The present invention relates generally to methods for one-step purification of organic polymers using tangential flow filtration. In one embodiment, the present invention relates to methods for size fractionation of heterogeneous organic polymers, such as poly(ethylene glycol) derivatives, using tangential flow filtration. The present invention also relates to purification of low molecular weight monofunctional organic polymer derivatives produced by procedures in which high molecular weight bifunctional poly(ethylene glycol) impurities are removed. The present invention further relates to purification of high molecular weight branched polymer products from low molecular weight starting materials.