Atom transfer radical polymerization thermodynamics

Lin, C. Y., M. L. Coote, A. Gennaro, and K. Matyjaszewski. Ah initio evaluation of the thermodynamic and electrochemical properties of alkyl halides and radicals and their mechanistic implications for atom transfer radical polymerization. J. Am. Chem. Soc. 130, 2008 12762-1277. 

Braunecker WA, Tsarevsky NV, Gennaro A, Matyjaszewski K. Thermodynamic components of the atom transfer radical polymerization equilibrium quantifying solvent effects. Macromolecules 2009 42 6348-6360. 

A certain kind of radical transfer can be modelled by the transfer of a hydrogen atom from an alkane molecule to a small alkyl radical. This reaction was studied in detail in the gas phase. With hydrocarbon partners, heats of reaction are a fairly safe measure of the relative rate of transfer, as the pre-exponential Arrhenius factors remain approximately constant for a series of transfers to a given radical. Tabulated thermodynamic data indicate, however, [31, 32] that the correlation between the heat of reaction and the transfer rate is not valid for reactions of a radical with polar substrates [32, 33], In condensed phases, transfer reactions have not been sufficiently studied. Polymerizations themselves are the source of the most valuable, though incomplete, information. 

DT relies on a thermodynamically neutral (degenerative) transfer reaction. The key for control is a minimal energy barrier for that reaction. Conventional free radical initiators are used, i.e., peroxides and diazenes, at temperatures typical for radical polymerization and the polymerization is carried out in the presence of a compound with a labile group or atom which can be either reversibly abstracted or added-fragmented by the growing radical. The simplest examples are reactions in the presence of alkyl iodides [33,183-184] Scheme 11 ... 

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