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 were examined in the RuCl 2 (PPh 3 ) 3 -mediated living radical polymerization of methyl methacrylate (MMA) with the use of model compounds of the polymer terminal, which include four authentic unimer and dimer halides [R−X: (CH 3 ) 2 C(CO 2 Me)Cl ( 1a ); (CH 3 ) 2 C(CO 2 Et)Br ( 1b ); (CH 3 ) 2 C(CO 2 Me)CH 2 C(CH 3 )(CO 2 Me)X, X = Cl ( 2a ) or Br ( 2b )]. These halides induced smooth polymerizations of MMA in conjunction with RuCl 2 (PPh 3 ) 3 and Al(O- i -Pr) 3 in toluene at 80 °C, where the rates were almost independent of R−X while the molecular weight distributions were broader in the order: 2b < 2a ∼ 1b < 1a . This shows that initiation is faster with the dimer and with the bromide than with the unimer and with the chloride, respectively. 1 H NMR analysis of the halogen exchange between the brominated initiator 1b or 2b and RuCl 2 (PPh 3 ) 3 shows that the carbon−halogen bond of the dimer is more reactive than that of the unimer. Added Al(O- i- Pr) 3 led to faster polymerizations and narrower MWDs but had no effects on the halogen exchange reactions.