovation in chemical products (three articles) (56).
Catalytic processes from the viewpoint of green chemistry include catalytic reductions and oxidations methods, solid-acid and solid-base catalysis, as well as carbon-carbon bond formation reactions (57).
Novel concepts and techniques such as bio-inspired polymer design, synthetically-inspired material development are now considered to contribute to the development of natural monomers and polymers as a sustainable resource. These concepts and techniques that integrate materials synthesis, process and manufacturing options with eco-efficiency have been documented (58–62).
1.2.1 Genetic Engineering
The direct production of novel compounds in biomass crops in order to produce bioenergy as a coproduct seems to be a promising way to improve the economics of transgenic plants as biofactories (63).
Genetic engineering of plants may be used for the production of novel polymers and basic chemicals. These methods may help to alleviate the demands for limited resources and provide a platform to produce some desired compounds in bulk quantities.
Recent advances in enhancing the production of novel compounds in transgenic plants consist of a multigene transformation and the direction of the biosynthetic pathways to specific intracellular compartments.
Basically it appears feasible to produce interesting proteins, such as spider silk or collagen, novel carbohydrates, and biopolymers using transgenic plants. These compounds could replace petroleum-based plastics (63). However, there are pro and con arguments. For example, if transgenic plant factories should compete with conventional production processes, economic efficiency and sustainability are important. These factors depend on the future development of oil and energy prices.
On the other hand, societal factors such as the public acceptance of transgenic plants are also key factors (63). Chemicals that may be produced from biomass or in transgenic plants are listed in Table 1.3.
Cyanophycin is an attractive biopolymer with chemical and material properties that are suitable for industrial applications in the fields of food, medicine, cosmetics, nutrition, and agriculture (65). For the efficient production of cyanophycin, cyanophycin synthetases have been investigated and also fermentations and downstream processes have been elucidated.
The strategies for cyanophycin production in microbial strains, including Escherichia coli, Pseudomonas putida, Ralstonia eutropha, Rhizopus oryzae, and Saccharomyces cerevisiae, heterologously expressing different cyanophycin synthetase genes have been reviewed.
Also, the chemical and material properties of cyanophycin and its derivatives produced through biological or chemical modifications were addressed in the context of their industrial applications (65).
The biochemical pathways for producing 2(E)-heptenedioyl-CoA methyl ester have been described (66). Also, methods for enzymatically converting hept-2-enedioyl-CoA methyl ester to pimeloyl-CoA using a polypeptide have been shown.
Table 1.3 Chemicals from biomass or occurring in transgenic plants (64).
Compound
Remark
Succinic acid
Natural substances in plants
Fumaric acid
Natural substances in plants
Malic acid
Natural substances in plants
2,5-Furandicarboxylic acid
Oxidative dehydration of C6 sugars
3-Hydroxypropionic acid
Fermentation from sugar
Aspartic acid
Amination of fumaric acid
Glucaric acid
Oxidation of starch
Glutamic acid
Byproduct of sucrose production
Itaconic acid
Conversion of aconitic acid
Levulinic acid
Acid-catalyzed dehydration of cellulose
3-Hydroxybutyrolactone
Oxidation of starch
Glycerol
In plant oil
Sorbitol
Hydrogenation of sugars
Xylitol
Hydrogenation of sugars
Lysine
Fermentation
Proline
Fermentation
Arginine
A component of cyanophycin
Isomaltulose
From sucrose
Inulin
From chicory
1.3 Commercial Situation
The chemistry, important applications, and the market potential of intrinsically biodegradable polymers have been reviewed (67). One method for evaluating the potential demand for biodegradable polymers is to review the applications and necessary pricing to penetrate various end uses. Each application end use has a price hurdle associated with it.
However, the true market potential for biodegradable plastics will depend on:
The selling price of the material,
Environmental pressure,
Legislation,
Establishment of standards for degradability,
The development of composting infrastructure, and
The ability to overcome the problem of potentially contaminating the pool of recyclable materials.
These factors are difficult to predict since there are external forces that may not be universally applied in the same manner. The interest in biodegradable plastics has continuously grown as the conventional resources based on petroleum are beginning to decrease. The last two decades of the twentieth century saw a paradigm shift from biostable to biodegradable materials.
For example, in the next couple of years, many of the permanent prosthetic devices used for temporary therapeutic applications will be replaced by biodegradable devices that could help the body to repair and regenerate the damaged tissues (68).
