Polymerisation, radiation induced

A third source of initiator for emulsion polymerisation is hydroxyl radicals created by y-radiation of water. A review of radiation-induced emulsion polymerisation detailed efforts to use y-radiation to produce styrene, acrylonitrile, methyl methacrylate, and other similar polymers (60). The economics of y-radiation processes are claimed to compare favorably with conventional techniques although worldwide iadustrial appHcation of y-radiation processes has yet to occur. Use of y-radiation has been made for laboratory study because radical generation can be turned on and off quickly and at various rates (61). 

Options 1 and 2 can be dismissed as highly unlikely. Option 3a can be set aside just because the kp+ in question which should be, but are not, the same as the k2, were selected as those (very few) which had been derived from reactions for which the concentration of propagating carbenium ions was deemed to have been well-established and which were clearly first-order with respect to the monomer. The much-quoted rate constants for radiation-induced polymerisations of bulk monomers, which are internally of zero order, were not included (see Plesch [1] for details). 

We emphasise that the strongest single argument against the view that the pseudocationic reactions are propagated by ions is the nil-effect of water on the rate. The strongly inhibiting effect of water on radiation-induced true cationic polymerisations is too well known to require further elaboration here. 

There is a sequence of experimental techniques ranging from the open beaker on the bench to the extremely rigorous h.v.t. used for studying the kinetics of radiation-induced cationic polymerisations. Each technique in the series has its characteristic level of extraneous materials, and as the level of cleanliness is raised, there is a concurrent loss of flexibility. 

Since 1965, Ueno and the present author (K. H.) have extensively studied the radiation-induced polymerization of rigorously dried styrene in bulk (39, 40, 41). They found that the rate of polymerisation was increased remarkably by drying the monomer. This was attributed to the radiation-induced cationic polymerization which was enhanced by the removal of water, a cation scavenger, from the polymerization system. Potter et al. reported the same findings independently (42). 

Qualitative evidence that ionic species were significant intermediates was obtained from a study of the radiation induced polymerisation of isobutene28,29. Since this monomer was known to be readily polymerised by ionic initiators, polymerisation by 2 MeV electrons at —80 °C seemed to indicate the existence of ionic intermediates. However, the polymerisation was inhibited by oxygen and benzoquinone which are known to be inhibitors for free radical polymerisations. It was subsequently suggested30 that polymerisation was caused by the positive ion (CH3)3C+ produced by the reactions... 

Cyclopentadiene undergoes ionic polymerisation by Friedel-Crafts catalysts32. Its polymerisation by y-rays is markedly suppressed by ammonia or amines but much less by diphenylpicrylhydrazyl or oxygen33. This again points to the ionic rather than the free radical nature of the radiation-induced polymerisation. The quenching effect of ammonia was postulated as due to reactions of the type... 

The importance of these studies extends beyond the ecific realm of radiation-induced polymerisation since many of these species are formed or thought to be formed in other types of cationic polymerisation. The development of even faster irradiating instruments such as the Febetron, which allows nanosecond pulses, will provide an even deeper insight into these processes. 

Tabata and collaborators reported more information on the spectra and reactivity of the cations and cation-radicals formed as intermediate in the radiation-induced polymerisation of N-vinylcarbazole. 

Williams, F. Principles of radiation-induced polymerisation. In Fundamental processes in radiation chemistry (Ausloos, P., ed.). New York Intersdence 1978, p. 515... 

Effect of high energy radiation This effect is particularly important in the case of organic solids due to the formation of free radicals. Radiation induced polymerisation is also known. 

M. Steinberg, Intern. Symp. Radiation-induced Polymerisation and Graft Polymerisation, Battelle Mem. Inst. 1962 Rept. TID 7643. 

In previous work, polyfunctional monomers such as divinyl-benzene (DVB) and trimethylol propane triacrylate (TMPTA) have been used to enhance the rates of radiation induced homopolymerisation of monomers such as methyl methacrylate. Since common intermediates may well exist in both radiation grafting and polymerisation processes, it is conceivable that these polyfunctional monomers when used in additive amounts ( 1%) would also accelerate radiation grafting. 

UV and radiation induced polymerisation techniques are shown to be useful processes for enhancing the range of available renewable resource materials. In particular, grafting reactions have been examined as a means for modifying polymer structures. 

Unsaturated polyester finishes of this type do not need to be stoved to effect crosslinking, but will cure at room temperature once a suitable peroxide initiator cobalt salt activator are added. The system then has a finite pot life and needs to be applied soon after mixing. Such a system is an example of a two-pack system. That is the finish is supplied in two packages to be mixed shortly before use, with obvious limitations. However, polymerisation can also be induced by ultra violet radiation or electron beam exposure when polymerisation occurs almost instantaneously. These techniques are used widely in packaging, particularly cans, for which many other unsaturated polymers, such as unsaturated acrylic resins have been devised. 

This same ability to recognise hidden, unexploited treasure induced me to develop the first comprehensive theory of the polymerisations by ionising radiations [146]. None of the original researchers had stood back from their own findings, seen that their rather primitive theory was incompatible with the results of others, and set about constructing the general theory that was evidently needed. My effort [146] eventually produced a much-refined model of the propagating carbenium ion in solution and its different modes of reaction. 

New views on the cationic polymerisations induced by ionising radiations... 

Low energy initiation techniques [179, 180, 181] (near infrared, ultrasonic radiation and line tuneable pulse laser) have lately emerged to be better alternatives to the high-energy radiations (y-irradiation and e-beam). Laser-induced polymerisation of monomers have attracted significant attention in recent years generating a considerable literature published on both pulsed... 

Set-off can be avoided by the use of anti-set-off compoimds. These are spray powders that reduce the frictional contact between the sheets. Materials like silica and starch, which have a particle diameter slightly greater than the printed ink film thickness, can be used. Slow setting problems may be reduced by the use of less solvent or by a higher ratio of resin to oil. Varnishing with a formula that utilises polymerisation induced by radiation is another way to avoid set-off. The varnish covers the printed surface and direct contact between the layer of print and the iimer food contact surface of the substrate is avoided. 

The photolytic excitation of charge-transfer complexes is another recent addition to the available physical expedients to promote cationic polymerisation. The cation radicals generated by the photolysis have been characterised in some systems. More recent still is the use of ultraviolet radiation to induce the photolysis of substances whose photoproducts are initiators of cationic polymerisation. These processes will be discussed in Chap. Vin. 

---