The present invention relates to substituted cellulose acetates and methods of use thereof. One embodiment of the present invention provides a drilling fluid having a base fluid and a viscosifier having a substituted cellulose acetate having a polar substituent that comprises an oxygen atom covalently bonded to a nonmetal selected from the group of sulfur, phosphorus, boron, and chlorine； where the nonmetal is present in at least about 0.01% by weight of the substituted cellulose acetate.