Finding applications for renewable polymers that lead to mass production and price reduction poses a major contemporary challenge. This can be attained by improving the end performance of the biodegradable polymers (69).
The complexities of renewable supply chains have been elucidated (70). In particular, polymers manufactured from renewable feedstocks will augment various industrial markets such as plant material used as a renewable ingredient in paint manufacture, partially substituting for crude oil derivative ingredients. Polymer industrial supply chains have been identified and the market opportunity for renewable polymers has been estimated.
The developments in the field of renewable polymers illustrate how business models can link producers and customers through the development of new technologies and products (71). Initially, the companies assumed that reducing the costs and increasing the production will guarantee success of biopolymers in the market. However, some unconventional hurdles emerged. Companies have build markets for biopolymers and to assure customers that biopolymers are in fact produced sustainably.
Several companies have identified new market opportunities for biopolymers, designed distinctive types of business models to seize these opportunities, and developed ways to create an increased value by communicating performance advantages and the reduction of the environmental impact to downstream entities.
However, because they did not include societal factors in their efforts to define the term sustainable, a significant risk emerges that their sustainable value propositions may not endure without further refinements (71).
1.4 Environmental Situation
Polymer waste management options are shown in Figure 1.2. The utilization of waste polymers by mechanical recycling and incineration has ecological limitations.
The impact of biodegradable polymers on the environment and on the society has been detailed (72).
Landfills contain a tremendous amount of plastic waste. As the plastics degrade, pollutants leach into the soil and gases escape into the air. In response to this issue, the concept of recycling has been introduced into the consumption cycle. Recycling generally involves processing of the used materials into new products. However, the processing of waste can be economically ineffective, as it entails various mechanisms, such as:
Collecting the waste,
Sorting the waste according to provided specifications, and
Processing the waste into materials that can be used in new products in the final stage.
Figure 1.2 Polymer waste management options (73, 74).
Aside from the ineffective processes, recycling is not widely available in all communities and if available, often is not mandatory. Thus, many individuals either do not have a convenient venue for recycling or simply choose not to recycle (75).
Recycling has its disadvantages as well. The sorting and shipping of the plastic waste to the appropriate recycling facility is costly, both monetarily and environmentally. Different types of plastics must be recycled separately because they do not cooperate to form a stable reusable plastic. In addition, many plastics have a limited recyclable life. For example, recycling plastic water bottles can result in a lower grade plastic that cannot be converted into a new plastic water bottle.
Other methods of preventing this pollution include the partial use of biodegradable materials in plastic products. Certain auxiliary elements, made of biodegradable material, are then incorporated within the container. However, the remainder of the container is substantially plastic based on petroleum. If these mixed products are included with other plastics for recycling, they can contaminate the product and render it unusable (75).
Materials such as paper, paperboard, plastics, and even metals are presently used in enormous quantities in the manufacture of articles such as containers, separators, dividers, lids, tops, cans, and other packaging materials.
Most of the currently used food packaging materials are non-degradable and are creating serious environmental problems. New technologies are being explored and developed to study the complex interaction between the food packaging materials and food (76). For example, a nanocomposite of cellulose constitutes environmentally friendly packaging, which is easily recycled by combustion and requires low power consumption in production.
There are several such biodegradable materials which are available at a low price, have good mechanical properties and allow disposal in the soil. This is advantageous because biological degradation produces only carbon dioxide, water, and inorganic compounds. Also, it has been discovered that biodegradable plastics made of such materials can be disposed of together with organic waste. The widespread use of biopolymers in the place of standard plastics would help to reduce the weight of waste. Therefore, biodegradable materials take part in the natural cycle from nature to nature and play an important role for environmental sustainability (76).
Modern processing and packaging technology allows a wide range of liquid and solid goods to be stored, packaged, and shipped in packaging materials while being protected from harmful elements such as gases, moisture, light, microorganisms, vermin, physical shock, crushing forces, vibration, leaking, or spilling. Many of these materials are characterized as being disposable, but actually have little, if any, functional biodegradability. For many of these products, the time for degradation in the environment can span decades or even centuries (77).
Each year, over 100 billion aluminum cans, billions of glass bottles, and thousands of tons of paper and plastic are used in storing and dispensing soft drinks, juices, processed foods, grains, beer and other products. In the United States approximately 5.5 million tons of paper are consumed each year in packaging materials, which represents about 15% of the total annual domestic paper production.
