Peroxy initiator

For a number of applications curing at room temperature is desirable. This so-called cold cure is brought about by using a peroxy initiator in conjunction with some kind of activator substance. The peroxy compounds in these cases are substances such as methyl ethyl ketone peroxide and cyclohexanone peroxide, which as used in commercial systems tend not to be particularly pure, but instead are usually mixtures of peroxides and hydroperoxides corresponding in composition approximately to that of the respective nominal compounds. Activators are generally salts of metals capable of undergoing oxidation/reduction reactions very readily. A typical salt for this purpose is cobalt naphthenate, which undergoes the kind of reactions illustrated in Reactions 4.6 and 4.7. 

Moad G, Solomon DH (1989) Azo and Peroxy Initiators In Allen G, Bevington JC, Eastmond AL, Russo A (eds), Comprehensive Polymer Science, Pergamon Press, Oxford 3(8) 97... 

The peroxy initiators bis(perfluoro-2-Ar-propoxypropionyl) peroxide (BPPP) and diethyl peroxydicarbonate (DEPDC) have had the greatest application in the heterogeneous polymerization of fluoroolefins in CO2. 

Finally, Chiefari et al. [315-317] suggested another technique leading to the synthesis of addition-fragmentation-type macromonomers but without the use of any CTA. This method, clean, easy, and economical, involves heating a mixture of acrylate or styrene monomer in an appropriate solvent with an azo or peroxy initiator. High temperatures (typically up to 150 °C) are required. To prove the expected mechanism, the authors studied the poly(alkyl acrylate) reactions in the presence (or not) of monomers and by using different conditions. They showed the reaction does not occur without the monomer. Moreover, an increase of the temperature leads to a better yield and a decrease of the molar mass. Macromonomers have been synthesized by this technique withMn between 103 and 104 gmol... 

Figure 6.27 Mechanism of RAFT process. Polymerizations can be carried out in bulk, solution, emulsion or suspension, using azo or peroxy initiators as in conventional free-radical polymerization. The moiety S=C(X)S- remains as the end group.

The chemistry for RAFT is illustrated in Scheme 3. The RAFT process is the newest of the living-radical processes and is reported not to have the limitations of the two previously described systems [45], It is essentially a degenerative transfer process in which a polymer chain (P ), initiated with an azo or peroxy initiator, reacts with a (thiocarbonyl)sulfanyl compound, S=C(Z)-S-R, to release R, an alkyl radical which can go on to initiate another polymer chain. Another propagating chain (Pm ) can subsequently react with P -S-C(Z)=S to release P which can go on to add more monomer. This cycle then repeats itself to produce polymer. 

LLDPE and amine-terminated butadiene nitrile liquid rubber (ATBN) with di-peroxy initiator into the rubber tire powder. The compounded blends were molded into test specimens. The data showed about 50% better tensile and impact strength after compatibilization [Duhaime and Baker, 1991]. 

The polymerisation of styrene with vegetable oils involves free radical initiated polymerisation. A free radical type initiator, such as benzoyl peroxide, azobisisobutryronitrile and ditertiarybutyl peroxide is normally used to accelerate the copolymerisation reaction (Rg. 8.4). Linseed, tung,soybean, sunflower and oiticica oils and dehydrated castor oil (DCO) are widely used in the preparation of styrenated-oil products. " The free radical polymerisation of methyl methacrylate or n-butyl methacrylate, using polymeric oil peroxy initiators from the auto-oxidation of linseed oil, soybean oU, and Unoleic acid has been carried out successfully. 

Thermal Initiation. Thermally decomposing initiators (mainly) fall into two classes azo- and peroxy-type molecules. The general structures of azo- and peroxy-initiators are represented by 1 and 2 respectively. 

Copolymer composition can be calculated as a function of monomer composition when the polymer is formed by free radical polymerization in a CSTR (continuous stirred tank reactor). Consider two monomers 1 and 2 as starting materials for forming a copolymer with repeat units of 1 and 2. The initiation can be effected by thermal means or by using a peroxy initiator. 

The inhibiting effect of radical acceptors, however, suggests that polymerization proceeds by a radical mechanism. The yield of polymer from such reactions depends on the tendency of the different monomers to be polymerized by ionic and radical catalysts. The reactivity ratio on grinding for styrene plus methyl methacrylate is similar to that for peroxy-initiated polymerization. Nuclear magnetic resonance data, however, show a different stereochemical configuration for the copolymers [200]. 

Initiator decomposition can be triggered in a variety of ways. The most conunon method for industrial free radical polymerization is thermal initiation (typically using azo or peroxy initiating species), while photoinitiation is more popular for laboratory scale kinetic studies. In either case. Equation 1.6 describes the decomposition of initiator into two radical species, which may or may not have equal reactivities, depending on the choice of initiator [1,2]. The concentration of initiator can then be calculated by ... 

The use of activators was recommended in conjunction with peroxy initiators. Examples included sodium bisulfite, sodium metabisulfite, sodium thiosulfate, sodium hydrosulfate, and water soluble reducing agents such as dextrose. The amount of the activator was generally in the range of 0.2-0.8 part by weight per hundred parts of total monomers. 

Some interest attaches to the unsaturated tetra-alkyls of germanium and their possible polymerization. Vinyl and allyl compounds of the type R GeR 4.n (where R is methyl or ethyl) have been polymerized at 6000 atm and 120°, using peroxi initiators. Monovinyl and monoallyl compounds gave oily liquids, while the polyfimctional compounds... 

As reactions (18-20) imply, the mechanism of induced decomposition does very much depend on the solvent. Furthermore, the extent to which it occurs changes with the type of peroxy initiator used and the monomer itself, because often induced decomposition may be triggered by any of the radicals present in the reaction mixture. 

The choice of the proper peroxy initiator largely depends on its decomposition rate at the reaction temperature of the polymerization. BPO is the major initiator for bulk polymerization of polystyrene or acrylic ester polymers, where temperatures from 90°C to 220°C are encountered. Dilau-royl, dicaprylyl, diacecyl, and di- err-butyl peroxides are also used. In the case of suspension polymerization of styrene, where temperatures between 85°C and 120 C are applied, the initiators also range in activity from BPO to di-tm-butyl peroxide. In suspension polymerization of vinyl chloride (reaction temperatures of 45-60°C for the homopolymer), thermally very labile peroxides such as diisopropyl peroxydicarbonate and rm-butyl peroxy-pavilate are used. 

Besides low-molecular-weight peroxides, there are also numerous works on macromolecular peroxide initiators, which are useful in the preparation of block copolymers [73]. As illustrated in Table 4, various compounds have been reacted via condensation or addition reaction to yield macro-peroxy-initiators. 

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