Modified natural polymers

Modification of natural polymers such as starch, cellulose, and proteins is a way of capitalizing on the well-accepted biodegradability of the base material with 

Protein substrates [172] are expected to be similar to the starch grafts the fundamental problem remains the control acrylic acid polymerization to the oligomers range, in order to have complete biodegradabiUty. 

Other grafts to natural materials are exemplified by Dordick s work [173] in which he produced polyesters from sugars and polycarboxylates by enzyme catalysis of the condensation polymerization. These polymers and the method of synthesis may well be the future of renewable resource chemistry. 

Simple chemical reactions on natural polymers are well known to produce polymers such as hydroxyethyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, cellulose acetates and propionates, and many others that have been in commerce for many years. Their biodegradabiUty is often taken for granted, but, in many cases, is not at aU well established. Carboxymethyl cellulose, for example, has been claimed as biodegradable below a degree of substitution of about 2, which is similar to the claim for cellulose acetate. More recently, there has been attempts to more rigorously quantify the biodegradation of the cellulose acetates [174,175] and to establish a property-biodegradation relationship. 

Rhone-Poulenc indicates that cellulose acetate with a degree of substitution of about 2 is biodegradable, in agreement with its earlier reference [176]. Cellulose has been discussed as a renewable resource [177], A recent publication [178] on chitosan reacted with citric acid indicates that the ampholytic product is biodegradable. Chitosan acetate liquid crystals [179], hydrophobic amide derivatives [180], and crossUnked chitosan [181] are also claimed to be biodegradable. 

The first man-made polymer was nitrocellulose (NC). The main use of the NC resins was a replacement of the natural and expensive materials, viz., ivory, tortoise shell products, amber, ebony, onyx, or alabaster. The use of cellulose acetate (CA), as a thermoplastic, began in 1926. Cellulose ethers and esters became commercially available in 1927. Casein cross-linked by formaldehyde gave hornlike materials - Galalith has been used to manufacture shirt buttons or as imitation of ivory and porcelain (Pontio 1919). 

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Thickener polymers include polyurethanes, polyesters, polyacrylamides, natural polymers, and modified natural polymers [510]. 

Macromolecules may be classified according to different criteria. One criterion is whether the material is natural or synthetic in origin. Cellulose, lignin, starch, silk, wool, chitin, natural rubber, polypeptides (proteins), polyesters (polyhydroxybutyrate), and nucleic acids (DNA, RNA) are examples of naturally occurring polymers while polyethylene, polystyrene, polyurethanes, or polyamides are representatives of their synthetic counterparts. When natural polymers are modified by chemical conversions (cellulose —> cellulose acetate, for example), the products are called modified natural polymers. 

In Chapter 4, we will discover that the field of polymer science essentially began when scientists chemically modified natural polymers to prepare new materials with improved properties. Some of these reactions are still commercially important today. However, the specificity in the structures of most biopolymers themselves makes their laboratory synthesis extremely difficult. 

Smart materials such as these illustrate an important technological direction for materials science the design of materials with sophisticated properties that behave more like biological systems. Let s briefly recap our history. In Chapter 4 we noted a significant period of discovery when people modified natural polymers to improve their properties. We can call this period, roughly before 1900, Stage 1, and it asked the question, How can I improve upon nature This was followed by a century of synthetic polymer science in which... 

Polar group Natural polymers Modified natural polymers Synthetic polymers... 

The present book is a follow-up of a previous one with the title Analytical Pyrolysis of Natural Organic Polymers published by Elsevier as vol. 20 in the series Techniques and Instrumentation in Analytical Chemistry. In addition to the discussion on pyrolysis of various natural polymers, the previous book contains information on chemically modified celluloses, modified starches, etc. For this reason, the present book does not include synthetically modified natural polymers. Information on the pyrolysis process and pyrolytic techniques in general also can be found in the book on natural polymers. 