Packaging materials are all, to varying extents, damaging to the environment. For example, the manufacture of poly(styrene) (PS) products involves the use of a variety of hazardous chemicals and starting materials, such as benzene, a known mutagen and a probable carcinogen. Chlorofluorocarbons have also been used in the manufacture of blown or expanded PS prod ucts. Chlorofluorocarbons have been linked to the destruction of the ozone layer.
Due to widespread environmental concerns, there has been significant pressure on companies to discontinue the use of PS products in favor of more environmentally safe materials. Some groups have favored the use of products such as paper or other products made from wood pulp. However, there remain drawbacks to the sole use of paper due to the tremendous amount of energy that is required to produce it. A strong need to find new, easily degradable materials that meet necessary performance standards remains (77).
The concept of sustainable bio-based products is as follows (78): A bio-based product derived from renewable resources should have a recycling capability and a triggered biodegradability. This means that it is stable in the course of service time. However, it should biodegrade after disposal under the specific conditions of composting. This composting procedure should also be commercially viable.
The general situation of compostable polymer materials has been described in detail in a monograph (79).
From a waste management perspective, high-barrier, multilayer, biodegradable food packaging could be a useful replacement for current multilayered packaging that is non-recyclable and non-degradable (80).
It has been envisioned that a biodegradable thermoplastic starch and poly(β-hydroxyalkanoate) (PHA) layered material could be a promising target. In a study an attempt was made to quantify the greenhouse gas trade-offs associated with using the proposed biodegradable packaging and identify the important design considerations (80).
The study also considered the impacts of landfill methane capture efficiency, which is an important aspect as biodegradable packaging may release methane when disposed of in a landfill whereas non-biodegradable packaging is inert (80). However, a key result is that when food waste is included in the system boundaries, it contributes over 50% of the greenhouse gas emissions associated with the system, regardless of whether the package is biodegradable or not.
This shows that even for biodegradable packaging, reducing food waste is a key design consideration. In fact, the negative environmental impacts associated with disposal of a PHA-biodegradable thermoplastic starch packaging in landfill with low gas capture rates can actually be offset if the package reduces food wastage (beef) by approximately 6%. The overarching result is that a PHA-biodegradable thermoplastic starch food packaging only delivers positive greenhouse gas outcomes if it reduces food wastage or increases the viability of biological food waste processing (80).
1.4.1 Problems with Bio-based Composites
Bio-based composites often exhibit unsatisfactory properties, such as, or resulting from (81):
Inadequate processing conditions, resulting in filler agglomeration and poor filler dispersion within the matrix,
Variations in natural fiber properties, often due to geographical and seasonal variability,
Anisotropy of the natural fibers themselves,
High linear coefficient of thermal expansion for natural fibers, and
Incompatibility between typically hydrophilic natural fibers and hydrophobic polymer matrices, resulting in poor interfacial adhesion between the phases.
The chemical modification of a natural fiber is often performed in order to enhance the properties of the interface between fiber and matrix. A more recent method of modification involves the deposition of a coating of nanosized cellulose onto the natural fibers or dispersing a nanosized cellulose in natural fiber reinforced composites. This method has been shown to improve the fiber-matrix interface and the overall mechanical performances. Such composites have been addressed as hierarchical, multiscale, nanoengineered, or nanostructured composites. The state-of-the-art in this field has been reviewed (81).
Natural polymer blends and nanocomposites as well as natural fiber reinforced composites and other aspects have been reviewed in a monograph (82).
1.4.2 Biodegradation
Biodegradable polymers are commonly those that undergo a microbially induced degradation, i.e., a chain scission. This will result in mineralization, photodegradation, oxidation, and hydrolysis. In this way, the polymer is altered in the course of the degradation process (83).
Biodegradation reactions are typically enzyme catalyzed and occur in aqueous media. Natural macromolecules containing hydrolyzable linkages, such as protein, cellulose and starch, are generally susceptible to biodegradation by the hydrolytic enzymes of microorganisms.
A few man-made polymers, however, are also biodegradable. The hydrophilic character of the polymers greatly affects their biodegradability. A general rule is that polar polymers are more readily biodegradable. Other important polymer characteristics that affect the biodegradability include crystallinity and chain flexibility.
Besides being able to biodegrade, it is often important for a polymer to exhibit certain physical properties such as stiffness, flexibility, water resistance, strength, elongation, temperature stability, or gas permeability (84).
The intended application often dictates the necessary properties. For example, in the case of sheets and films for usage as packaging materials, the desired criteria of performance include elongation, printability, imperviousness to liquids, temperature stability, etc.
