Influence of the chemical structure of MADIX agents on the RAFT polymerization of styrene

Xanthates (MADIX agents) are an important class of RAFT agents. Currently, they are the only "living" agent that can be successfully used in a classical ab inito emulsion polymerization (i.e., where surfactant micelles are used as mircoreactors for polymerization), in which novel nanostructures can be prepared. The composition and molecular weight distribution (MWD) controls to a large extent the morphology of these nanostructures. A key factor that controls the MWD is the chain transfer ability of the MADIX agents. Three MADIX agents with different leaving groups (methyl benzyl or ethyl propionyl) and different activating moieties (O-ethyl or O-trifluoroethyl) were examined for their chain transfer ability in the homopolymerization of styrene. The chain transfer constants, C-tr, were determined using both the Mayo and chain length distribution methods. This is the first comprehensive study in which accurate activation parameters for MADIX agents have been elucidated for styrene polymerizations. The activation parameters from both methods showed that changing the Z group on the MADIX agent from a OCH2CH3 (C-tr = 0.69) to a OCH2CF3 (C-tr=3.5) lowered the E-a,E-tr by at least 5 kJ mol(-1). This suggests that when the electron-withdrawing power on the Z group is increased, the reactivity of polystyrene radicals toward the S=C bond is also increased. This is opposite to what is found when electron-donating Z groups are used (e.g., phenyl or benzyl groups), in which the C-tr values are actually orders of magnitude greater than for OCH2CF3, suggesting that the radical reactions to RAFT agents is more complex than to C=C bonds. It is postulated that the transition state of the intermediate radical plays an important role in the overall C-tr value. The confidence contour plots showed that the activation parameters for the O-ethyl xanthates with different leaving groups (C(CH3)Ph and C(CH3)CO2Et) were completely overlapping, suggesting that these leaving groups have little or no effect on the C-tr values. The O-trifluoroethyl xanthate allows the preparation of polymers with controlled molecular weights and low polydispersities close to 1.4 to be prepared, which is a distinct improvement on the first generation MADIX agents, O-ethyl xanthates (polydispersity close to 2).