UTILIZING HALIDE EXCHANGE TO IMPROVE CONTROL OF ATOM-TRANSFER RADICALPOLYMERIZATION

Halide exchange during atom transfer radical polymerization (ATRP) usi ng mixed halide initiation systems, R-X/Cu-Y (X, Y = Cl or Br), was in vestigated. Model studies of mixed halide initiation systems (i.e., R- X/Cu-Y, X not equal t Y) demonstrated that exchange occurs rapidly at 90 degrees C, and there is a clear preference for alkyl chlorides to b e formed in over alkyl bromides. This was attributed mainly to the car bon-chlorine bond being stronger than the corresponding carbon-bromine bond. This implies that, in ATRP with a mixed halide initiator/cataly st system, the bulk of the polymer chain ends are terminated by chlori ne if[CuCl](0) greater than or equal to [RBr](0). Examples of using th is information to improve the control in ATRP of methyl methacrylate ( MMA) are presented. It was shown that, when benzyl halides were used a s the initiator in the ATRP of MMA, the rate of initiation was increas ed relative to the rate of propagation, thus providing better control by using the benzyl bromide/copper chloride mixed halide system. Bette r molecular weight control at high conversions of monomer to polymer w as observed when using ethyl 2-bromoisobutyrate and copper chloride as initiator/catalyst in comparison to using ethyl 2-bromoisobutyrate/co pper bromide, indicating that side reactions are less significant in t he former.