Valency platform molecules comprising high molecular weight polyethylene oxide groups are provided, as well as conjugates thereof with biologically active molecules, and methods for their preparation. The high molecular weight polyethylene oxide group has, for example, a molecular weight of greater than 22,000 Daltons, for example at least 40,000 Daltons. In one embodiment, a composition comprising the valency platform molecules is provided, wherein the molecules have a polydispersity less than about 1.2. Conjugates of the valency platform molecule and a biologically active molecule, such as a saccharide, poly(saccharide), amino acid, poly(amino acid), nucleic acid or lipid also are provided. Also provided are pharmaceutically acceptable compositions comprising the conjugates disclosed herein and a pharmaceutically acceptable carrier, as well as methods of making and using the conjugates and compositions.