Chemically modified natural monomers

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... 

There is a good amount of data on the transformations of natural polymers, but the literature concerning chemical modifications of natural monomers is rather scanty. However, there are a few natural monomers that have found applications. The monomers in these cases are either modified into other suitable monomers of industrial importance or are polymerized directly into polymers. A few examples are given below for illustration. 

Finally, nature also takes polysaccharides and chemically modifies them to make use-fill structures. Chitin is a classic example and can be thought of as cellulose with one hydroxyl group on each monomer replaced by an acetylamino group, as illustrated in Figure 9-38. This allows for increased hydrogen ... 

The understanding of the mechanisms involved in the polymer synthesis with natural precursors is definitively a key factor for their appropriate exploitation. Taking into account this need, Ronda et al. explained recently different pathways to modify natural resources. These authors proposed three routes to modify vegetable oils to transform them into polymers (1) direct polymerization (cationic, radical, or thermal polymerization) (2) functionalization and polymerization and (3) monomer synthesized, chemical modification and polymerization [32]. 

However, lack of good interfacial adhesion, low melting point, and poor resistance towards moisture make the use of natural fibre-reinforced composites less attractive. Pre-treatments of the natural fibre can clean the fibre surface, chemically modify the surface, stop the moisture absorption process and increase the surface roughness. Among the various pre-treatment techniques, graft copolymerization and plasma treatment are the best methods for surface modification of natural fibres. Graft copolymers of natural fibres with vinyl monomers provide better adhesion between matrix and fibre. 

The data presented in Figure 8 graphically illustrate the tremendous and rapid growth in interest in FOSS chemistry, especially for patented applications. This looks set to continue with current applications in areas as diverse as dendrimers, composite materials, polymers, optical materials, liquid crystal materials, atom scavengers, and cosmetics, and, no doubt, many new areas to come. These many applications derive from the symmetrical nature of the FOSS cores which comprise relatively rigid, near-tetrahedral vertices connected by more flexible siloxane bonds. The compounds are usually thermally and chemically stable and can be modified by conventional synthetic methods and are amenable to the usual characterization techniques. The recent commercial availability of a wide range of simple monomers on a multigram scale will help to advance research in the area more rapidly. 

Oleochemical based dicarboxylic acids - azelaic, sebacic, and dimer acid (Figs. 4.5 and 4.6) - amount to ca. 100000 tonnes year-1 as components for polymers. This is about 0.5% of the total dicarboxylic acid market for this application, where phthalic and terephthalic acids represent 87%. The chemical nature of these oleochemical derived dicarboxylic acids can alter or modify condensation polymers, and, used as a co-monomer, will remain a special niche market area. Some of these special properties are elasticity, flexibility, high impact strength, hydrolytic... 

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