Cellulose alkoxide

Cellulose (VIII) is spun into fiber or cast into film by using a chemical reaction to convert it into a soluble xanthate derivative (Turbak, 1988). This is achieved by treating cellulose with 18-20% aqueous sodium hydroxide solution at 25-30°C for about 0.5-1 h. Much of the sodium hydroxide is physically absorbed into the swollen polymer some of it may be in the form of cellulose alkoxides. The excess alkali is pressed out of the cellulose pulp and the mass aged to allow oxidative degradation of the polymer chains to the desired molecular weight. The alkali cellulose is then treated with carbon disulfide at about 30° C and the resulting mass dissolved in dilute sodium hydroxide to form the sodium... 

It should also be noted that some nonradical ionic and condensation reactions of monomers with cellulose are used to modify the properties of cellulosic products. In one type of anionic-initiated reaction of monomers, cellulose is reacted with concentrated aqueous solutions of alkali metal hydroxides to yield cellulose copolymer. Free alkali metal in liquid ammonia or alkali metal alkoxides in nonaqueous systems may also be used as initiators of cellulose alkoxide derivatives. In cationic-initiated formation of copolymers, cellulose is reacted with an acid, such as boron trifluoride, to yield a cellulosic carbonium ion which initiates reactions with vinyl monomers. Condensation reactions of cyclic monomers with cellulose also form copolymers. Cellulose is usually slightly oxidized and also has reactive hydroxyl groups on carbons C-2, C-3 and C-6 of the anhydroglucose unit. The reactions of cyclic monomers are initiated at these carbonyl groups. A heating step may increase cellulosic oxidation and thereby increase the yield of these condensation products of cellulose and cyclic monomers." ... 

Zinc chloride is a Lewis acid catalyst that promotes cellulose esterification. However, because of the large quantities required, this type of catalyst would be uneconomical for commercial use. Other compounds such as titanium alkoxides, eg, tetrabutoxytitanium (80), sulfate salts containing cadmium, aluminum, and ammonium ions (81), sulfamic acid, and ammonium sulfate (82) have been reported as catalysts for cellulose acetate production. In general, they require reaction temperatures above 50°C for complete esterification. Relatively small amounts (<0.5%) of sulfuric acid combined with phosphoric acid (83), sulfonic acids, eg, methanesulfonic, or alkyl phosphites (84) have been reported as good acetylation catalysts, especially at reaction temperatures above 90°C. 

Strong basic solutions, such as sodium hydroxide, penetrate the crystalline lattice of a-cellulose producing an alkoxide called alkali or soda cellulose. Mercerized cotton is produced by aqueous extraction of the sodium hydroxide. 

Because the cellulose ether alkoxide is present entirely in the aqueous phase, the rate-limiting step may be the partitioning (phase transport) of the hydrophobic electrophile across the interface from the organic to aqueous phase. If the reaction rate is controlled by diffusion of the electrophile across the interface, then one would expect a correlation between water solubility of the hydrophobe and its alkylation efficiency. The fact that the actual alkylation reaction is probably occurring in the aqueous phase (or at the interface) yet the electrophile itself is principally soluble in the organic phase has important mechanistic ramifications. This type of synthetic problem, in which one reactant is water soluble and the other organic soluble, should be amenable to the techniques of phase transfer catalysis (PTC) to yield significant improvements in the alkylation efficiency. 

Theory would predict that PTC should be useful in increasing the alkylation efficiency of hydrophobic electrophiles with cellulose ether alkoxides. However, there is very little previous work reported in using PTC in the preparation of cellulose ethers. Daly and coworkers10 reported that quaternary ammonium salts were useful in catalyzing the heterogeneous benzylation of cellulose, but when we applied this technique to the DPGE alkylation of nascent HEC in aqueous /-butyl alcohol, the presence of catalytic amounts of tetramethylammonium chloride or tetrabutylammonium bromide actually afforded lower alkylation efficiencies. 

The sodium alcoholate of cellulose prepared from a sodium alkoxide is probably similar to, if not identical with, that prepared from sodium hydroxide. Monomethylation of the alcoholate prepared with either sodium methoxide or sodium l-butoxide140 occurs preferentially at the hydroxyl groups on C-2 and C-6. Very little methylation occurs at the hydroxyl group on C-3. 

A much more extensive investigation of the effect of alkalies has been made in the case of polysaccharides, especially cellulose this is understandable in view of the industrial importance of mercerization, of the viscose process, and of cellulose ethers. Various complexes have been reported for cellulose and alkalies depending upon the nature of the alkali, upon its concentration, upon the washing treatment used, and upon the pretreatment of the cellulose. A discussion of this subject has been published by Nicoll and Conaway.84 There is general agreement on the formation of several compounds, which are susceptible to hydrolysis. The question as to whether these compounds are molecular complexes (XLVII), true alkoxides (XLVIII), or an equilibrium mixture of the two has not been answered. In recent studies Lauer65 has reached... 

TBTO undergoes a condensation reaction with cellulose to form a tributyltin alkoxide (57). This reaction with cellulose could prevent metabolism by inhibiting the extracellular enzymes of the wood-destroying fungi. However, such a reaction is unlikely to be initiated in situ because the alkoxide is highly susceptible to hydrolysis (58). 

Again, cationic differences are evident. For instance, potassium hydroxide is more effective in methylation (with dimethyl sulfate) than sodium hydroxide. In alkali-cellulose, addition products are formed by the interaction of alkalis with hydroxyl groups, and the tendency for alkoxide formation increases in the order LiOH < NaOH < KOH < RbOH < CsOH < organic quaternary bases. ... 

Cellulose derivative fibers are commercially available and extensively used for the preparation of dialysis modules. Cellulose acetate fibers functionalized with metal alkoxides for the immobilization of enzymes was described by Kurokawa and Hanaya [60], In this example, the presence of metal alkoxides induced the gelifica-tion of cellulose acetate due to the coordination of the polyvalent metal on the hydroxyl groups on pyranose rings. The strength of the fiber strongly depends on alkoxide content. 

The interaction of boron alkoxides with cellulose and 0-methylcellulose has been investigated. An unsaturated cellulose, namely, 5,6-cel-lulosene (25) and its acetic ester have been prepared by the thermal decomposition of an allylxanthate and benzylxanthate of cellulose. ... 

A similar reaction can be used to form graft copolymers of poly(ethylene oxide) on cellulose acetate. Poly (ethylene oxide) can also be grafted to starch. For instance, a preformed polymer terminated by chloroformate end groups can be used with potassium starch alkoxide ... 

---