Radiation-induced polymerization free-radical mechanisms

The creation of active sites as well as the graft polymerization of monomers may be carried out by using radiation procedures or free-radical initiators. This review is not devoted to the consideration of polymerization mechanisms on the surfaces of porous solids. Such information is presented in a number of excellent reviews [66-68]. However, it is necessary to focus attention on those peculiarities of polymerization that result in the formation of chromatographic sorbents. In spite of numerous publications devoted to problems of composite materials produced by means of polymerization techniques, articles concerning chromatographic sorbents are scarce. As mentioned above, there are two principle processes of sorbent preparation by graft polymerization radiation-induced polymerization or polymerization by radical initiators. We will also pay attention to advantages and deficiencies of the methods. 

It was found in this experiment that both anionic and cationic species reacted efficiently with methanol in bulk styrene. The bonded dimer cations and the radical anions were converted to long-lived benzyl radicals, which initiated the radical polymerization. The G value of the propagating benzyl radical was only 0.7 in pure styrene, but it increased up to 5.2 in the presence of methanol. A small amount of methanol converted almost all the charge carriers to propagating free radicals this explains why the mechanism of radiation-induced polymerization is changed drastically from cationic to radical processes on adding methanol. 

Systematic investigation of radiation-induced polymerization was conducted in Leeds by F. S. Dainton and his group (8) and in Paris by M. Magat and co-workers (9). The results of these studies convincingly demonstrated that radiation could initiate polymerization at any desired (low) temperature and led to a fundamental conclusion, viz., radiation-induced polymerizations occurred by conventional free radical mechanisms. This concept was extended to other radiation-chemical processes, and in the 1950 s most radiation chemists used free radical theories for interpreting their experimental data. 

Until 1950s, radiation-induced polymerization was considered to proceed only by the free-radical mechanism. In fact, the rate of ion generation by ionizing radiation is one to two orders of magnitude lower than that of free-radical formation. In contrast, the recombination constants for ions (ion and counter-ion) are approximately two orders of magnitude higher than those for free radicals. Hence, the stationary concentration of ions is approximately 100 times lower than that of free radicals. Consequently, radiation polymerization proceeds mainly by a free-radical mechanism. 

The great variation in the types of active centres generated in the irradiated monomer makes it possible to initiate polymerization by different mechanisms. In each specific case, the nature of the monomer determining the formation of a certain type of active centre which ensures effective initiation and the polymerization conditions, mainly the temperature and the medium (solvents), are of the greatest importance. Hence, the polymerization process usually occurs by a certain definite mechanism. Since in the course of secondary radiation-chemical transformations, in practice, particles with a longer lifetime form free radicals, the free-radical mechanism is the simplest process of radiation-induced initiation. 

The main features of the free-radical mechanism in radiation-induced polymerization are reported below. 

First, suitable monomers are required for radiation-induced polymerization proceeding by a cationic mechanism. Isobutylene, vinyl ethers, cyclopentadiene and p-pinene polymerize only by a cationic mechanism, whereas a-methyl styrene polymerizes by both cationic and anionic mechanisms. Second, it is necessary to use the conditions of the existence of ions M+ (M—>M+ + e) and the stabilization of secondary electrons capable of neutralizing M+. This is achieved (a) by carrying out polymerization at low temperatures when the lifetime of ions increases and the activity of free radicals drastically decrease, and (b) by using electron-accepting solvents or additives. 

Polymerizations induced by y radiation proceed by both cationic and free-radical mechanisms. Predominance of the cationic process can be assured by exhaustive purification of monomers and solvents or by employing high radiation dose rates (from electron beam sources). The rate of cationic polymerization has been shown to be strongly influenced by the dielectric constant of the medium, giving rise to discrepancies between radiation-induced bulk polymerizations and chemically initiated solution polymerizations. These discrepancies have been attributed to specific solvation of propagating ions by monomer and polymer. 

Studies on the radiation-induced polymerization were carried out mainly on the example of vinyl monomers, which polymerize by free radical mechanism, leading both reactions in the mass of monomer as well... 

The solid n-hexadecene-1 system has also been studied.76 It was concluded that the experimental results were incompatible with a free-radical reaction and an ionic mechanism was proposed. An enhancement in the polymerization yield by a factor of 2 on going from the liquid at 20 °C. to the solid at 0°C. was observed. Solid-state ionic polymerizations induced by high-energy radiation have been reviewed by Magat.76... 

A basic requirement of the ESR technique is the presence of molecules or atoms containing unpaired electrons. Such species can be generated in polymeric systems by homolytic chemical scission reactions or by polymerization processes involving unsaturated monomers. These reactions can be initiated thermally, photochemically, or with a free-radical initiator, and, in the case of scission, by mechanical stress applied to the system. Therefore, ESR can be used to study free-radical-initiated polymerization processes and the degradation of polymers induced by heat, light, high-energy radiation, or the application of stress. 

Sufficient experimental data from several laboratories now exist to describe the conditions under which the radiation-induced ionic propagation of many pure liquid vinyl monomers can be observed. The kinetic data and electrical conductivity measurements establish the ionic nature of the reaction scavenger studies appear to establish the preponderant role played by the carbonium ion in propagating the polymerization. On the basis of a single propagating species, it is possible to write a simple mechanism to describe the process. Limiting values of several of the kinetic rate constants can be estimated, notably the rate constant for reaction between a bare carbonium ion and a vinyl double bond. These rate constants are compared with similar constants arrived at in chemically initiated free radical, carbonium ion and carbanion polymerization. Several shortcomings of the present scheme are discussed. 

In 1982, Crlvello and co-workers published a report on the UV Initiated cationic polymerization of vinyl ether monomers using onlum salt catalysts(14). Vinyl ethers are among the most reactive monomers which polymerize by a cationic mechanism. The radiation Induced cationic curing of vinyl ethers occurs much faster than the cationic curing of epoxy coatings. In fact, cure rates that are at least as fast as the free radical polymerization of acrylates can be achleved(8,14). A recent report Indicates that the cationic polymerization of vinyl ethers can occur even in the presence of certain polar functional groups(15). 

Another mode of dual curing involves the simultaneous occurrence of free radical and cationic radiation-induced cross-linking polymerization of formulations containing appropriate initiators [20, 23, 28]. This method, which is called hybrid curing, leads to coatings with imique properties. A typical hybrid-cure system contains a diacrylate and a diepoxide, the former polymerizing by a free radical and the latter by a cationic mechanism. Exposure of the system to in-... 

Radiation sensitizers are multifunctional vinyl monomers (MFA) that are highly reactive towards free radicals. Since all common MFAs contain terminal unsaturation, it can be expected that addition/polymerization is the principal mechanism by which they react in the polymer compound. These additives are used mainly to accelerate the radiation-induced crosslinking in the polymers. The addition of MFA to the polymer formulations suppresses the chain scission reactions and allows more crosslinking to occur. 

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