The synthesis of new series of PPH (poly(phosphorhydrazone)) dendrimers with amino-PEG phosphonates or the corresponding amino-PEG phosphonic acids as terminal groups is presented, from generations 1-3. The size of PEG-terminated dendrimers is experimentally measured by diffusional NMR, and by means of dynamic light scattering. Classical molecular dynamics and well-tempered metadynamics simulations are used to assess or confirm the formation of aggregates in some cases. The influence of PEG capping on the cytotoxicity profiles of the dendrimers is evaluated on human peripheral blood mononuclear cells by means of LIVE/DEAD assays, and confirms the importance of PEG capping to ensure low cytotoxicity. (C) 2014 Wiley Periodicals, Inc.