Starch derivatives with high degree of functionalization. III. Influence of reaction conditions and starting materials on molecular structure of carboxymethyl starch

Carboxymethyl starch (CMS) was prepared in a completely heterogeneous proce dure in a methanol/water slurry activated with aqueous sodium hydroxide (45 %, w/v) using monochloroacetic acid as the etherifying agent. The influence of the reaction conditions and the type of starting starch (amylose conten t and preactivation) was evaluated in regard to the formation of the main r epeating units (i.e., unfunctionalized and mono-, di-, tri-, and tetra-O-ca rboxymethylated) and the pattern of functionalization within the anhydroglu cose units (AGU). The reproducible synthesis gave products with. a maximal. degree of substitution of CM groups (DSCM) of 0.66, which was reached in a one-step synthesis. Repeated carboxymethylation led to products with a DSC M of 0.88. As revealed by means of HPLC analysis after complete acidic depo lymerization, in any sample the mono-O-carboxymethylated glucose (mono-O-CM glc) was preferably present while the di-O-CMglc was formed to a very low e xtent only. The tri-O-CMglc was found in some samples while tetra-O-CMglc w as not detected. The mole fractions determined did not follow the simple Sp urlin statistic as shown for CM cellulose synthesized under comparable cond itions. Within the carboxymethylated AGUs a preferred functionalization at position 2 was analyzed by means of H-1-NMR spectroscopy after hydrolytic c hain degradation. Consequently, the CMS samples synthesized contained mainl y 2-mono-O-CM-AGU. (C) 2001 John Wiley & Sons, Inc.