Since the number of biodegradable polymers is limited, it is often difficult, or impossible, to select a single polymer or copolymer that meets all the desired performance criteria.
Polymers that have a high glass transition temperature (Tg) are difficult to be blown into films or are too brittle for use as a packaging material. In contrast, polymers with a low glass transition temperature show low softening and melting temperatures. This makes the fabrication of sheets and films difficult without self- adhesion. In addition, such sheets may lack adequate water permeation, which is necessary for packaging applications (84).
1.4.2.1 Details of the Mechanism of Degradation
In general, biodegradable plastics are believed to undergo enzymatic biodegradation through the following processes (85):
A polymer degrading enzyme is first adsorbed onto the surface of the polymer material. This enzyme is a substance secreted extracellularly by a certain kind of microorganism.
The enzyme then breaks down chemical bonds in polymer chains, such as ester, glycosidic, and peptide bonds, by a hydrolysis reaction.
As a result, the polymer material is reduced in molecular weight and even decomposes to a low molecular weight compound unit by the degrading enzyme.
Finally, decomposed products are further metabolized and utilized by various microorganisms and converted into carbon dioxide, water, and bacterial cell components.
1.4.2.2 Controlling the Rate of Biodegradation
The rate of biodegradation can be controlled. The essential ingredients for such a composition are a biodegradable polymer, a carbodiimide compound, an ultraviolet ray absorbent, and an antioxidant (85).
Examples of carbodiimide compounds are summarized in Table 1.4.
N,N´-Dicyclohexylcarbodiimide and N,N´-diisopropylcarbodiimide are particularly preferable because of their industrial availability. Polycarbodiimides can be synthesized from isocyanates by conventional methods (86). As carbodiimidization catalyst, 3-methyl-1-phenyl-2-phospholene-1-oxide is used.
Antioxidants are collected in Table 1.5 and examples of ultraviolet absorbers are summarized in Table 1.6. Some ultraviolet absorbers are also shown in Figure 1.3.
These compounds can be used as additives to tailor the properties of the polymers if desired.
Table 1.4 Carbodiimides (85).
Monocarbodiimides
N
,
N
´-dicyclohexylcarbodiimide
N
,
N
´-Diisopropylcarbodiimide
Dimethylcarbodiimide
Diisobutylcarbodiimide
dioctylcarbodiimide
tert
-Butylisopropylcarbodiimide
Diphenylcarbodiimide
Di-
tert
-butylcarbodiimide
Di-β-naphthylcarbodiimide
Isocycanate Monomers for Polycarbodiimides
4,4´-Dicyclohexylmethane diisocyanate
Isophorone diisocyanate
Tetramethylxylylene diisocyanate
Table 1.5 Antioxidants (85).
Hindered Phenol-based Antioxidants
4,4´-Methylene-bis-(2,6-di-
tert
-butylphenol)
Octadecyl-3-(3,5-di-
tert
-butyl-4-hydroxyphenyl)propionate
Phosphite-based Antioxidants
Tris-(2,4-di-
tert
-butylphenyl)phosphite
Bis-(2,4,di-
tert
-butylphenyl)pentaerythritol-diphosphite
Bis-(2,6-di-
tert
-butyl-4-methylphenyl)pentaerythritol-diphosphite
Table 1.6 Ultraviolet absorbers (85).
Benzotriazole-based Compounds
2-(2´-Hydroxy-5´-methylphenyl)benzotriazole
2-(2´-Hydroxy-5´-
tert
-butylphenyl)benzotriazole
2-(2´-Hydroxy-3´-
tert
-butyl-5´-methylphenyl)-5-chlorobenzotriazole
2-(3,5-Di-
tert
-butyl-2-hydroxyphenyl)-5-chlorobenzotriazole
2-[2´-Hydroxy-3´,5´-bis(α,α-dimethylbenzyl)phenyl]benzotriazole
Triazine-based Compounds
2-(4,6-Diphenyl-1,3,5-triazine-2-yl)-5-[(hexyl)oxy]phenol
2-[4,6-Bis(2,4-dimethylphenyl)-1,3,5-triazine-2-yl]-5-[(octyl)oxy]phenol
2,4,6-Triamino-1,3,5-triazine
2,4-Diamino-6-phenyl-1,3,5-triazine
2,4-Diamino-6-methyl-1,3,5-triazine
2,4-Diamino-6-(2-(dodecasylamino)ethyl)-1,3,5-triazine
2,4-Diamino-6-(o-methoxyphenyl)-1,3,5-triazine
4,6-Diamino-1,2-dihydro-2,2-dimethyl-1-(2,6-xylyl)-1,3,5-triazine
2,4-Diamino-6-(2-methoxyethyl)-1,3,5-triazine
2-Amino-4-ethyl-1,3,5-triazine
2-Amino-4-phenyl-1,3,5-triazine
2-Amino-4-ethyl-6-methyl-1,3,5-triazine
Figure 1.3 Ultraviolet absorbers.
