Polysaccharides, for example xanthan, diutan, scleroglucan, gums, celluloses, and starches, and derivatives of the foregoing, are reacted with organic carbonates forming products that: (1) have increased viscosifying efficiency, i.e., higher viscosities at reduced concentrations compared to the unmodified polysaccharide; or (2) are insolubilized at temperature ranges for which the unmodified polysaccharide would hydrate fully; or (3) both properties. In some cases, the insolubilized polysaccharides can be activated to dissolve and viscosify at specific temperature ranges by using pH modifiers. A method of treating a well is provided, including the steps of: (a) obtaining a modified polysaccharide characterized by having been made by the process of combining: (i) a water-soluble polysaccharide, wherein the polysaccharide is in a solid form during the combining step; and (ii) a transesterification agent, wherein the transesterification agent is in liquid form; wherein the step of combining is in the presence of less than 10% by weight water relative to the water-soluble polysaccharide and wherein any water present is at a pH of 8 or less; (b) forming a treatment fluid comprising the modified polysaccharide and water; and (c) introducing the treatment fluid into the well.