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

M.M.F. Adamy, A.M. Herk, van, M. Destarac, M.J. Monteiro

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Abstract

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 polymn. (i.e., where surfactant micelles are used as microreactors for polymn.), in which novel nanostructures can be prepd. The compn. and mol. wt. distribution (MWD) controls to a large extent the morphol. 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 (Me benzyl or Et propionyl) and different activating moieties (O-Et or O-trifluoroethyl) were examd. for their chain transfer ability in the homopolymn. of styrene. The chain transfer consts., Ctr, were detd. 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 polymns. The activation parameters from both methods showed that changing the Z group on the MADIX agent from a OCH2CH3 (Ctr = 0.69) to a OCH2CF3 (Ctr=3.5) lowered the Ea,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., Ph or benzyl groups), in which the Ctr 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 Ctr value. The confidence contour plots showed that the activation parameters for the O-Et 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 Ctr values. The O-trifluoroethyl xanthate allows the prepn. of polymers with controlled mol. wts. and low polydispersities close to 1.4 to be prepd., which is a distinct improvement on the first generation MADIX agents, O-Et xanthates (polydispersity close to 2). [on SciFinder (R)]
Original languageEnglish
Pages (from-to)2293-2301
JournalMacromolecules
Volume36
Issue number7
DOIs
Publication statusPublished - 2003

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