1.5 Properties of Biodegradable Polymers
Biopolymers are polymers that degrade through the action of living organisms. Such polymers include a range of synthetic polymers such as poly(ester)s, poly(ester amide)s, poly(carbonate)s, and others (84). In Table 1.7 the mechanical properties of E-glass and some natural fibers are compared.
Table 1.7 Mechanical properties (78).
Material
Density /[
g cm
−3
]
E-Modulus /[
G Pa
]
E-glass
2.55
73
Hemp
1.48
70
Flax
1.4
60–80
Jute
1.46
10–30
Sisal
1.33
38
Coir
1.25
6
Cotton
1.51
12
As can be seen from Table 1.7, in the case of hemp and flax, the elastic modulus is comparable to E-glass and the density is even smaller than that of E-glass.
1.6 Special Methods of Synthesis
The methods of synthesis of biodegradable polymers can be subdivided into conventional methods, click chemistry, enzymatic polymerization and polycondensation, chemoenzymatic polymerization, vine-twining polymerization, and bacterial synthesis. These methods will be detailed subsequently.
1.6.1 Conventional Methods
Biodegradable polymers can be synthesized using renewable resources by conventional methods (33). The recent advances in the synthesis of polymers from renewable resources have been shown (87).
For example, yttrium complexes are suitable compounds for the synthesis of poly(lactide)s (33).
Also, a chiral Schiff base/aluminium alkoxide can be used as initiator for the stereoelective polymerization of rac-(D,L)-lactide (88). The compound is shown in Figure 1.4.
Figure 1.4 Chiral Schiff base (88).
A rather high stereoelectivity is observed, as a polymer with an 88% enantiomeric enrichment in the D units can be isolated at 19% conversion. At higher conversions a stereo-complex from D- and L-enriched stereocopolymers is formed. A narrow molecular weight distribution of Mw/Mn in the range of 1.05–1.30 is observed even at very high conversions. Thus, the polymerization reaction shows living type features (88).
1.6.2 Click Chemistry
Click chemistry is a powerful strategy that relies mainly on the construction of carbon-heteroatom bonds. It has been described in the present meaning in 2001 (89).
Applications are found in a wide variety of methods in modern chemistry, including biocompatible synthesis (90). For example, adhesive polymers can be formed when polyvalent azides and alkynes are assembled into crosslinked polymer networks by copper-catalyzed 1,3-dipolar cycloaddition (91). The polycondensation is efficiently promoted by copper ions.
As another example, poly(glycolide)s have been described that include a polymeric alkynyl-substituted glycolide (92). The alkynyl groups provide reactive sites for further functionalization of the polymer, for example, by reaction with azide derivatives. The alkynyl and azide groups react via the click chemistry mechanism to form functional groups covalently bonded to the polymer via a triazole link. The polymers are biodegradable and can be used to deliver drugs or other therapeutic substances at controlled release rates.
The thiol-ene click reactions are considered a versatile and broadly applicable tool in polymer science (93). This reaction can be used as a click reaction for the synthesis of monomers as well as of dendrimers. Also, polysaccharides can be derivatized via a click reaction (94).
1.6.3 Enzymatic Polymerization
The field of enzymatic polymerization has been reviewed in detail (95–97). Enzymes have been industrially established in large-scale synthesis and degradation, such as the use of:
Glucose isomerase in food industry,
Cellulase for textile finishing,
Lipase, protease, and cellulase in detergents, and
Catalysts in chemical and pharmaceutical industries.
Enzymatic polymerization is an important issue for green polymer chemistry to save energy in production processes and to reduce the formation of undesired byproducts since the reaction is mostly selective. In general, an enzyme catalyzed reaction proceeds much faster than a conventional reaction, by lowering the activation energy.
Typical polymers that can be synthesized by an in-vitro enzymatic catalysis are summarized in Table 1.8. The basic concept of the in-vitro enzymatic polymerization can be explained as follows: In the course of an in-vitro enzymatic polymerization, a monomer is treated by the catalyst enzyme as an unnatural or artificial substrate. But the substrate must be recognized and form a complex with the enzyme in order to allow a reaction.
