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 procedure 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 content and preactivation) was evaluated in regard to the formation of the main repeating units (i.e., unfunctionalized and mono-, di-, tri-, and tetra-O-carboxymethylated) and the pattern of functionalization within the anhydroglucose 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 DSCM of 0.88. As revealed by means of HPLC analysis after complete acidic depolymerization, in any sample the mono-O-carboxymethylated glucose (mono-O-CMglc) was preferably present while the di-O-CMglc was formed to a very low extent only. The tri-O-CMglc was found in some samples while tetra-O-CMglc was not detected. The mole fractions determined did not follow the simple Spurlin statistic as shown for CM cellulose synthesized under comparable conditions. Within the carboxymethylated AGUs a preferred functionalization at position 2 was analyzed by means of H-1-NMR spectroscopy after hydrolytic chain degradation. Consequently, the CMS samples synthesized contained mainly 2-mono-O-CM-AGU. (C) 2001 John Wiley & Sons, Inc.