Co-oligomer compounds, complexes of the same with polyanions, such as siRNAs, and methods for using the same are provided. the delivery of polynucleotides, into a cell. The subject co-oligomers include at least a liphopilic monomer and at least a hydrophilic monomer (e.g., a guanidinium containing monomer). In some embodiments, the co-oligomer compounds are capable of complexing a siRNA of interest, thereby increasing the cell permeability of the siRNA, prior to release of the siRNA into the cell. In some embodiments, the subject method is a method of delivery a siRNA into a cell. In some embodiments, the subject method is a method of reducing expression of a protein target of a siRNA of interest. The subject co-oligomer/siRNA complexes may be formulated and administered to a subject to treat a condition resulting from expression of a protein target of the siRNA of interest.