Termination, of radical polymerization

The termination of radical polymerization cannot be prevented under normal conditions. This would be possible only in a polymerization initiated in rigid media, assuming that no chain transfer occurs, or if the radicals are trapped, for instance, by precipitation of the polymer during the process of its formation. Both methods have been used, and indeed the termination was considerably slowed down or even prevented permanently. However, such systems are of little value for synthesizing polymers according to a preconceived pattern. 

To date, by far the greatest experience has been accumulated on the termination of radical polymerizations. 

Head-to-Head (H-H) and tail-to-tail (T-T) linkages may also be present in polymer structures. They have been identified in addition polymers prepared by radical or coordination polymerization (polypropylene). Introduction of H-H linkages into addition polymers by radical polymerization can proceed by two mechanisms (a) terminations of radical polymerization (recombination or disproportionation of the polymer radicals) (3). The recombination introduces one H-H linkage, disproportionation of a vinyl and a saturated polymer end. Recombination and disproportionations are not always equally important termination reactions what type of termination depends on the type of monomer and polymerization conditions, (b) H-H linkages are also introduced into polymers by reverse addition of the monomer in radical polymerization. This behavior is particularly noticeable in radical polymerizations of hal-... 

Before any chemistry can take place the radical centers of the propagating species must conic into appropriate proximity and it is now generally accepted that the self-reaction of propagating radicals- is a diffusion-controlled process. For this reason there is no single rate constant for termination in radical polymerization. The average rate constant usually quoted is a composite term that depends on the nature of the medium and the chain lengths of the two propagating species. Diffusion mechanisms and other factors that affect the absolute rate constants for termination are discussed in Section 5.2.1.4. 

Quinones (Q) are well known as inhibitors of radical polymerization they terminate chains by the addition of alkyl radicals via the following reactions [7] ... 

In the present context the word termination is applied not to the breaking-off of a physical chain, i.e., the cessation of growth of a particular molecule, but to the complete destruction of a kinetic unit, which means the irreversible annihilation of one ion pair. This kinetic termination, which is a well-understood feature of radical polymerizations, is a comparatively rare event in cationic polymerizations it may occur in several different ways and in some systems not at all. 

Ionic polymerizations are characterized by a wide variety of modes of initiation and termination. Unlike radical polymerization, termination in ionic polymerization never involves the bimolecular reaction between two propagating polymer chains of like charge. Termination of a propagating chain occurs by its reaction with the counterion, solvent, or other species present in the reaction system. 

The rate of copolymerization, unlike the copolymer composition, depends on the initiation and termination steps as well as on the propagation steps. In the usual case both monomers combine efficiently with the initiator radicals and the initiation rate is independent of the feed composition. Two different models, based on whether termination is diffusion-controlled, have been used to derive expressions for the rate of copolymerization. The chemical-controlled termination model assumed that termination proceeds with chemical control, that is, termination is not diffusion-controlled [Walling, 1949]. But this model is of only historical interest since it is well established that termination in radical polymerization is generally diffusion-controlled [Atherton and North, 1962 Barb, 1953 Braun and Czerwinski, 1987 North, 1963 O Driscoll et al., 1967 Prochazka and Kratochvil, 1983] (Sec. 3-10b). 

Figure 3.3-10. Rate constants of chain propagation (left) and termination (right) of radical polymerization of ethylene. X, 189°C O, 153°C X, 132°C.

Commonly in radical polymerizations, initiation occurs continuously at a steady rate and is balanced by termination so lhal a steady concentration of growing centers (usually in the region of 10-8 mole/1) is established. The number of propagation reactions greatly exceeds the number of reactions of other types so that macromolecules are built up. The life-time of an active center is very much less than the duration of the whole process of polymerization and so the macromolecules are produced even in the earliest stages there is not a continuous rise in the molecular weight of the polymeric product as found in polymerizations of certain other types. It is instructive to consider in some detail the component reactions in the overall process of radical polymerization. 

The initiation of radical polymerizations, various transfer, as well as termination reactions all lead to a variety of products and the makeup of the mixture can only be slightly influenced by varying the reaction conditions or the monomer concentration, the initiator or the solvent. Furthermore, radical block copolymerization leads inevitably to more or less homopolymer so that the products require careful separation before the block copolymer can be characterized. Nevertheless, the synthesis of block copolymers via a radical mechanism has several important advantages ... 

In ionic polymerization a hydride (H-) transfer or a proton transfer are the analogues of the hydrogen atom transfer in radical polymerization. A hydride (H-) ion transfer is observed in many isomerizations and dimerizations of hydrocarbons which proceed via carbonium-ion mechanism. A similar process is responsible for chain transfer ip some carbonium-ion polymerizations. The transfer of negative ions like Cl- is also common, e.g. triphenyl methyl chloride is an efficient transfer agent in such a polymerization. Transfer of a proton is, on the other hand, a very common mode of termination of anionic polymerization. Indeed, this mode of termination was discussed previously in connection with branching reactions, and it was postulated in the earliest studies of anionic poly-... 

Processes such as cross-linking, second-order termination of radical species, and polymerization are all examples of recombination. Here, we consider second-order recombination, where two molecules or radical fragments may join to form a new molecule of higher degree of polymerization. 

One more option for increasing time scale tj of the formation of macromolecules in the processes of radical polymerization is the employment of nontraditional initiators, such, as iniferters. The term "iniferter," introduced by Otsu and Yoshida (1982), is a result of the fusion of three words initiator, transfer agent, and terminator. A special feature of an iniferter is its participation in each of the three reactions mentioned. 

Two-phase polymerization is modeled here as a Markov process with random arrival of radicals, continuous polymer (radical) growth, and random termination of radicals by pair-wise combination. The basic equations give the joint probability density of the number and size of the growing polymers in a particle (or droplet). From these equations, suitably averaged, one can obtain the mean polymer size distribution. 

Some quantum yields of radical polymerization (< >p for definition see Eq. (2b)) with the model monomer MM A are presented in Table 5. It is obvious that a variety of parameters acts on this value. Mainly, the participation of initiator molecules and of primary radicals on chain termination are responsible for the differences in the < >p-values. 

Another synthetic approach based on pyridium salt photochemistry involves the use of alkoxy radicals which are formed in both direct and sensitized decomposition of pyridinium ions in free radical polymerization [78]. Obviously, polytetrahydrofuran (PTHF), terminated by JV-alkoxy pyridinium ions, can act as macrophotoinitiator for the polymerization of monomers such as methyl methacrylate (MMA) that readily polymerize by a free-radical mechanism. PTHF macrophotoinitiators were prepared by termination of living polymerization of THF by the corresponding IV-oxides, The well-defined macrophotoinitiators with exact functionalities, confirmed by H-NMR, UV-visible and g.p.e. analysis, were obtained. Upon irradiation of macroinitiators at suitable wavelengths, polymeric alkoxy radicals are produced. The overall process is shown for the pyridinium macrophotoinitiator in the following Scheme 21. 

THE PRE-EFFECT OF RADICAL POLYMERIZATION ACCOMPANIED BY MUTUAL RADICAL TERMINATION... 

THE DECAY OF RADICAL POLYMERIZATION DUE TO MUTUAL TERMINATION OF RADICALS... 

A general mathematical solution of transfer and the corresponding retardation is not available. Many years ago, a solution was published for stationary radical polymerizations (with mutual termination of radicals). The derived relations remain valid. Other cases of transfer and of degradative transfer are solved individually. 

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