An interfacial process for preparing a poly(aliphatic ester-carbonate) includes providing an initial polymerization reaction mixture comprising an aliphatic C6-20 dicarboxylic acid, a bisphenol, an alkali hydroxide, and optionally a catalyst in a solvent system comprising water and an immiscible organic solvent, adding an initial portion of a carbonyl dihalide over a first time period while maintaining the reaction at a first pH from 7 to 8; and adding a second portion of the carbonyl dihalide over a second, subsequent time period while maintaining the reaction pH at a second pH from 9 to 12, to provide a product polymerization mixture, wherein the amount of alkali hydroxide in the initial polymerization reaction mixture is effective to increase the fraction of the first time period at a measured pH of 7 to 8 compared to the same reaction mixture with a higher amount of alkali hydroxide in the initial polymerization mixture.