Reversible activation of carbon-halogen bonds by RuCl2(PPh3)(3): Halogen exchange reactions in living radical polymerization

Influences of the structures of the initiating radical species and terminal halogens of their precursors mere examined in the RuCl2(PPh3)(3)-mediated living radical polymerization of methyl methacrylate (MMA) with the use of model compounds of the polymer terminal, which include four authentic unime r and dimer halides [R-X: (CH3)(2)C(CO2Me)Cl (la); (CH3)(2)C(CO2Et)Br (1b); (CH3)(2)C-(Co2Me)CH2C(CH3)(Co2Me)X, X = Cl (2a) or Br (2b)]. These halides induced smooth polymerizations of MMA in conjunction with RuCl2(PPh3)(3) a nd Al(O-i-Pr)(3) in toluene at 80 degrees C, where the rates were almost in dependent of R-X while the molecular weight distributions were broader in t he order: 2b < 2a similar to Ib < la. This shows that initiation is faster with the dimer and with the bromide than with the unimer and with the chlor ide, respectively. H-1 NMR analysis of the halogen exchange between the bro minated initiator Ib or 2b and RuCl2(PPh3)(3) shows that the carbon-halogen bond of the dimer is more reactive than that of the unimer. Added Al(O-i-P r)a led to faster polymerizations and narrower MWDs but had no effects on t he halogen exchange reactions.