Polymer radicals radical combinations

As with the previous reported conclusion concerning the polymeric photoinitiators based on hydroxyalkyl acetophenone moiety (KIP), in this case rm (Eq. 4) also is much higher in the polymeric systems than in the models (Table 22), indicating that the B radicals anchored to the polymer backbone are less prone to give radical-radical combination, thus favouring flieir reaction with the acrylic monomers. 

The resulting poly(styrene), which is expected to have tertiary amine groups attached at each end of the polymer chain, due to the well established termination mechanism by radical-radical combination, are used, under UV irradiation, in conjunction with 9-fluorenone as a photo-redox system for the free radical polymerization of MMA to yield MMA/St/MMA block copolymers (Scheme 37) ... 

The most important free-radical chain reaction conducted in industry is the free-radical polymerization of ethylene to give polyethylene. Industrial processes usually use (/-Bu())2 as the initiator. The t-BuO- radical adds to ethylene to give the beginning of a polymer chain. The propagation part has only one step the addition of an alkyl radical at the end of a growing polymer to ethylene to give a new alkyl radical at the end of a longer polymer. The termination steps are the usual radical-radical combination and disproportionation reactions. 

R2 is a generalized form, and the successive addition of maleimide and styrene to a radical will produce a polymer. The solvent radicals perform not only as the initiator (R2) but also as the terminator (R4) of the polymerization. Concerning the reaction of the polymer radical, the combination between polymer radicals (R5) competes with that of the solvent radical (R4). 

For irradiation at a higher dose rate, the radical-radical combination reactions (R6) wiU efficiently occur and compete with the addition reactions of radicals and solute molecules to initiate the polymerization (R2), while the addition reactions (R2) effectively occur during irradiation at a lower dose rate, because of the reduction of radical loss (Nakagawa 2010). This will lead to an increased polymer yield with a decreasing dose rate. As solvent radicals work not only as an initiator (R2) but also the terminator (R4) of polymerization, the probability for polymer radicals to terminate with solvent radicals (R4) will be less by irradiation at a lower dose rate. This will make it easy for polymer radicals (R5) to produce a polymer with a higher molecular weight. 

This procedure has been adapted to transformation reactions however, most of the reported transformations were achieved from AM polymerization of cyclic ethers to conventional radical polymerization by using thermal or photochemical activation. For instance, AM polymerization of epichlorohydrin (ECH) was performed in the presence of 4,4 -azobis(4-cyanopentanol) yielding polymers with azo linkages in the main chain. Polymerization was conducted under typical conditions, that is, by slow addition of ECH to the solution of initiator containing catalyst. The reaction was considerably slower than in the presence of simple diols (e.g., EO) and only 28% conversion was achieved under conditions sufficient to reach complete conversion in the polymerization initiated by EO. Poly(epichlorohydrin) (PECH) prepared this way was consequently used in the polymerization of St to produce block polymer (Scheme 59). This polymerization yielded PSt with PECH segments at each end since termination occurs through radical-radical combinations. 

Crosslinking can also affect photodegradation rates by locking the polymer structure and preventing lamellar unfolding. The consequence is to prevent separation of photoproduced radicals, which favors radical-radical combination. Crosslinked systems therefore generally have smaller quantum yields of degradation relative to non-crossUnked systems. ... 

The mechanism of the reaction is first an abstraction of a hydrogen atom from the polymer chain, leading to the formation of a reactive radical site [6]. Then two polymer radicals can combine, which results in a polymer network. The network formed by those reactions is very irregular, fn order to improve the network the addition of compounds with multiple double bonds, for example, triaUyl cyanurate (TAG), triallyl isocyanurate (TAIC), trimethylol propane trimethacrylate (TMPTA), or ethylene dimethaciylate (EDMA) is necessary [6]. They are often referred to as activators. The effect is described as an addition to the radical site at the polymer chain and transfer of the radical to the activator. The network is formed by reaction of the transferred radical with another chain. 

This situation is expected to apply to radical termination, especially by combination, because of the high reactivity of the trapped radicals. Only one constant appears which depends on the diffusion of the polymer radicals, so it cannot cancel out and may be the source of a dependence of the rate constant on the extent of reaction or degree of polymerization. 

This expression is plotted in Fig. 6.7 for several large values of p. Although it shows a number distribution of polymers terminated by combination, the distribution looks quite different from Fig. 5.5, which describes the number distribution for termination by disproportionation. In the latter Nj,/N decreases monotonically with increasing n. With combination, however, the curves go through a maximum which reflects the fact that the combination of two very small or two very large radicals is a less probable event than a more random combination. 

