Cellulose derivatization reactions

Grafting reactions alter the physical and mechanical properties of the polymer used as a substrate. Grafting differs from normal chemical modification (e.g., functionalization of polymers) in the possibility of tailoring material properties to a specific end use. For example, cellulose derivatization improves various properties of the original cellulose, but these derivatives cannot compete with many of the petrochemically derived synthetic polymers. Thus, in order to provide a better market position for cellulose derivatives, there is little doubt that further chemical modification is required. Accordingly, grafting of vinyl monomers onto cellulose or cellulose derivatives may improve the intrinsic properties of these polymers. 

Several solvent systems dissolve cellulose, a process that may, or may not lead to cellulose derivative formation. Both types of solvent systems will be considered, although the important derivatizing reaction employed in the... 

Cellulosic substances have been used in various fields from commodities to industrial materials after mechanical and chemical modifications. Especially because chemically modified cellulosics have some unique functional properties, and also because of their biodegradability in most instances, the chemistry of cellulose has become one of the major areas in cellulose science. Figure 12 illustrates the chemical structure of cellulose in terms of chemical modifications. "" Three hydroxyl groups in the glucose residue, one primary and the other two secondary, are the sites for substitution reactions, which are the most common in cellulose derivatizations. The (1 4)-/3-glycoside bonds and other functional groups such as carboxyls and aldehydes present in most cellulosic material as minor groups are also possible sites for chemical modifications. 

Each glucose unit in a molecule of cellulose has three hydroxyl groups that can be used to derivatize the cellulose by reactions common to all alcohols. It is uncommon, and for some derivatives impossible, to achieve a degree of substitution (DS) of three. Most important derivatives of cellulose have a DS that is somewhat below that value. For a given derivative, the DS must be specified since the properties of the derivative depend almost as much on DS as they do on the substituting agent. 

Many possible complications (29,81) associated with the use of the trinitrate derivative for the SEC analysis of cellulose have been reported. The major complication arises from the distinct possibility of hydrolysis of the polymeric cellulose chain as a result of the acid reaction conditions employed during derivatization. That the derivatization reaction does not give a completely derivatized material and that the cellulose trinitrate is not stable are also complicating... 

In contrast to - cellulose derivatives, which are modified to a greater extent to make them water-soluble, low - DS are sufficient in most of the s. to have the wanted effect on their functional properties. Derivatization reactions are mostly performed in heterogeneous aqueous phase at alkaline pH, keeping the s. in the granular state. After completing the reactions, the starch granules are washed free from reagent residues and other solubles, dewatered and dried. 

At first glance, the HRC scheme appears simple the polymer is activated, dissolved, and then submitted to derivatization. hi a few cases, polymer activation and dissolution is achieved in a single step. This simplicity, however, is deceptive as can be deduced from the following experimental observations In many cases, provided that the ratio of derivatizing agent/AGU employed is stoichiometric, the targeted DS is not achieved the reaction conditions required (especially reaction temperature and time) depend on the structural characteristics of cellulose, especially its DP, purity (in terms of a-cellulose content), and Ic. Therefore, it is relevant to discuss the above-mentioned steps separately in order to understand their relative importance to ester formation, as well as the reasons for dependence of reaction conditions on cellulose structural features. 

The basic requirement for cellulose dissolution is that the solvent is capable of interacting with the hydroxyl groups of the AGU, so as to eliminate, at least partially, the strong inter-molecular hydrogen-bonding between the polymer chains. There are two basic schemes for cellulose dissolution (i) Where it results from physical interactions between cellulose and the solvent (ii) where it is achieved via a chemical reaction, leading to covalent bond formation derivatizing solvents . Both routes are addressed in details below. 

Another important aspects of solubilization are the physical state of the dissolved polymer as well as the thermo-chemistry and kinetics of the dissolution reaction. It is known that a clear cellulose solution is a necessary, but not sufficient condition for the success of derivatization. The reason is that the polymer may be present as an aggregate, as will be discussed below. Additionally, dissolution of activated cellulose requires less time at low temperature, e.g., 2 h at 40 °C, and more than 8 h at 70 °C [106]. These aspects will be commented on below. 

