Ethylene oxide, reaction with starch

The two main types of hydroxyalkyl ethers industrially produced are hy-droxyethyl starch and hydroxypropyl starch. These compounds are synthesized by reaction of ethylene oxide or propylene oxide respectively with starch under alkaline conditions (134). 

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 ceUulose leads to the formation of a new hydroxyl group with which another alkylene oxide molecule can react to form an oligomeric side chain. Therefore, theoreticaUy, 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 usuaUy derivatized to a considerably lesser degree than is ceUulose, formation of substituent poly(alkylene oxide) chains does not usuaUy occur when starch is hydroxyalkylated and DS = MS. 

Graft copolymers of nylon, protein, cellulose, starch, copolymers, or vinyl alcohol have been prepared by the reaction of ethylene oxide with these polymers. Graft copolymers are also produced when styrene is polymerized by Lewis acids in the presence of poly-p-methoxystyrene. The Merrifield synthesis of polypeptides is also based on graft copolymers formed from chloromethaylated PS. Thus, the variety of graft copolymers is great. 

Hydroxyethylation on extrusion is also possible.941 It was reported that enzymic hydrolysis of both 2- and 6 - () - (h y dro xy c thy I) s larch with alpha amylase decreased with increasing DS. The product having 2-substitution was more resistant to that enzyme than a 6-substituted product.942 The reaction of ethylene oxide with starch is accompanied by concurent hydrolysis of ethylene oxide to ethylene glycol.943... 

Extended studies on starch etherification with C2 to C5 aliphatic alkylene oxides in alkaline slurries have been performed 944,945 This reaction was subsequently re-examined for hydroxypropylation.946 No significant effects were observed as a result of using added hydrogen peroxide, benzoyl peroxide, azodiisobutyronitrile, or K2S2O8.945 In all instances, including those studied without such additives, led to water- and alcohol-soluble thermoplastic materials. Further improvements involved etherification in either acetone or butanone in the presence of aqueous NaOH. Ethylene oxide was introduced incrementally.947 Etherification of starch... 

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 ... 

Electroneutral starch ethers, i.e., hydroxyethyl starch (HES) and hydroxypropyl starch (HPS) are manufactured in analogy to the corresponding cellulose ethers by reaction of alkalinized starch with ethylene oxide or propylene oxide (Figure 4A.40). 

For these and a variety of special non-food purposes esters with higher fatty acids, succinic, adipic and citric acids and carbamates (reaction products with urea), have also been prepared. Examples of starch ethers are 2-hydroxyethyl and 2-hydroxypropyl starches prepared by reaction of starch with oxirane (ethylene oxide) and methyloxirane (propylene-l,2-oxide). The reaction occurs preferentially at the secondary hydroxyl groups at C-2, with less on the C-3 and C-6 hydroxyl groups. The most common products are those shown in Figure 4.15. The degree of substitution tends to be <0.2. According to the reaction conditions, polyoxaalkyl starches ... 

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