Free-radical polymerization thermodynamic equilibria

Free-radical polymerization reactions are equilibrium reactions. The equilibrium between the monomer and the growing polymer is subject to thermodynamic conditions. At equilibrium, therefore, the change in free energy is zero ... 

Strictly speaking, any model based on the time-independent thermodynamics cannot be used to adequately predict the concentration of monomer in latex particles during Smith-Ewart Interval II. This is because the free radical polymerization of monomer in the discrete latex particles is governed by the simultaneous kinetic events such as the generation of free radicals in the continuous aqueous phase, the absorption of free radicals by the particles, the propagation of free radicals with monomer molecules in the particles, the bimolecular termination of free radicals in the particles, and the desorption of free radicals out of the particles. The equilibrium (or saturation) concentration of monomer in the growing latex particles may not be achieved if the rate of consumption of monomer in the major reaction loci is much faster than that of diffusion of monomer molecules from the monomer droplets to the reaction loci. Therefore, the equilibrium concentration of monomer in the latex particles represents an upper limit that is ultimately attainable in the course of polymerization. Nevertheless, the general... 

All the above schemes include, in essence, different variants of empirical linear equations in which the rate constants for chain propagation in the free radical polymerization are brought into correlation with thermodynamic (heat, Hammett constant (a), the change in the Gibbs energy in the equilibrium reaction) and kinetic (loga-ridims of the rate constants of the reference reaction and the reaction under study) characteristics of the addition reaction. 

This equation permits the calculation of equilibrium constants for polymerization-depolymerization from copolymer composition data extrapolated to zero Mi feed. The agreement between equilibrium constants calculated in this manner from free radical copolymerizations and those obtained from anionic homopolymerizations is shown in Table II, and again emphasizes the thermodynamic character of this work. 

The requirement of the presence of the polymerization catalyst in the depolymerization process stems from the principle of microscopic reversibility. If, for example, all free radicals are removed from the system by simply endcapping the polymer, the thermodynamic equilibrium dictates that depolymerization state cannot be reached and the system will be stable. This was the approach employed by Ito and Willson in stabilizing polyphthaldehyde resists. [See for example, C.G. Willson, H. Ito, J.M.J. Frechet, T.G. Tessier, F.M. Houlihan, Approaches toward the design of radiation sensitive polymeric imaging systems with improved sensitivity and resolution, J. Electro chem. Soc. 133, 181 (1986)]. 

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