We describe here a new methodology for the synthesis of well-defined phosphonic acid-terminated poly(ethylene glycol) (PEG) and RAFT-derived poly(N-vinylpyrrolidone) (PVP) and poly(N-vinylcaprolactam) (PVCL) by amine–thiol–ene and amine–thiol–thiosulfonate conjugation strategies using a phosphonated thiolactone and their use to prepare stable, water-dispersible multifunctional upconverting luminescent nanohybrids.