An esterified cellulose ether of increased weight average molecular weight is produced in a process for reacting a cellulose ether with an aliphatic monocarboxylic acid anhydride and a dicarboxylic acid anhydride, wherein the process comprises the steps of a) preparing a reaction mixture comprising the cellulose ether, the dicarboxylic acid anhydride and a reaction diluent and heating the reaction mixture to a temperature of from 60 C to 110 C prior to, during or after mixing the components of the reaction mixture, and b) adding continuously or in at least three portions the monocarboxylic acid anhydride to the reaction mixture of step a) and completing the reaction.

An esterified cellulose ether of increased weight average molecular weight is produced in a process for reacting a cellulose ether with an aliphatic monocarboxylic acid anhydride and a dicarboxylic acid anhydride, wherein the process comprises the steps of a) preparing a reaction mixture comprising the cellulose ether, the dicarboxylic acid anhydride and a reaction diluent and heating the reaction mixture to a temperature of from 60 C to 110 C prior to, during or after mixing the components of the reaction mixture, and b) adding continuously or in at least three portions the monocarboxylic acid anhydride to the reaction mixture of step a) and completing the reaction.

A modified process for producing an esterified cellulose ether is provided which comprises the stages of i) preparing a reaction mixture comprising a cellulose ether, an aliphatic monocarboxylic acid anhydride, a dicarboxylic acid anhydride, and a reaction diluent and heating the reaction mixture to a temperature of from 60 C. to 110 C. prior to, during or after mixing the components of the reaction mixture to conduct an esterification reaction, and ii) before the esterification reaction is completed, adding an additional amount of reaction diluent continuously or in one or more portions to the reaction mixture and allowing the esterification reaction to further proceed.

A modified process for producing an esterified cellulose ether is provided which comprises the stages of i) preparing a reaction mixture comprising a cellulose ether, an aliphatic monocarboxylic acid anhydride, a dicarboxylic acid anhydride, and a reaction diluent and heating the reaction mixture to a temperature of from 60 C. to 110 C. prior to, during or after mixing the components of the reaction mixture to conduct an esterification reaction, and ii) before the esterification reaction is completed, adding an additional amount of reaction diluent continuously or in one or more portions to the reaction mixture and allowing the esterification reaction to further proceed.

1. An esterified cellulose ether which i) comprises groups of the formula C(O)RCOOH or a combination of aliphatic monovalent acyl groups and groups of the formula C(O)RCOOH, wherein R is a divalent hydrocarbon group, ii) has a weight average molecular weight M.sub.w of up to 70,000 Dalton, iii) has a degree of neutralization of the groups C(O)RCOOH of not more than 0.4, and iv) has a solubility in water of at least 2.0 weight percent at 2 C., is useful for preparing enteric capsules.

1. An esterified cellulose ether which i) comprises groups of the formula C(O)RCOOH or a combination of aliphatic monovalent acyl groups and groups of the formula C(O)RCOOH, wherein R is a divalent hydrocarbon group, ii) has a weight average molecular weight M.sub.w of up to 70,000 Dalton, iii) has a degree of neutralization of the groups C(O)RCOOH of not more than 0.4, and iv) has a solubility in water of at least 2.0 weight percent at 2 C., is useful for preparing enteric capsules.

An esterified cellulose comprising aliphatic monovalent acyl groups and groups of the formula C(O)RCOOH, R being a divalent hydrocarbon group, has the following properties: i) the degree of neutralization of the groups C(O)RCOOH is not more than 0.4, ii) the total degree of ester substitution is from 0.03 to 0.38, and iii) the esterified cellulose ether has a solubility in water of at least 2.0 weight percent at 20 C.

An esterified cellulose comprising aliphatic monovalent acyl groups and groups of the formula C(O)RCOOH, R being a divalent hydrocarbon group, has the following properties: i) the degree of neutralization of the groups C(O)RCOOH is not more than 0.4, ii) the total degree of ester substitution is from 0.03 to 0.38, and iii) the esterified cellulose ether has a solubility in water of at least 2.0 weight percent at 20 C.

An esterified cellulose ether comprising aliphatic monovalent acyl groups and groups of the formula C(O)RCOOH, R being a divalent hydrocarbon group, has the following properties: i) the degree of neutralization of the groups C(O)RCOOH is not more than 0.4, ii) the total degree of ester substitution is from 0.10 to 0.70, and iii) the esterified cellulose ether has a solubility in water of at least 2.0 weight percent at 2 C.

An esterified cellulose ether comprising aliphatic monovalent acyl groups and groups of the formula C(O)RCOOH, R being a divalent hydrocarbon group, has the following properties: i) the degree of neutralization of the groups C(O)RCOOH is not more than 0.4, ii) the total degree of ester substitution is from 0.10 to 0.70, and iii) the esterified cellulose ether has a solubility in water of at least 2.0 weight percent at 2 C.