Bulk Polymerization. The bulk polymerization of acryUc monomers is characterized by a rapid acceleration in the rate and the formation of a cross-linked insoluble network polymer at low conversion (90,91). Such network polymers are thought to form by a chain-transfer mechanism involving abstraction of the hydrogen alpha to the ester carbonyl in a polymer chain followed by growth of a branch radical. Ultimately, two of these branch radicals combine (91). Commercially, the bulk polymerization of acryUc monomers is of limited importance. 

Cross-linked PVP can also be obtained by cross-linking the preformed polymer chemically (with persulfates, hydrazine, or peroxides) or with actinic radiation (63). This approach requires a source of free radicals capable of hydrogen abstraction from one or another of the labile hydrogens attached alpha to the pyrrohdone carbonyl or lactam nitrogen. The subsequently formed PVP radical can combine with another such radical to form a cross-link or undergo side reactions such as scission or cyclization (64,65), thus ... 

Low-molecular weight azo compounds have frequently been used in cationic polymerizations producing azo-containing polymers. Thus, the combination of ionically and radically polymerizable monomers into block copolymers has been achieved. Azo compounds were used in all steps of cationic polymerization without any loss of azo function as initiators, as monomers and, finally, as terminating agents. 

Even though the rate of radical-radical reaction is determined by diffusion, this docs not mean there is no selectivity in the termination step. As with small radicals (Section 2.5), self-reaction may occur by combination or disproportionation. In some cases, there are multiple pathways for combination and disproportionation. Combination involves the coupling of two radicals (Scheme 5.1). The resulting polymer chain has a molecular weight equal to the sum of the molecular weights of the reactant species. If all chains are formed from initiator-derived radicals, then the combination product will have two initiator-derived ends. Disproportionation involves the transfer of a P-hydrogen from one propagating radical to the other. This results in the formation of two polymer molecules. Both chains have one initiator-derived end. One chain has an unsaturated end, the other has a saturated end (Scheme 5.1). 

Early reports37 157 167 suggested that termination during VAc polymerization involved predominantly disproportionation. However, these investigations did not adequately allow for the occurrence of transfer to monomer and/or polymer, which are extremely important during VAc polymerization (Sections 6.2.6.2 and 6.2.7.4 respectively). These problems were addressed by Bamford et who used the gelation technique (Section 5.2.2,2) to show that the predominant radical-radical termination mechanism is combination (25 °C). 

Chains with uttdesired functionality from termination by combination or disproportionation cannot be totally avoided. Tn attempts to prepare a monofunctional polymer, any termination by combination will give rise to a difunctional impurity. Similarly, when a difunctional polymer is required, termination by disproportionation will yield a monofunctional impurity. The amount of termination by radical-radical reactions can be minimized by using the lowest practical rate of initiation (and of polymerization). Computer modeling has been used as a means of predicting the sources of chain ends during polymerization and examining their dependence on reaction conditions (Section 7.5.612 0 J The main limitations on accuracy are the precision of rate constants which characterize the polymerization. 

Prior to the development of NMP, nitroxides were well known as inhibitors of polymerization (Section 5.3.1). They and various derivatives were (and still are) widely used in polymer stabilization. Both applications are based on the property of nitroxides to efficiently scavenge carbon-centered radicals by combining with them at near diffusion-controlled rates to form alkoxyamines. This property also saw nitroxides exploited as trapping agents to define initiation mechanisms (Section 3.5.2.4). 

The free radicals combine to form a carbon-to-carbon bond and give a saturated polymer molecule with initiator fragments on both ends. Termination by disproportionation produces two polymer molecules, one of which will contain a double bond ... 

This new free radical adds to the double bond of another monomer molecule, growing the polymer chain. The polymerization process ends as the unpaired electrons of two free radicals combine to form a single bond ... 

As for any chain reaction, radical-addition polymerization consists of three main types of steps initiation, propagation, and termination. Initiation may be achieved by various methods from the monomer thermally or photochemically, or by use of a free-radical initiator, a relatively unstable compound, such as a peroxide, that decomposes thermally to give free radicals (Example 7-4 below). The rate of initiation (rinit) can be determined experimentally by labeling the initiator radioactively or by use of a scavenger to react with the radicals produced by the initiator the rate is then the rate of consumption of the initiator. Propagation differs from previous consideration of linear chains in that there is no recycling of a chain carrier polymers may grow by addition of monomer units in successive steps. Like initiation, termination may occur in various ways combination of polymer radicals, disproportionation of polymer radicals, or radical transfer from polymer to monomer. 

In this two growing polymer radicals combine to yield a dead polymer. 

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