Table 1.8 Polymers by in-vitro enzymatic catalysis.
Enzyme types
Polymers
Oxidoreductases
Polyphenols, polyanilines, vinyl polymers
Transferases
Polysaccharides, cyclic oligosaccharides, polyesters
Hydrolases
Polysaccharides, polyesters, poly(carbonate)s, polyamides, polyphosphates, polythioesters
The polymerization of catechol under high-pressure homogenization was catalyzed by laccase from Myceliophthora thermophila (98). This polymer was used for the green coloration of textile substrates.
The oxidation reactions were conducted using different forms of laccase, i.e., native laccase, PEGylated laccase and PEGylated laccase immobilized onto an epoxy resin. These three enzyme forms were deposited inside a polyester fabric bag during the experiments. The amount of polymer obtained was similar when using the three enzyme forms and its dispersion in a water/dimethyl sulfoxide mixture led to powder particles of about 30–60 nm.
It could be shown that the oxidation of catechol conducted under high-pressure homogenization can be an efficient methodology for the in-situ coloration of textiles. The polymers produced by this methodology strongly stained the textile container, revealing this experimental setup as a promising greener coloration/coating methodology involving milder conditions than that normally used in textile processes (98).
2,5-Furandicarboxylic acid-based semi-aromatic poly(amide)s were synthesized using enzymatic polymerization (99). These polymers are bio-based alternatives to poly(phthalamide)s, which are petrol-based semi-aromatic polyamides. From a commercial perspective, they have interesting properties as high-performance materials and engineering thermoplastics. It is even more appealing to explore novel 2,5-furandicarboxylic acid-based polyamides with added functionality for the development of sustainable functional materials.
Here, a set of 2,5-furandicarboxylic acid-based heteroatom polyamides have been successfully produced via a Novozyme 435 (N435)-catalyzed polymerization of bio-based dimethyl 2,5-furandicarboxylate with potentially heteroatom diamines, i.e., 4,9-dioxa-1,12-dodecanediamine, diethylenetriamine, and 3,3-ethylenediiminopropylamine (99). The enzymatic polymerization reactions were performed both in solution and in bulk. The latter approach is more sustainable and results in high- er molecular weight products. Among the tested heteroatom diamines, N435 showed the highest catalytic activity toward 4,9-dioxa-1,12-dodecanediamine. Furthermore, it was found that all obtained 2,5-furandicarboxylic acid-based heteroatom polyamides are amorphous materials with a relatively high thermal stability. These heteroatom polyamides show glass transition temperatures ranging from 41°C to 107°C (99).
Enzymatic ring-opening polymerization is an appealing method for the preparation of poly(lactide)s and poly(lactone)s (100). These reactions are typically carried out at relatively high temperatures of 60–130°C.
However, there is a deficiency of enzyme-compatible solvents for such thermally demanding biocatalytic processes. A series of short-chained glycol-grafted ionic liquids were prepared based on a phosphonium, imidazolium, pyridinium, ammonium, or piperidinium cationic headgroup.
Most of these glycol-grafted ionic liquids exhibit relatively low dynamic viscosities (33–123 mPa s at 30°C), coupled with excellent short-term thermal stabilities with decomposition temperatures in the 318°C–403°C range (100). Significantly, the long-term thermal stability under conditions matching those for enzymatic ring-opening polymerization synthesis (130°C for 7 d) is excellent for several of these task-specific ionic liquids.
Using a Novozym 435-catalyzed ring-opening polymerization, these ionic liquids were demonstrated to be viable solvents for the enzymatic production of reasonable yields (30–48%) of high molecular mass (Mw ∼ 20 kDa) poly(L-lactide) and poly(ε-caprolactone) compared to solventless conditions (12–14 kDa) (100).
1.6.3.1 Polycondensation
The recent developments in lipase-catalyzed synthesis of polyesters have been reviewed (101). A series of diacids, such as succinic acid, glutaric acid, adipic acid, and sebacic acid, and diols, such as 1,4-butanediol, 1,6-hexanediol, and 1,8-octanediol have been polymerized in solution and in bulk using lipase as a catalyst (102, 103).
Polymerization reactions with longer chain length monomers show a higher reactivity than reactions of shorter chain length monomers (103). The lipase-catalyzed reaction of an alcohol with a vinyl ester proceeds much faster than with an alkyl ester (104).
