The theoretical approach to free-radical polymerization in the presence of polyfunctional transfer agents can be derived in a simple way if the effects of primary chains and termination reactions are neglected. Under these conditions, and for values of the chain transfer constant C = 1, the analytical expressions for the number-and weight-average degree of polymerization, dispersion index and weight distribution function were derived through the elementary probability theory and generating functions. Numerical examples are reported on the relationship between distribution parameters, functionality and monomer conversion.
Polymerization of methyl methacrylate was carried out in the presence of polyfunctional chain transfer agents: 1,6-hexanedithiol, trimethylolpropane b, tris(mercaptoglyco1ate) and pentaerythritol tetrakis(3-mercaptopropionate). The theoretical results reported in the preceding papers are completely confirmed the experimental values of the dispersion index (D = &?, , , / &?, , ) are 1.62 for f = 2, 1.39 for f = 3 and 1.28 for f = 4. In the case of bifunctional transfer agents the molecular weights calculated are in very good agreement with those obtained by size exclusion chromatography. The formation of star-shaped macromolecules was confirmed by cleavage of the C-S-Cbonds. The number-average molecular weight decreases and the dispersion index is approximately equal to 2 after exposure to UV rays. The phenomenon of "pseudo-living" polymerization in the presence of polyfunctional chain transfer agents is also discussed.
In this part of the series, the influence of polyfunctional chain transfer agents with transfer constant C 5 1 on the molecular weight distribution was studied. The analytical expressions for the number-and weight-average degree of polymerization, and dispersion index were derived by kinetic and statistical methods. The expression for the molecular weight distribution can only be obtained by statistical methods. Some numerical examples on the dependence of distribution parameters as a function of the functionality of transfer agents f and transfer constants are illustrated. A critical value of the chain transfer constant was found toexist, which permits the synthesis of linear (forf = 2) or branched polymers cf > 2) with DP,/DP, approximately equal to 2 during the entire course of the polymerization. a) Part I: cf. ref. ') 0 1994, Huthig & Wepf Verlag, Base1
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