Polysaccharide Building Blocks

A Sustainable Approach to the Development of Renewable Biomaterials

Youssef Habibi, Lucian A. Lucia

• Publisher: Wiley
• Year: 2012
• Print ISBN:
  9780470874196
• Edition: 1
• eBook ISBN:
  9781118229477
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

This book is an archival reference for the evolving field of biomaterials and their applications in society, focusing on their composition, properties, characterization, chemistry and applications in bioenergy, chemicals, and novel materials and biomaterials.  It has broad appeal due to the recent heightened awareness around bioenergy and biomass as potential replacements for petroleum feedstocks.  The book is divided into three parts: cellulose-based biomaterials, chitin and chitosan biomaterials, and hemicelluloses and other polysaccharides.  Each chapter addresses a separate biomaterial, discussing its chemical, physical, and biological attributes, and hones in on each compound's intrinsic tunability for numerous chemical transformations.  In the current quest for a "green" economy and resources, this book will help inspire scientists towards novel sources for chemicals, materials, and energy in the years to come.

Chapter 1

Recent Advances in Cellulose Chemistry

Thomas Heinze and Katrin Petzold-Welcke

1.1 Introduction

The chemical modification of polysaccharides is still underestimated regarding the structure and hence property design of materials based on renewable resources. At present, the cellulose derivatives commercially produced in large scale are limited to some ester with C₂–C₄ carboxylic acids, including mixed esters and phthalic acid half-esters as well as ethers with methyl-, hydroxyalkyl-, and carboxymethyl functions. In general, organic chemistry of cellulose opens a wide variety of products by esterification and etherification. In addition, novel products may be obtained by nucleophilic displacement reactions, unconventional chemistry like “click reactions,” introduction of dendrons in the cellulose structure, and regiocontrolled reactions within the repeating units and along the polymer chains. The aim of this chapter is to highlight selected recent advances in chemical modification of cellulose for the synthesis of new products with promising properties as well as alternative synthesis paths in particular under homogeneous conditions, that is, starting with dissolved polymer considering own research results adequately.

1.2 Technical Important Cellulosics

The application of the glucane cellulose as a precursor for chemical modifications was exploited extensively even before its polymeric nature was determined and well understood. Cellulose nitrate (commonly misnomered nitrocellulose) of higher nitrogen content was one of the most important explosives. Its partially nitrated ester was among the first polymeric materials used as a “plastic” well known under the trade name of Celluloid. Today, cellulose nitrate is the only inorganic cellulose ester of commercial interest (Balser et al., 1986). Further cellulose products like methyl-, ethyl-, or hydroxyalkyl ethers or cellulose acetate, and, in addition, products with combinations of various functional groups, for example, ethylhydroxyethyl and hydroxypropylmethyl cellulose, cellulose acetopropionates, and acetobutyrates are still important, many decades after their discovery. Ionic cellulose derivatives are also known since a long time. Carboxymethyl cellulose, up to now the most important ionic cellulose ether, was first prepared in 1918 and produced commercially in the early 1920s in Germany (Brandt, 1986). Various cellulose derivatives are produced in large quantities for diversified applications. Their properties are primarily determined by the type of functional group. Moreover, they are influenced significantly by adjusting the degree of functionalization and the degree of polymerization (DP) of the polymer backbone (Table 1.1).

Table 1.1 Examples of Important Cellulose Esters and Ethers Commercially Produced.

1.3 Nucleophilic Displacement Reactions (S_N)

It is well known from the chemistry of low molecular alcohols that hydroxyl functions are converted to a good leaving group for nucleophilic displacement reactions by the formation of the corresponding sulfonic acid esters (Heinze et al., 2006a). Moreover, cellulose derivatives obtained by S_N reactions are suitable starting materials for the preparation of novel products by unconventional chemistry like “click reactions.” Even selectively dendronized celluloses could be prepared.

1.3.1 Cellulose Sulfonates

Typical structures of sulfonic acid esters used in polysaccharide chemistry are shown in Figure 1.1. The synthesis of sulfonic acid esters is realized heterogeneously by reaction of cellulose with sulfonic acid chlorides in aqueous alkaline media (NaOH, Schotten–Baumann reaction), or is most efficiently completely homogeneous in a solvent like N,N-dimethylacetamide (DMA)/LiCl. The main drawback of heterogeneous procedures is a variety of side reactions, including undesired nucleophilic displacement reactions caused especially by long reaction times and high temperatures required. In contrast, the homogeneous process using cellulose dissolved in DMA/LiCl yields well soluble sulfonic acid esters (McCormick and Callais, 1987).

Figure 1.1 Typical sulfonic acid esters of cellulose.

