The mechanism of copper(I) mediated living radical polymerisation (ATRP) of vinyl monomers has been widely assumed to be relatively simple. However, monomer reactivity ratios in ATRP can be significantly different from those in conventional radical polymerisation, and the exact nature of the catalyst is not well understood due to rapid exchange of ligand and monomer at the copper centre under the polymerisation conditions. This paper illustrates aspects of this catalyst system, which complicate the accepted mechanism of this new and important reaction. Coordination of aminoethyl methacrylates and methoxy[poly(ethylene glycol)] methacrylate monomers is demonstrated by NMR with r_MMA = 0.96(2), 0.98(1) and 0.97(1) for dimethylaminoethyl methacrylate (DMAEMA), (diethylamino)ethyl methacrylate (DEAEMA) and (tert-butylamino)ethyl methacrylate (TBAEMA) under free radical polymerisation and 0.74(3), 0.79(3) and 0.69(3) for transition-metal mediated polymerisation.