Free-radical polymerisation inhibition

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... 

However, these systems offer some important advantages when compared with traditional free-radical polymerisation processes simpler polymerisation kinetics milder reaction conditions insensitivity to oxygen inhibition [2, 3]. These characteristics have made the thiol-ene photo-polymerisation reaction the focus of extensive research, and its application in oleochemistry is of considerable importance in terms of polymer science and technology. 

In cationic curing, high curing speeds can be achieved even though the photoinitiator requirement is less. Though cationic polymerisation process is slower than free radical polymerisation, higher belt speeds can be achieved in cationic systems as there is no problem of air inhibition. 

In dry air and in the presence of polymerisation inhibitors methyl and ethyl 2-cyanoacrylates have a storage life of many months. Whilst they may be polymerised by free-radical methods, anionic polymerisation is of greater significance. A very weak base, such as water, can bring about rapid polymerisation and in practice a trace of moisture on a substrate is enough to allow polymerisation to occur within a few seconds of closing the joint and excluding the air. (As with many acrylic monomers air can inhibit or severely retard polymerisation). 

This reaction has been shown to be very rapid77. Sulphuric and acetic acids sup press the polymerisation. Evidently their anions are ineffective as initiators, and the enhanced proton concentration provided by them must reduce the chain lifetime. The slight retarding effect of oxygen could be due to electron scavenging. However, the authors suggest that there may be a small free radical component of the chain reaction, which is inhibited in the presence of oxygen. 

Free-radical polymerization processes are used to produce virtually all commercial methaerylie polymers. Usually free-radical initiators tqv > such as a/o compounds or ieroxides are used to initiate the polymerisations. Photochemical and radiation-initiated polymerizations are also well known. At it constant temperature, the initial rate of the hulk or solution radical polymerization of methaerylie monomers is first-order with respect to monomer eoneentration. anil one-half order with respect to the initiator concentration. Methacrylate polymerizations are markedly inhibited by-oxygen therefore considerable care is taken to exclude air during the polymerization stages of manufacturing. 

In this century it has been shown that mastication of rubber produces free radicals with high chemical reactivity. Thus, for example, macroradicals from cis-polyisoprene initiate vinyl polymerisation when oxygen is rigorously excluded from a high shear mixer, giving rise to block copolymers, typically containing rubber and methyl methacrylate (Scheme 3.2). In the presence of oxygen this process is inhibited and the... 

Pressures during emulsion polymerisation are typically low (near atmospheric pressure). Since the atmospheric oxygen present in the headspace of a reactor is an excellent free radical scavenger, a nitrogen purge or blanket is introduced into the reactor to flush out the oxygen. Otherwise, an induction period (a period in which the polymerisation is inhibited) or a retardation effect (a reduction in the rate of polymerisation) may be observed. Not all emulsion polymerisations are at low pressure. For example, polymerisations of vinyl acetate-ethylene (88), vinyl chloride, and styrene-butadiene mbber are typically run at high pressure. 

To inhibit polymer formation and oxidative degradation during shipment and subsequent storage, an inhibitor TBC (4-tert-butylcatechol), is added. TBC prevents polymerisation by reacting with oxidation products (peroxides forming free radicals) in the presence of a small amount of oxygen. The inhibitor level must be maintained above a minimum concentration at all times which is 4 to 5 ppm. The standard level of TBC is 10 to 15 ppm. 

These inhibitors have very great practical importance, because they allow many products to be stabilized (e.g. antioxidants in the rubber industry). Radical polymerisations can be inhibited by various substances often the presence of impurities causes a longer or shorter induction period (in which polymerisation is halted), until the impurities have been entirely consumed by reaction with free radicals. 

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