Propylene 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. 

Natural Products. Many natural products, eg, sugars, starches, and cellulose, contain hydroxyl groups that react with propylene oxide. Base-cataly2ed reactions yield propylene glycol monoethers and poly(propylene glycol) ethers (61—64). Reaction with fatty acids results ia a mixture of mono- and diesters (65). Cellulose fibers, eg, cotton (qv), have been treated with propylene oxide (66—68). 

Starches have been chemically modified to improve their solution and gelling characteristics for food applications. Common modifications involve the cross linking of the starch chains, formation of esters and ethers, and partial depolymerization. Chemical modifications that have been approved in the United States for food use, involve esterification with acetic anhydride, succinic anhydride, mixed acid anhydrides of acetic and adipic acids, and 1-octenylsuccinic anhydride to give low degrees of substitution (d.s.), such as 0.09 [31]. Phosphate starch esters have been prepared by reaction with phosphorus oxychloride, sodium trimetaphosphate, and sodium tripolyphosphate the maximum phosphate d.s. permitted in the US is 0.002. Starch ethers, approved for food use, have been prepared by reaction with propylene oxide to give hydroxypropyl derivatives [31]. 

Etherification of starch dialdehyde is possible. The reaction of starch dialdehyde with propylene oxide and other etherification reactions were described.530 Esterification of starch dialdehyde with carboxylic anhydrides stabilizes the viscosity and adhesiveness of starch dialdehyde.531 This adhesive is additionally blended with urea. 

The reaction of starch with propylene oxide in alkali forms 0-(2-hydroxypropyl)starch.892,930-938 A granular derivative can be prepared by impregnating starch with acetic acid prior to its alkali treatment with and reaction propylene oxide.939 Etherification at 0-3 and 0-2 occurs at approximately the same rate, and the reaction at 0-6 is slightly less favorable.933 However, etherification at 0-2 has priority.940... 

Allylstarch can be blended with lower alkyl itaconates in the ratio from 2 1 to 10 1 and then copolymerized with a variety of coating materials to enhance coating plasticity, drying, and hardening.1028 Allylstarch can be polymerized into insoluble products using either sulfur chloride or disulfur dichloride.1029 The reaction product of allylstarch with propylene oxide was utilized as a dispersant for petroleum emulsions.1030 Novel resinous materials have been obtained from adducts of unsaturated esters of dienophilic dioic acids and cyclic polyenic hydrocarbons with mono- and di-allyl starch.1031... 

J. -A. Han and J. N. BeMiller, Influence of reaction conditions on MS values and physical properties of waxy maize starch derivatized by reaction with propylene oxide, Carbohydr. Polym., 64 (2006) 158—162. 

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 ... 

Da Roz et al. synthesized and characterized new starch cross-linked PUs produced by the reaction of native corn starch with a propylene oxide toluene diisocyanate oligomer (PTD) [126]. They studied the thermogravimetric behavior of the unmodified native starch, the cross-linked starches and the PTD by TGA (Figure 7.4). 

The chemical substitution of hydroxyl groups with functional groups yields modified starches with unique functionalities and uses. Acetylated and hydroxypropylated starches are produced after treatment with anhydrous acetic and propylene oxide under an alkaline pH, respectively. After the reaction, the slurry is neutralized, filtered, and dried. These derivatized starches are less prone to retrogradation, produce weaker and clearer gels, and have improved water-holding capacity (Mauro et al. 2003, Thomas and Atwell 1999). 

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