INTRODUCTION AND OBJECTIVES

1 Introduction and Objectives Polysaccharides are unique biopolymers with an enormous structural diversity. Huge amounts of polysaccharides are formed biosynthetically by many organ- isms including plants, animals, fungi, algae, and microorganisms as storage poly- mers and structure forming macromolecules due to their extraordinary ability for structure formation by supramolecular interactions of variable types. In addi- tion, polysaccharides are increasingly recognised as key substances in biotransfor- mation processes regarding, e.g., activity and selectivity. Although the naturally occurring polysaccharides are already outstanding, chemical modification can improve the given features and can even be used to tailor advanced materials. Etherification and esterification of polysaccharides represent the most versatile transformations as they provide easy access to a variety of bio-based materials with valuable properties. In particular, state-of-the-art esterification can yield a broad spectrumofpolysaccharidederivatives,asdiscussedintheframeofthisbookfrom a practical point of view but are currently only used under lab-scale conditions. In contrast, simple esterification of the most abundant polysaccharides cellulose and starch are commercially accepted procedures. Nevertheless, it is the author’s intention to review classical concepts of esterification, such as conversions of cellulose to carboxylic acid esters of C 2 to C 4 acids including mixed derivatives of phthalic acid and cellulose nitrate, which are produced in large quantities. These commercial paths of polysaccharide esterification are carried out exclusively under heterogeneous conditions, at least at the beginning of the conversion. The majority of cellulose acetate (about 900 000 t per year) is based on a route that includes the dissolution of the products formed [1–3]. Research and development offers new opportunities for the synthesis of polysaccharide esters resulting from: – New reagents (ring opening, transesterification), enzymatic acylation and in situ activation of carboxylic acids – Homogeneous reaction paths, i.e., starting with a dissolved polysaccharide and new reaction media – Regioselective esterification applying protecting-group techniques and pro- tecting-group-free methods exploiting the superstructural features of the polysaccharidesaswellasenzymaticallycatalysedprocedures With regard to structure characterisation on the molecular level most important are NMR spectroscopic techniques including specific sample preparation. Having 2 1 Introduction and Objectives been extensively involved in polysaccharide research, we would like to stress that a clear description of structure–property relationships is conveniently accessible notonlyforthecommercialderivatives,butalsoforproductsofimprovedoreven new features by this technique. The combination of new esterification techniques, comprehensive structure characterisation and detailed structure–property relationships is the key for nanoscience and nanotechnology, smart and responsive materials with polysac- charides and also opens new applications in the field of biosensors, selective separation, bioengineering and pharmaceutics. The objective of the book is not to supplement or replace any of the several review articles and books in the field of polysaccharide chemistry and in particu- 1 Introduction and Objectives 3 lar esterification, but rather to describe the important features of typical synthetic routes, efficient structure characterisation, and unconventional polysaccharide esters including structure–property relationships. Additionally, comments about selected new application areas are included. The methods of modification and analysis described are mainly focused on glucans because they represent a large part of naturally occurring polysaccharides. Moreover, glucans are structurally most uniform. In contrast, polysaccharides consisting of various monosaccha- rides and substructures, e.g., galactomannans, or algal polysaccharides exhibit a broad diversity in properties caused by a large number of irreproducible factors. Thus, the features of the algal polysaccharides vary extensively depending on the seaweed species, the part of the plant the alginate was extracted from, and climatic conditions at the time of growth [4]. Modification multiplies the structural and property broadness, and is therefore of limited relevance for these polymers up to now. Structure analysis is hardly achievable. Because most analytical strategies and synthesis paths are adapted from the conversion of glucans, more complex polymers will only be discussed if specific treatment is applied, e.g., esterification of carboxylic acid moieties of alginates [5]. Among the broad variety of these complex polysaccharides, the most important galactomannan guar gum, the algal polysaccharide alginate, the aminoglucane chitin, the hemicellulose xylan, and the fructan inulin are discussed to demonstrate the specifics of these polymers. Although recently the chemical (ring opening polymerisation) and enzymatic synthesis of polysaccharides and polysaccharide derivatives was experimentally achieved (up to now rather low DP values of maximum 40 have been obtained) the polymeranalogous modification of polysaccharides isolated from natural sources is the most important route to new products today and will continue to be the most important in the foreseeable future. Consequently, polymeranalogous reactions are discussed exclusively. It should be pointed out that not necessarily a strictly poly- meranalogous reaction (no change in DP) is required. On the contrary, a certain degradation prior or during the reaction may be a desired goal. We hopethis book fills a gap between various aspects ofpolysaccharide research concerning biosynthesis and isolation, on one hand, and material science, on the other hand. It is hoped that this book will be accepted by the scientific community as a means to stimulate scientists fromdifferent fields to use the chemical modification of polysaccharides as basis for innovative ideas and new experimental pathways. . characterisation and detailed structure–property relationships is the key for nanoscience and nanotechnology, smart and responsive materials with polysac- charides and. articles and books in the field of polysaccharide chemistry and in particu- 1 Introduction and Objectives 3 lar esterification, but rather to describe the