The present invention is directed membrane active poly(vinyl ester) polymers and compositions for targeted delivery of RNA interference (RNAi) polynucleotides to cells in vivo. RNAi polynucleotides are conjugated to the poly(vinyl ester) polymers and the polymers are reversibly modified to enable in vivo targeted delivery. Membrane activity of the poly(vinyl ester) provides for movement of the RNAi polynucleotides from outside the cell to inside the cell. Reversible modification provides physiological responsiveness.