The dependence of the origin of the particular lipase on the conversion in the ring-opening polymerization of 15-pentadecanolactone is shown in Table 1.9.
Table 1.9 Origin of lipase and conversion of 15-pentadecanolactone (101).
Lipase Type
Conversion/[%]
None
0
Rhizopus japanicus
(lipase RJ)
<5
Hog liver (HLE)
<5
Penicillium roqueforti
(lipase PR)
12
Aspergillus niger
(lipase A)
16
Candida rugosa
(lipase CR)
21
PPL
27
Lipase CC
54
Pseudomonas cepacia
(lipase PC)
90
Lipase PF
97
1.6.4 Chemoenzymatic Polymerization
The mechanism of catalysis of green biocatalysts and metal catalysts is quite different, but if both are mutually compatible, then it allows the application concurrently in the same reaction system. The combination of these different types of catalysts is also known as chemoenzymatic method. Utilizing the advantages of enzymes, the chemoenzymatic method has been developed for the synthesis of various block copolymers, which are otherwise difficult to prepare.
The combination of the lipase-catalyzed ring-opening polymerization of lactones and the atom transfer radical polymerization allows a versatile synthesis of block copolymers consisting of a polyester chain and a vinyl polymer chain (105). Also, branched polymers have been produced by the chemoenzymatic technique (106).
1.6.4.1 Cyclodextrins
Cyclodextrins can polymerize cyclic esters such as lactones and lactides (107). They can initiate the polymerization of cyclic esters in bulk without any solvents to give products in high yields.
Cyclodextrins are cyclic oligosaccharides. α-Cyclodextrin is a 6-membered ring, β-cyclodextrin is a 7-membered ring, and γ-cyclodextrin is an 8-membered ring. The structure of α-cyclodextrin is shown in Figure 1.5.
Figure 1.5 α-Cyclodextrin.
It has been found that cyclodextrins are active not only as initiators and catalysts but also support certain architectures of the final product similar to chaperone proteins.
Chaperones are proteins that assist the folding or unfolding process and thus the assembly or disassembly of other macromolecular structures. However, they do not appear in those macromolecular structures (108).
The cyclodextrin may encircle a linear polymer chain so that the chain assumes the proper conformation and avoids coagulation. So, a cyclodextrin mimics the strategy that a living system uses to form polymers. It is believed that such a system could provide an environmentally friendly route to produce biodegradable functional polymers (107).
Multi-responsive cyclodextrin vesicles have been prepared and shown to be self-assembled by supramolecular bola-amphiphiles, consisting of (N,N´-bis(ferrocenylmethylene)-diaminohexane a guest, and as host, i.e., γ-hydroxybutyric-β-cyclodextrin (109). The vesicles may serve as redox-responsive systems.
Bola-amphiphiles are amphiphilic molecules that have hydrophilic groups at both ends of a sufficiently long hydrophobic hydrocarbon chain. Basically, a bola-amphiphile looks like the schematic shown in Figure 1.6.
Figure 1.6 Schematic illustration of a bola-amphiphile.
1.6.5 Vine-Twining Polymerization
The Vine-twining polymerization is a method for the preparation of well-defined supramolecules, which are amylose polymer inclusion complexes (110). The method consists of the enzymatic polymerization of α-D-glucose-1-phosphate catalyzed by phosphorylase in the presence of various synthetic polymers such as poly(ether)s, poly(ester)s, poly(ester ether)s, and amphiphilic block copolymers. The 1H-NMR spectra of the polymers indicated structures composed from amylose and guest polymers.
The preparation of inclusion complexes composed of amylose and hydrophobic poly(carbonate)s have been achieved by vine-twining polymerization (111, 112). The structure of such complexes is shown in Figure 1.7.
This is a phosphorylase-catalyzed enzymatic polymerization of an α-D-glucose-1-phosphate from a maltoheptose in the presence of poly(carbonate)s. Poly(carbonate)s with a shorter methylene chain length, such as poly(tetramethylene carbonate), is more favorable as the guest polymer to form the inclusion complex with amylose.
Figure 1.7 Inclusion complexes (112).
It has been shown that amylose selectively includes poly(tetrahydrofuran) or poly(δ-valerolactone) from a mixture of two resemblant poly(ether)s, i.e., poly(oxetane) and poly(tetrahydrofuran), or also a mixture of two resemblant polyesters in vine-twining polymerization (113). The highest priority on the inclusion of amylose showed poly(tetrahydrofuran). Also, a selective inclusion according to the molecular weight of a poly(tetrahydrofuran) was demonstrated (114).
