Alkylation of the phosphorus coordination of the diphenyl(dithioformato)phosphine ligand in [W(CO)(5) (PPh2CS2)]NEt4 (1) at the S atom results in the formation of the novel RAFT agent S C[W(CO)(5)PPh2]S-R (2a, R = CH2Ph; 2b, R = CH2CH CH2). These compounds have been shown to be highly effective in reversible addition-fragmentation chain transfer (RAFT) polymerization to produce polymers (homopolymer and diblock copolymer) of predetermined molecular weight and narrow polydispersity (< 1.3). Electron-withdrawing organometallic substituents can increase the activity of RAFT agents. To the best of our knowledge. this is the first report of their use as RAFT agents in polymerization. (C) 2010 Elsevier B.V. All rights reserved.