Radical chain polymerization depropagation

Copolymerization with a properly chosen comonomer, however, may be a way to introduce into the polymer chain monomers that are otherwise unable to polymerize, or to enhance the conversion of monomers showing limited polymerizability. The previous section describes one of the possibilities related to retardation of depropagation. An analogous system is the radical copolymerization of S02 with olefins. 

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 ... 

Chain Scission with Depropagation. Many carbon-chain polymers and other simple chains, such as acetal resins (polyethers), are produced by chain reaction polymerization, either via double bonds or by ring opening. Such polymerizations involve repeated addition of a monomer molecule to an active center, which may be a radical, an ion, or a coordination complex. 

In the ideal case, where only depropagation occurs, the mechanism can be deduced from the dependence of the observed first-order rate constant for weight loss on the initial degree of polymerization of the polymer. If depolymerization is both chain-end and chain-scission initiated and termination is first-order then application of the steady-state assumption to the concentration of depolymerizing radicals leads to the following relation (14,15) ... 

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. 

Initiation is very likely owing to several causes, such as thermal effects, catalyst fragments, chemically incorporated oxygen, and weak bonds produced in the terminal step of the polymerization reaction. Althou initiation is prohahly a composite of all these factors, treatment here is limited to random and terminal initiation. Depropagation is taken to he the reverse of growth, and transfer is presumed to consist in the abstraction of hydrogen from another molecule. It is also likely that a radical can react with a hydrogen from its own chain the most probable position of attack is from the third to the seventh carbon atom adjacent to the radical end. In order to facilitate the mathematical treatment, disproportionation has been assumed to be the only termination step (28). 

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