The p-toluenesulfonic (tosyl) and the methanesulfonic (mesyl) acid esters of cellulose are the most widely used sulfonic acid esters, due to their availability and hydrolytic stability (Heinze et al., 2006a). The homogeneous reaction of cellulose in DMA/LiCl with p-toluenesulfonyl chloride permits the preparation of cellulose tosylate with defined degree of substitution (DS) easily controlled by the molar ratio reagent to anhydroglucose unit (AGU) with almost no side reactions (McCormick and Callais, 1986, 1987; Rahn et al., 1996; Siegmund and Klemm, 2002). The structure of the product may depend on both the reaction conditions and the workup procedure used (McCormick et al., 1990). The tosyl chloride may react with DMA in a Vilsmeier–Haak-type reaction forming the O-(p-toluenesulfonyl)-N,N-dimethylacetiminium salt, which attacks the OH groups of the cellulose depending on the reaction conditions used. For a higher efficiency of tosylation of cellulose, stronger bases such as triethylamine (pK_a 10.65) or 4-(dimethylamino)-pyridine (pK_a 9.70) are necessary, which react with the O-(p-toluenesulfonyl)-N,N-dimethylacetiminium salt building a quaternary ammonium salt and hence lead to the formation of tosyl cellulose without undesired side reactions (Figure 1.2) (McCormick et al., 1990). On the contrary, the use of a weak organic base like pyridine (pK_a 5.25) or N,N-dimethylaniline (pK_a 5.15) for the reaction with cellulose yields a reactive N,N-dimethylacetiminium salt, which may form chlorodeoxy celluloses at high temperatures or cellulose acetate after aqueous workup (Heinze et al., 2006a).

Figure 1.2 Mechanism of the reaction of cellulose with p-toluenesulfonyl chloride in DMA/LiCl in the presence of triethylamine. Adapted from McCormick et al. (1990).

Various cellulose materials with degree of polymerization in the range of 280–1020 were transformed to the corresponding tosyl esters (Rahn et al., 1996). DS values in the range of 0.4–2.3 with negligible incorporation of chlorodeoxy groups were obtained at reaction temperatures of 8–10°C for 5–24 h (Table 1.2).

Table 1.2 Results and Conditions of the Reaction of Cellulose with p-Toluenesulfonyl Chloride (TosCl) in DMA/LiCl Applying Triethylamine as Base (2 mol/mol TosCl) for 24 h at 8°C.

Cellulose tosylates are soluble in various organic solvents; beginning at DS of 0.4, solubility in aprotic dipolar solvents like DMA, N,N-dimethylformamide (DMF), and dimethylsulfoxide (DMSO) occurs. The cellulose tosylates become soluble in acetone and dioxane at a DS value of 1.4 and solubility in chloroform and methylene chloride appears at DS of 1.8. Position 6 reacts faster compared to the secondary OH groups at positions 2 and 3, which can be characterized by means of FTIR and NMR spectroscopy of cellulose tosylate (Rahn et al., 1996).

1.3.2 S_N Reactions with Cellulose Sulfonates

Cellulose sulfonates are studied for a broad variety of S_N reactions, as discussed in various review papers (Belyakova et al., 1971; Hon, 1996; Siegmund and Klemm, 2002). Usually the S_N reaction occurs selectively at the primary sulfonates. The mechanism (S_N1 versus S_N2) of nucleophilic substitution reaction of cellulose derivatives is still a subject of discussion. A remarkable finding is that a treatment of partially substituted cellulose tosylates (DS 1.2–1.5) with strong nucleophiles like azide or fluoride ions leads to a substitution of both primary and secondary tosylates (Siegmund and Klemm, 2002; Koschella and Heinze, 2003).

Water-soluble 6-deoxy-6-S-thiosulfato celluloses (Table 1.3) form S–S bridges by oxidation with H₂O₂—in analogy to nonpolymeric compounds of this type (Milligan and Swan, 1962)—leading to waterborne coatings (Klemm, 1998).

Table 1.3 Examples ...

Table of contents

• Cover
• Title Page
• Copyright
• Foreword
• Preface
• Contributors
• Chapter 1: Recent Advances in Cellulose Chemistry
• Chapter 2: Cellulosic Aerogels
• Chapter 3: Nanocelluloses: Emerging Building Blocks for Renewable Materials
• Chapter 4: Interactions of Chitosan with Metals for Water Purification
• Chapter 5: Recent Developments in Chitin and Chitosan Bio-Based Materials Used for Food Preservation
• Chapter 6: Chitin and Chitosan as Biomaterial Building Blocks
• Chapter 7: Chitosan Derivatives for Bioadhesive/Hemostatic Applications: Chemical and Biological Aspects
• Chapter 8: Chitin Nanofibers as Building Blocks for Advanced Materials
• Chapter 9: Electrical Conductivity and Polysaccharides
• Chapter 10: Polysaccharide-Based Porous Materials
• Chapter 11: Starch-Based Bionanocomposites: Processing and Properties
• Chapter 12: Starch-Based Sustainable Materials
• Chapter 13: The Potential of Xylans as Biomaterial Resources
• Chapter 14: Micro- and Nanoparticles from Hemicelluloses
• Chapter 15: Nonxylan Hemicelluloses as a Source of Renewable Materials
• Index