Reaction behavior, living radical

Reaction Behavior and Kinetic Modeling Studies of Living Radical Photopolymerizations... 

From these experimental and modeling studies, the mechanism of the living free radical polymerizations initiated by a combination of TED and DMPA have been elucidated. The TED produces DTC radicals that preferentially cross-terminate with the propagating carbon radicals. By this cross-termination reaction, the carbon radical concentration is kept low (as was shown in figure 6) and the rate of polymerization is decreased, as is the autoacceleration effect. This suppression of the autoacceleration peak in HEM A polymerizations and, interestingly, in DEGDMA polymerization has been observed to increase as the TED concentrations are increased. This behavior has been predicted successfully by the model as well. 

In a number of reactions that are written as dissociative electron attachments, short-lived radical anions are in fact intermediates. A case in point is 5BrUra (Chap. 10). An interesting behavior is shown by the radical anion of N-bromo-succinimide which does not release a bromide ion but rather fragments into a bromine atom and a succinimide anion [reactions (17) and (18)] (Lind et al. 1991). 

Klumperman and coworkers [259] observed that while it is lately quite common to treat living radical copolymerization as being completely analogous to its radical counterpart, small deviatiOTis in the copolymerization behavior do occur. They interpret the deviations on the basis of the reactions being specific to controlled/living radical polymerization, such as activation—deactivation equilibrium in ATRP. They observed that reactivity ratios obtained from atom transfer radical copolymerization data, interpreted according to the conventional terminal model deviate from the true reactivity ratios of the propagating radicals. 

The scavenger molecule is by itself a radical and reacts with any other radicals in the system to generate nonreactive products. Because of the stability of the radical compounds employed for such inhibition/retardation reactions, the generated bond is very weak and may homolytically cleave at elevated temperatures to give back the radical reactants. This reaction behavior is exploited in the living free radical polymerization technique using nitroxides as mediators (434,435). 

It was not until the invention of iodonium and sulfonium salts as photo-initiators by Crivello (1975) that cationic photo-polymerization became practical (see Crivello et al., 1977,1990,2000). Upon irradiation of these Crivello salts, acids are generated. Another significant difference between free radical and cationic polymerizations is the latter process is a living polymerization— once the acid species is formed, it remains active even after the irradiation is stopped. In contrast to this behavior free radicals die soon after irradiation is stopped. Also, unhke free radical polymerizations cationic reactions are not inhibited by oxygen. Quite often the dark reaction following irradiation can play an important role in enabhng a cationic system to develop its full properties and this leads manufacturers of commercial cationic photopolymers to often recommend a thermal (dark) postcure after carrying out the photo-irradiation process. 

Whether the first or the second factor dominates depends on the type of polymerization process involved. If the period during which the polymer molecule is growing is short compared to the residence time of the molecule in the reactor, the first factor dominates. This situation holds for many free radical and ionic polymerization processes where the reaction intermediates are extremely short-lived. Figure 9.11, taken from Denbigh (10), indicates the types of behavior expected for systems of this type. 

One-electron transfer reactions are typical in living organisms. Ion-radicals are acting participants of metabolism. Of course, such ion-radicals are instantly included in further biotransformations. Therefore, it is reasonable to consider the problem of ion-radical formation together with the data on their behavior in biosystems. Chapter 3 contains a special section covering this topic. However, the issue of competition for an electron during ion-radical formation deserves to be mentioned here. 

Radical homopolymerization and copolymerization of macromonomers are fairly well understood and reveal their characteristic behaviors that have to be compared with those of conventional monomers. A detailed mechanism of the polymer-polymer reactions involved, however, appears still to be an issue. Ionic or, desirably, living polymerization and copolymerization are still an important... 

The development of the two-color and laser jet approaches has also allowed the study of the photochemical behavior of excited states of reaction intermediates, i.e., transient species that are chemically distinct from the original ground or excited state, such as neutral and ion radicals, biradicals, carbenes, and ylides. In fact, the study of excited reaction intermediates has been more comprehensive than the study of upper states. Originally, the short-lived nature of the ground-state transient itself led to the incorrect assumption that the excited transient would be too short-lived to participate in any chemical or photophysical processes other than deactivation to the ground state. However, this is now known not to be the case and some surprising differences between the ground- and excited-state behavior of reaction intermediates have been observed. 

Most of the known cationic photoinitiators produce acid species in an irreversible reaction, and once formed these species continue to promote the polymerization reaction even after the end of irradiation. This behavior is of living type and is in contrast to the radical photoinitiated... 

An examination of the behavior of peroxides in the course of their rapid thermal decanposition, provides some insight into the nature of the decomposition intermediates and/or products. NMR spectra taken during the thermolysis of various peroxides and per-oxyesters in solution at temperatures where they have short half lives, e.g. benzoyl peroxide and t-butyl peroxypivalate at 110 C, contain emission peaks and/or enhanced absorption peaks. These peaks are presumed to be those of the reaction products resulting from interactions involving the transient decomposition products, e.g. interactions between pairs of caged radicals or from radical pair encounters. The NMR spectra are indicative of the occurrence of chemically induced dynamic nuclear polarization (CIDNP). 

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