Free radical polymerization deviation from ideal kinetics

But studies of the kinetics of polymerization of different monomers, under different conditions and chemical environments, indicate that ideal behavior is probably more an exception than the rule. Most practical free-radical polymerizations will deviate to a greater or lesser extent from the standard conditions outlined in the reaction scheme (6.3) to (6.10) either because the actual reaction conditions are not entirely as postulated in the ideal kinetic scheme or because some of the assumptions that underlie the ideal scheme are not valid. 

Practical free-radical polymerizations often deviate from Eq. (6.126) because the assumptions made in the ideal kinetic scheme are not fuUy satisfied by the actual reaction conditions or because some of these assumptions are not valid. For example, according to the ideal kinetic scheme that leads to Eq. (6.126), the initiation rate (Rf) and initiator efficiency (/) are independent of monomer concentration in the reaction mixture and primary radicals (i.e., radicals derived directly from the initiator) do not terminate kinetic chains, thoughrifi reality R may depend on [M], as in the case of cage effect (see Problem 6.7) and, at high initiation rates, some of the primary radicals may terminate kinetic chains (see Problem 6.25). Moreover, whereas in the ideal kinetic scheme, both kp and kt are assumed to be independent of the size of the growing chain radical, in reahty k[ may be size-dependent and diffusion-controlled, as discussed later. 

The polymerization rate is directly proportional to the monomer concentration for ideal free radical polymerization kinetics. Deviations from this first-order kinetics can be caused by a whole series of effects which must be checked by separate kinetic experiments. These effects include cage effects during initiator free radical formation, solvation of or complex formation by the initiator free radicals, termination of the kinetic chain by primary free radicals, diffusion controlled termination reactions, and transfer reactions with reduction in the degree of polymerization. Deviations from the square root dependence on initiator concentration are to be primarily expected for termination by primary free radicals and for transfer reactions with reduction in the degree of polymerization. 

FIGURE 12.13 M versus conversion, /from ACOMP for several BA polymerization reactions by RAFT ( 1 ) and a free radical polymerization reaction ( 5). For high [DoPAT]/[AIBN] ( 1-3), the reactions exhibit typical CRP behavior with nearly linear increase of mass versus /. The downward curvature in reaction 4 indicates significant deviations from the ideal living mechanism. Reaction 5 shows classical uncontrolled radical polymerization behavior of with conversion. Reprinted (adapted) with permission from Alb AM, Serelis AK, Reed WF. Kinetic trends in RAFT homopolymerization from online monitoring. Macromolecules 2008 41 332-338. 2008 American Chemical Society. 

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