Free radical initiators initiator efficiency

C - Quasi-steady concentration of free radicals f Initiator efficiency... 

We have also investigated the kinetics of free radical initiation using azobisisobutyronitrile (AIBN) as the initiator [24]. Using high pressure ultraviolet spectroscopy, it was shown that AIBN decomposes slower in C02 than in a traditional hydrocarbon liquid solvent such as benzene, but with much greater efficiency due to the decreased solvent cage effect in the low viscosity supercritical medium. The conclusion of this work was that C02 is inert to free radicals and therefore represents an excellent solvent for conducting free radical polymerizations. 

Fig. 10 Synthesis of a mixed brush may be accomplished by two strategies (a) A free radical initiator with low efficiency is used. This guarantees that some of the initiator is left after deposition of the first brnsh. Immersion into a second monomer yields a mixed brush where the two polymers are intercalated between each other, (b) A mixed monolayer could be used whereby A and B may be initiated independently (e.g. one is a photoinitiator and the other is a thermal initiator)...

Free radicals should initiate polymerization efficiently. Some peroxides such as dialkyl peroxides and peresters tend to abstract hydrogen from the monomer more readily than they react to initiate polymerizations. Consequently, their efficiency as initiators is reduced. 

The use of free radical initiators in such reactions can be very useful. They allow the reactions to be run at much lower temperatures and generally make them more efficient [60],... 

The chief objective of all research in the field of macromonomers is to get an easy access to a wide choice of graft copolymers. The main limitation of the earlier procedures to synthesize graft copolymers is the small number of systems to which grafting onto or grafting from techniques could be efficiently applied. The random copolymerization of a macromonomer with another monomer offers a much broader choice, and it is also much easier to carry out, in most cases by means of free-radical initiators. 

One example of an efficient free radical initiator generating reaction is the reduction of... 

Reduction of aryl and alkyl halides. This reaction can be conducted in generally good yield with LiAlHi by a free-radical process initiated by irradiation of di-t-butyl peroxide. The order of reactivity of aryl halides is Arl > ArBr > ArCI > ArF. Alkyl and cycloalkyl halides are reduced efficiently, but vinyl bromides are reduced in only modest yield. 

Since the 1960 s many researchers have been concerned with the development of feasible and industrially useful methods for the synthesis of cellulose graft copolymers3, 4. Recent investigations have shown that the most efficient approach to this problem involves free radical polymerization initiated by redox systems5. An impressive example is the industrial production of mtilon (cellulose-polyacrylonitrile graft copolymer) and other fibers, particularly those with ion-exchange and acid-resistant properties6"8. ... 

For efficient cationic polymerization of vinyl monomers, it is necessary that the carbon-carbon double bond be the strongest nucleophile in the molecule. Thus vinyl acetate would be classed as an electron-donor-type monomer (Section 7.10.2) but it cannot be polymerized cationically because the carbonyl group complexes the active center. (It is polymerized only by free radicals anionic initiators attack the ester linkage.)... 

The main function of metal deactivators (MD) is to retard efficiently metal-catalyzed oxidation of polymers. Polymer contact with metals occur widely, for example, when certain fillers, reinforcements, and pigments are added to polymers, and, more importantly when polymers, such as polyolefins and PVC, are used as insulation materials for copper wires and power cables (copper is a pro-oxidant since it accelerates the decomposition of hydroperoxides to free radicals, which initiate polymer oxidation). The deactivators are normally poly functional chelating compounds with ligands containing atoms like N, O, S, and P (e.g., see Table 1, AOs 33 and 34) that can chelate with metals and decrease their catalytic activity. Depending on their chemical structures, many metal deactivators also function by other antioxidant mechanisms, e.g., AO 33 contains the hindered phenol moiety and would also function as CB-D antioxidants. 

With the aim to study PA transformations Makarov [40] used method of free radicals initiation by thermal and photochemical decomposition of peroxides. The author succeeded in finding high efficiency of PA maeromolecules breakages under the action of free radicals at the temperature 20-98 ° C (within the limits of operational temperatures). He also succeeded in determination of reactions sequence and revealing the phase directly responsible for the acts of polymer chains destruction. It is shown that in the conditions of thermal initiation transformations of peroxides are caused by macroradicals and photochemical - by own radicals of peroxides. 

We have also studied the kinetics of free radical initiation in CO2 using azobis(isobutyronitrile) (AIBN) as an initiator [35]. These experiments were accomplished using high pressure UV spectroscopy, and illustrated that AIBN decomposes more slowly in CO2 than in traditional hydrocarbon solvents, yet the initiator efficiency is much greater in CO2 due to the reduced solvent cage effect in the low viscosity supercritical medium. The main conclusion drawn from this work was that CO2 can therefore be employed effectively as a solvent for free radical polymerizations and remains an inert solvent even in the presence of highly electrophilic hydrocarbon radicals. 

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