As previously discussed, solvents that dissolve cellulose by derivatization may be employed for further functionahzation, e.g., esterification. Thus, cellulose has been dissolved in paraformaldehyde/DMSO and esterified, e.g., by acetic, butyric, and phthalic anhydride, as well as by unsaturated methacrylic and maleic anhydride, in the presence of pyridine, or an acetate catalyst. DS values from 0.2 to 2.0 were obtained, being higher, 2.5 for cellulose acetate. H and NMR spectroscopy have indicated that the hydroxyl group of the methy-lol chains are preferably esterified with the anhydrides. Treatment of celliflose with this solvent system, at 90 °C, with methylene diacetate or ethylene diacetate, in the presence of potassium acetate, led to cellulose acetate with a DS of 1.5. Interestingly, the reaction with acetyl chloride or activated acid is less convenient DMAc or DMF can be substituted for DMSO [215-219]. In another set of experiments, polymer with high o -celliflose content was esterified with trimethylacetic anhydride, 1,2,4-benzenetricarboylic anhydride, trimellitic anhydride, phthalic anhydride, and a pyridine catalyst. The esters were isolated after 8h of reaction at 80-100°C, or Ih at room temperature (trimellitic anhydride). These are versatile compounds with interesting elastomeric and thermoplastic properties, and can be cast as films and membranes [220]. 

Heterogeneous catalysts, particularly zeolites, have been found suitable for performing transformations of biomass carbohydrates for the production of fine and specialty chemicals.123 From these catalytic routes, the hydrolysis of abundant biomass saccharides, such as cellulose or sucrose, is of particular interest. The latter disaccharide constitutes one of the main renewable raw materials employed for the production of biobased products, notably food additives and pharmaceuticals.124 Hydrolysis of sucrose leads to a 1 1 mixture of glucose and fructose, termed invert sugar and, depending on the reaction conditions, the subsequent formation of 5-hydroxymethylfurfural (HMF) as a by-product resulting from dehydration of fructose. HMF is a versatile intermediate used in industry, and can be derivatized to yield a number of polymerizable furanoid monomers. In particular, HMF has been used in the manufacture of special phenolic resins.125... 

In a typical experiment the isocyanate (0.006 moles) was reacted with 1.5 g of the polysaccharide in 150 ml of a 5% LiCl/ N,N-dimethylacetamide solution at 90°C under nitrogen for two hours. The appearance of a strong infrared absorbance at 1705 cm l was an indication of carbamate formation. The derivatized polymer was isolated as a white powder by precipitation of the reaction solution into a nonsolvent such as methanol. Alternatively thin films were cast directly from solution the lithium salt could be removed by rinsing with acetone. Figure 1 illustrates the reaction of cellulose with phenyl isocyanate. 

Regioselective enzymatic acylation of large, insoluble polysaccharides is still a quite difficult task and therefore it is not surprising that only scant data have been reported up to now, most of them describing reaction outcomes which met with limited success. Nevertheless, enzymatic derivatization of polysaccharides has been performed in nonpolar organic solvents using insoluble polysaccharides with soluble [51] or suspended enzymes [52]. Chemically modified celluloses with either enhanced solubility or more readily accessible hydroxyl groups, like cellulose acetate or hydroxypropyl cellulose, were acylated by CalB, as reported by Sereti and coworkers [53]. However, the same authors failed to modify crystalline cellulose under the same reaction conditions. 

These products are characterized in terms of moles of substitution (MS) rather than DS. MS is used because the reaction of an ethylene oxide or propylene oxide molecule with cellulose leads to the formation of a new hydroxyl group with which another alkylene oxide molecule can react to form an oligomeric side chain. Therefore, theoretically, there is no limit to the moles of substituent that can be added to each D-glucopyranosyl unit. MS denotes the average number of moles of alkylene oxide that has reacted per D-glucopyranosyl unit. Because starch is usually derivatized to a considerably lesser degree than is cellulose, formation of substituent poly(alkylene oxide) chains does not usually occur when starch is hydroxyalkylated and DS = MS. 

The final section addresses degradation and oxidation reactions in a commonly used derivatization system for cellulose, a mixture of DMSO and phenyl isocyanate to achieve cellulose carbanilation, e.g. for analytical purposes. Mechanistic studies were aimed at verifying the assumed oxidative action of this reaction system, and trapping methodology was employed to detect responsible intermediates. 

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