The classification of polymers previously described has been used in this book for the discussion of pyrolysis results. An important class of polymers that is not discussed here is that of chemically modified natural polymers (or semisynthetic polymers). Examples of such polymers are the modified celluloses (carboxymethyl cellulose, ethyl cellulose, etc ), modified starches, casein plastics (Galalith), etc. These types of compounds were discussed in the book on pyrolysis of natural organic polymers [2]. 

Transformations with immobilized enzymes or cells Often the stability of the biocatalyst can be increased by immobilization and many different enzymes and cells have been immobilized by a variety of different methods. The most popular method for the fixation of whole cells is entrapment or encapsulation with calcium alginate. Other natural gels e.g., carrageenan, collagen, chemically-modified natural polymers e.g., cellulose acetate and synthetic gels and polymers e.g., polyacrylamide or polyhydroxyethylmethacrylate can also be used for this type of immobilization. 

Cationic starch is modified natural polymer. Molar mass about 100,000,000. 

Although chitosan is the most important and well-studied natural cationic polymer, other natural cationic polymers have also been investigated for their applieations in gene and drug delivery. Different from ehitosan, these polymers are eationic modified natural polymers or cationic peptides, sueh as eationie eellulose, cationie dextrin or starch, cationic gelation, and others. 

The chemical industry s interest in polymers dates back to the 19th century. In those days it was a case of synthetically modifying natural polymers with chemical reagents to either improve their properties or produce new materials with desirable characteristics. Notable examples were nitration of cellulose giving the explosive nitrocellulose, production of regenerated cellulose (rayon or artificial silk) via its xanthate derivative, and vulcanization of rubber by heating with sulphur. Manufacture of acetylated cellulose (cellulose acetate or acetate rayon) developed rapidly from 1914 onwards with its use both as a semi-synthetic fibre and as a thermoplastic material for extrusion as a film. 

More recently, in the last 25 years, it has become increasingly apparent that, in addition to the major commodity synthetic plastics, water-soluble commodity and specialty polymers and plastics, such as poly(acrylic acids), polyacrylamide, poly(vinyl alcohol), poly(aIkylene oxides), and even some modified natural polymers, for example, cellulosics and starch, may potentially contribute to environmental problems and should also be targets for biodegradable substitutes. 

This suggests that future research is in the direction of condensation polymers and modified natural polymers, especially starch as a very cheap and widely available natural resource. The research will require close attention to cost-performance characteristics to develop competitive products since current nonbiodegradable products are not apparently creating environmental problems, though there is a nagging doubt since all the evidence is based on the absence of negative responses to date. 

Many chemical modifications have been applied to natural polymers to create new materials and applications. A very large number of compovmds made of modified cellulose or other modified natural polymers is currently in... 

A large number of polymers caimot be obtained by polymerisation of monomers but are obtained by chemical modification of polymers from either natural or synthetic origin. The first objects manufactured from polymers were obtained from chemically modified natural polymers. The chemical modifications are usually performed through classical reactions of organic chemistry, such as esterification, hydrolysis or etherification. Fiowever, the reactivity of chemical groups linked to polymers is not similar to the reactivity of the same groups present in small molecules, as the distance between polymer-linked... 

Results from biodegradation studies on numerous synthetic polymers, polymer blends, modified natural polymers and biopolymers have been reported. However, as pointed out earlier, biodegradability of a polymer is a function of several variables, including, but not limited to, the following. 

Thermoplastics (c) Natural products and modified natural polymers... 

Resin, synthetic n. Originally, a member of a group of synthetic substances which resemble and share some of the properties of natural resins, but now used for materials which bear htde resemblance to natural resins. The term is generally understood to mean a member of the heterogeneous group of compounds produced from simpler compoimds by condensation and/ or polymerization. Chemically modified natural polymers, such as cellulose acetate and hardened casein, are not considered to be synthetic resins. Flick EW (1991) Industrial synthetic resins handbook. Williams Andrews Publishing/Noyes, New York. 

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