Depropagation radical polymerization

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). 

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. 

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

With most common monomers, the rate of the reverse reaction (depropagation) is negligible at typical polymerization temperatures. However, monomers with alkyl groups in the a-position have lower ceiling temperatures than monosubstituted monomers (Table 4.10). For MMA at temperatures <100 °C, the value of is <0.01 (Figure 4.4). AMS has a ceiling temperature of <30 °C and is not readily polymerizable by radical methods. This monomer can, however, be copolymerized successfully (Section 7.3.1.4). 

While for many alkene monomers the position of the propagation-depropagation equilibrium is far to the right under the usual reaction temperatures employed (that is, there is essentially complete conversion of monomer to polymer for all practical purposes), there are some monomers for which the equilibrium is not particularly favorable for polymerization. For example, a-methylstyrene in a 2.2 M solution will not polymerize at 25°C and pure a-methylstyrene will not polymerize at 61°C (see Table 6.14). In the case of methyl methacrylate, though the monomer can be polymerized below 220° C, the conversion will be appreciably less than complete. For example, the value of [M]g at 110°C is found to be 0.139 M [3] which corresponds to about 86% conversion of 1 M methyl methacrylate. Since Eqs. (6.195) and (6.196) contain no reference to the mode of initiation, they apply equally well to ionic and ring-opening polymerizations. Thus the lower temperatures of ionic polymerizations often offer a useful route to the polymerization of many monomers that cannot be polymerized by radical initiation because of their low ceiling temperatures. 

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. 

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) ... 

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