Outer-sphere electron transfer metal-catalyzed polymerization of styrene using a macrobicyclic ligand

The CuBr-catalyzed polymerizations of styrene in the presence of a macrobicyclic mixed donor (N and S) encapsulating ligand, NH(2)capten, were carried out in toluene at 60 and 100 degrees C. The macrobicyclic nature of the ligand ensures that a transition metal ion is effectively encapsulated (caged) within the three-dimensional cavity, resulting in activation of radicals through an outer-sphere electron transfer mechanism. The kinetic data showed that the polymerizations were uncontrolled with little living behavior. The external orders of reaction in [CuBr], [NH(2)capten], and [CuBr2] were 0.5, 0.5, and close to zero, respectively, in agreement with the postulated mechanism of little or no deactivation of polymeric radicals and a significant amount of bimolecular termination. Although living behavior was not found using the cage ligand, it was decided that it would provide an ideal method for radical coupling experiments to make high-molecular weight multiblock copolymers from a difunctional PSTY (Br-PSTY-Br, PDI = 1.11). The coupling reaction of Br-PSTY-Br using CuBr/NH(2)capten and excess Cu(0) in toluene at 100 degrees C gave no loss of the starting Br-PSTY-Br. Changing the solvent to the aprotic DMSO resulted in a significant increase in the rate of consumption of starting Br-PSTY-Br, with over 87% being used in under 10 min at 60 degrees C. In addition, higher molecular weight species were also formed, suggesting that OSET gives little or no side reactions on this time scale. It was initially thought that to get such high rates of reaction that the SET-LRP disproportionation mechanism (2Cu(I), Cu(0) + Cu(II)) was at play. However, UV-Vis experiments of the CuBr/NH2capten showed little or no disproportionation products. This important result suggests that DMSO catalyzes the OSET reaction through the stabilization of the radical-anion intermediate, which then rapidly fragments to a polymeric radical. (c) 2007 Wiley Periodicals, Inc.