Free radical polymerization depropagation

Dorschner D, Multicomponent free radical polymerization model refinements and extensions with depropagation [M.S. thesis]. Waterloo, Ontario, Canada University of Waterloo 2010. 

It should be noted that, whereas the preceding discussion has been cast in terms of free-radical polymerizations, the thermodynamic argument is independent of the nature of the active species. Consequently, the analysis is equally valid for ionic polymerizations. A further point to note is that for the concept to apply, an active species capable of propagation and depropagation must be present. Thus, inactive polymer can be stable above the ceiling temperamre for that monomer, but the polymer will degrade rapidly by a depolymerization reaction if main chain scission is stimulated above T.. 

For most free-radical polymerization reactions there are some elevated temperatures at which the chain growth process becomes reversible and depropagation takes place ... 

When depropagation takes place at an elevated temperature, at a rate that is equal to the propagation in a free-radical polymerization, then the temperature of the reaction is a ceiling temperature (see Chap. 3). Termination can take place by disproportionation. Secmidary reactions, however, may occur in the degradation process depending upon the chemical structure of the polymer. Such side reactions can, for instance, be successive eliminations of hydrochloric acid, as in poly(vinyl chrolide), or acetic acid as in poly(vinyl acetate). 

At higher temperature in free-radical polymerization systems, a reverse reaction (depropagation) takes place. For such a situation, the disappearance of monomer... 

Random Scission without Depropagation. In radical polymerization, the simple chain-growth mechanism is complicated by side reactions of the propagating free radicals, which may undergo chain-transfer reactions in which the radical activity is transferred from one center to another, typically by hydrogen atom abstraction. 

The values of Tc for styrene and methyl methacrylate are typical for monomers used in free-radical addition polymerization. The low value for a-methyl styrene ( 60 C) is due to severe steric problems which arise during addition and reduce the value of kp. A lot of fruitless experimental work was done with a-methyl styrene in an attempt to find suitable initiators until it was realized that the difficulties with polymerization were caused by depropagation reactions taking place. 

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