Termination mechanism, bimolecular

Since the sensitivity towards water in many organic reactions lies in the order carbanion > carbonium ion > free radical, it appears likely that as water is progressively removed from a-methylstyrene—and, perhaps, other vinyl monomers—the free radical propagation is augmented or supplanted by a carbonium ion mechanism, which, in turn, is further enhanced at low water content, by a carbanion mechanism. Under the latter conditions, one would expect a termination mechanism which is bimolecular with regard to the total concentration of propagating species and hence a square-root dependence of the polymerization rate on the dose rate. This is the order dependence observed in a-methylstyrene at the highest polymerization rates and lowest water content. 

The photospeed increases linearly with incident light intensity (Figure 4). The linear relationship between the photospeed and the incident light intensity is explained by considering that in this viscous monomer the main termination mechanism is radical occlusion instead of bimolecular reaction between macroradicals. We will comment more extensively on the intensity... 

Anionic reactions have no bimolecular termination mechanism and in the absence of impurities (water, alcohols, etc.) or transfer agents, the end remains active indefinitely (living center). Termination reactions are significant for both cationic and free-radical polymerizations. 

Therefore, the main difference from the previous two systems is that this living mechanism does not form new radicals and a conventional initiator is needed to start and sustain the reaction. The initial amount of this species has to be properly selected. In fact, since the living reaction (3) does not affect the radical concentration, the final concentration of the chains terminated by bimolecular combination will be half of the initial concentration of the initiator. Therefore, the initial concentration of the species carrying the iodine group (in the following simply called the transfer agent ) determines the final DP of the polymer provided that the initiator concentration is small compared with that of the transfer agent. 

Photopolymerization of these systems appears to proceed by the conventional mechanism where termination is bimolecular. 

Another possible termination mechanism is unimolecular trapping of radicals which has been demonstrated [9, 10] using electron spin resonance spectroscopy (ESR). Anseth and Bowman [9] have shown that bimolecular termination through reaction diffusion is generally the dominant mechanism even in highly crosslinked methacrylates. 

The two models presented above may be considered to be two extreme cases. However, the most probable situation is that both t5q5es of termination reaction, the usual bimolecular interaction of polymer radicals (bimolecular termination) and a first-order process involving only one polymer radical (monomolecular termination), occur in parallel. Thus, we can have not two, but three possible termination mechanisms ... 

Autoacceleration, where the rate of polymerization increases with conversion in isothermal conditions, is observed in both thermal- and photoinitiated free-radical polymerizations because the termination mechanisms are the same for both. As the chains grow longer, it becomes more difficult for the active centers to diffuse and imdergo bimolecular termination thus, termination frequency decreases and active centers at the chain ends can become trapped. In cases where termination is controlled by diffusion, the pseudo-steady-state assumption is no longer valid and chain length dependent termination (CLDT) may occur (67). As is discussed for chain cross-linking photopolymerizations below, more complicated kinetic treatments must then be considered, including unsteady-state kinetics. 

To determine the prevailing termination mechanism in SI-PMP over a broad range of relevant reaction conditions, experimental data on film thickness evolution such as that shown in Figure 12.4 were fit in the brush regime (transitions marked by arrows in Figure 12.4) by the kinetic models that incorporate one or more termination mechanisms. For example, Rahane et al. combined Equation 12.1 with expressions for STR based on either bimolecular termination or chain transfer to monomer to develop models for how layer thickness should evolve as a function of exposure time. These models, shown as Equations 12.3 and 12.4, respectively, can be compared to experimental data of polymer layer thickness as a function of time to deduce which irreversible termination mechanisms are prevalent. 

In this stage, growth of the polymer chain is terminated. The two most common mechanisms of termination involve bimolecular reaction of growing polymer chains. Combination involves the coupling together of two growing chains to form a single polymer molecule... 

As stated in section I, the termination mode of the particular monomer determines the number of functionalities per macromolecular chain. Most monomers undergo both unimolecular and bimolecular termination reactions. It is often observed that both respective monofunctional and bifunctional polymers are formed and well-defined functional polymers cannot be prepared. The use of allylmalonic acid diethylester in free-radical polymerization has been proposed to overcome the problems associated with the aforementioned functionality. In the presence of the allyl compound, the free-radical polymerization of monomers, regardless of their termination mode, proceeds entirely with the unimolecular termination mechanism, as shown in Scheme 9. Because allyl compounds lead to degradative chain transfer, the resulting allyl radical is quite stable due to the allyl resonance. Monofunctional polystyrene, polyvinylacetate, and poly(t-butyl methacrylate) were prepared by using this approach [33]. Subsequently, various macromonomers were derived from these polymers. 

The most important mechanism for the decay of propagating species in radical polymerization is radical-radical reaction by combination or disproportionation as shown in Scheme 5.1. This process is sometimes simply referred to as bimolecular termination. However, this term is misleading since most chain termination processes are bimolecular reactions. 

The polymerizations (a) and (b) owe their success to what has become known as the persistent radical effect."1 Simply stated when a transient radical and a persistent radical are simultaneously generated, the cross reaction between the transient and persistent radicals will be favored over self-reaction of the transient radical. Self-reaction of the transient radicals leads to a build up in the concentration of the persistent species w hich favors cross termination with the persistent radical over homotermination. The hoinolermination reaction is thus self-suppressing. The effect can be generalized to a persistent species effect to embrace ATRP and other mechanisms mentioned in Sections 9.3 and 9.4. Many aspects of the kinetics of the processes discussed under (a) and (b) are similar,21 the difference being that (b) involves a bimolecular activation process. 

Phosphites can react not only with hydroperoxides but also with alkoxyl and peroxyl radicals [9,14,17,23,24], which explains their susceptibility to a chain-like autoxidation and, on the other hand, their ability to terminate chains. In neutral solvents, alkyl phosphites can be oxidized by dioxygen in the presence of an initiator (e.g., light) by the chain mechanism. Chains may reach 104 in length. The rate of oxygen consumption is proportional to v 1/2, thus indicating a bimolecular mechanism of chain termination. The scheme of the reaction... 

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