The enzymatic polymerization was investigated at 80°C using a primer-grafted poly(γ-glutamic acid) in the presence or absence of poly(L-lactic acid) as a guest polymer for the inclusion by amylose (115). The produced amylose-grafted poly(γ-glutamic acid)s formed microparticles by cooling the mixtures at room temperature after the enzymatic polymerization in either the presence or absence of poly(L-lactic acid). The particle sizes, which were evaluated by scanning electron microscopy (SEM), were dependent on the feed ratios of poly(L-lactic acid) (115).
It has been found that amylosic inclusion complexes, which were obtained by vine-twining polymerization using a designed guest polymer, i.e., an amphiphilic triblock copolymer poly(2-methyl-2-oxazoline-block-tetrahydrofuran-block-2-methyl-2-oxazoline), exhibited gel and film formation properties (116).
The characterization of the products suggested that enzymatically elongated amylose chains were complexed with the poly(tetrahydrofuran) block in the triblock copolymer (116).
Here, the outer poly(2-methyl-2-oxazoline) blocks constructed hydrophilic spaces among the inclusion complex segments. Furthermore, the presence of such outer blocks affected the low- er regularity of crystalline alignment among the inclusion complex segments in the products. Probably, such higher-order structures induced the formation of supramolecular soft materials such as gels and films (116).
1.6.6 Bacterial Synthesis
Bacteria can synthesize a wide range of biopolymers. The key aspects of the production of bacterial biopolymers have been reviewed (117, 118). It is expected that a better understanding of polymer biosynthesis and material properties can lead to an increased use of bacterial biopolymers.
PHAs are biopolymers that can be synthesized by microorganisms such as the bacterium Burkholderia xenovorans LB400 (119). In particular, LB400 cells are capable of synthesizing poly(3-hydroxybutyrate) (PHB) from glucose.
An attempt has made to produce and characterize electrospun fibers obtained from bacterial PHB (119). Here, the bacterial strain LB400 was grown in M9 minimal medium using xylose and mannitol (10 gl−1) as the sole carbon sources and NH4Cl (1 gl−1) as the sole nitrogen source.
The so-obtained biopolymer-based films were used to produce fibers by electrospinning. The diameter and the morphology of the microfibers were analyzed by SEM and also their thermogravimetric properties were investigated. Bead-free fibers using both PHBs were obtained with diameters of less than 3 µm. The surface morphology of the microfibers based on PHBs obtained from both carbon sources was different, even though their thermogravimetric properties were found to be similar. These results indicate that the carbon source may determine the fiber structure and properties (119).
1.7 Biodegradability Standards
There are several standards available in order to determine the biodegradability of plastic materials. Technical reports have been given that analyze a set of standards, documents and other reports, related to bio-based products (120, 121). The report is limited to matters concerning bio-based products, and hence excludes traditional products, energy applications and food.
In January 2018, the European Union released its strategy for a more sustainable plastics industry to be achieved by the year 2030 (122). Besides promoting plastic recycling, the strategy discusses the opportunities and the risks of biodegradable plastics.
These standards are summarized in Table 1.10. In addition, other certification schemes that are used in various other countries have been collected in the literature (123).
Table 1.10 Biodegradability Standards.
Number
Title
ISO 10210:2012
Plastics – Methods for the preparation of samples for biodegradation testing of plastic materials (
124
)
ISO 13975:2019
Plastics – Determination of the ultimate anaerobic biodegradation of plastic materials in controlled slurry digestion systems – Method by measurement of biogas production (
125
)
ISO 14851:2019
Determination of the ultimate aerobic biodegradability of plastic materials in an aqueous medium – Method by measuring the oxygen demand in a closed respirometer (
126
)
ISO 14852:2018
Determination of the ultimate aerobic biodegradability of plastic materials in an aqueous medium – Method by analysis of evolved carbon dioxide (
127
)
ISO 14853:2005
Plastics – Determination of the ultimate anaerobic biodegradation of plastic materials in an aqueous system – Method by measurement of biogas production
ISO 16929:2018-04
Plastics – Determination of the degree of disintegration of plastic materials under defined composting conditions in a pilot-scale test (
128
)
ISO 15270:2008
Plastics – Guidelines for the recovery and recycling of plastics waste
ISO 14855-1-2012
Determination of the ultimate aerobic biodegradability of plastic materials under controlled composting conditions – Method by analysis of evolved carbon dioxide – Part 1: General method (
129
)
ISO 14855-2-2018
