Azonitrile initiators

The conclusion that chain-breaking inhibition by zinc dialkyl dithiophosphates involves electron transfer was reached independently by Burn (6) following a more detailed qualitative study of the inhibition of the azonitrile-initiated oxidation of squalane and cumene and the noninitiated oxidation of indene by metal dialkyl dithiophosphates and related compounds (I to IV) ... 

Table 2.2. Decomposition Rates of Some Azonitrile Initiators ...

Table 3.2 Decomposition rates of some azonitrile initiators...

The reason why the primary propagating radicals produced by azonitrile initiators have different stability is not yet clear, but it is of great interest in regard to the pronounced penultimate effect observed in the copolymerization of AN. 

The azonitrile 19 also shows similar decomposition kinetics to AIBN (Table 3.2). The initiators 19 and AIBMe also have greater solubility in organic solvents than AIBN. 

The first published examples of hydrosilation, which appeared about 30 years ago, noted that they were observed to proceed by free-radical mechanisms initiated thermally (about 300°C) (J), by acyl peroxides (4), by azonitriles (5), by ultraviolet light (6), or by y radiation (7). The first hint that catalysts known to be effective for hydrogenation might also be effective for hydrosilation was found in a French patent (8) (1949) which stated that catalysts may be chosen from compounds and salts of the elements of Groups IIA, IVA, IB, and IIB of the periodic table and metals of Group VIII and certain of their salts. No example to demonstrate this was included in the patent. 

Azoic soluble dyes, 7 373t Azo initiators, 14 293, 294-295t handling, 14 296 worldwide producers of, 14 303 Azolla, in nitrogen fixation, 17 299 Azo-methine dyes, 9 503 Azomethine ylides, 17 51, 56 Azonitriles, 14 293... 

Orotic acid readily forms dimers even when irradiated in liquid medium [582, 583]. 5-Bromouracil (5-BrU) in DNA is dehalogenated, rather than forming cyclobutane-type dimers. Such DNA derivatives are more sensitive to ultraviolet irradiation than normal DNAs [584-594], Irradiation of 5-bromo-uracil and derivatives in aqueous medium produces 5,5 -diuracil [590, 591]. However, derivatives such as 3-sbutyl-5-bromo-6-methyluracil have been reported to yield cyclobutane dimers either by irradiation of frozen aqueous solutions, or by catalysis with free radical initiators, such as aluminium chloride, ferric chloride, peroxides or azonitriles [595]. 5-Hydroxymethyluracil is reported to dimerize very slowly in frozen water at 2537 A [596]. The fundamental research in the photochemistry of the nucleic acids, the monomeric bases, and their analogues has stimulated new experiments in certain micro-organisms and approaches in such diverse fields as template coding and genetic recombination [597-616]. 

Azonitriles have generally been considered to be cleaner initiators in the sense of being devoid of transfer, but this is not true [Braks and Huang, 1978]. The transfer with azonitriles probably occurs by the displacement reaction... 

The synthesis of special initiators is another route to block copolymers. Reaction of XXXVI with an HO-terminated polyester or polyether yields a polymeric azonitrile, which initiates polymerization of a monomer such as styrene [Laverty and Gardland, 1977 Walz and Heitz, 1978],... 

This decomposition usually shows little dependence on solvent, so if Ed for decomposition in chloroprene is likewise 30.9 kcal. per mole, then since Eox = 25.1 kcal. per mole Ep = 9.6 kcal. per mole, assuming termination to require no energy of activation. This is 1.2 kcal. per mole larger than kp for styrene oxidation (8). Values of e for azobisisobutyronitrile in oxidation systems usually lie in the range 0.6 to 0.8 if e = 0.7, the above equation for the decomposition of the azonitrile and that given earlier for the initiated oxidation of chloroprene permit calculation of kp/kt1/2 for chloroprene and also the kinetic chain lengths of the oxidations (Table IV). 

Generally, the commercially available azo initiators are of the symmetrical azonitrile type ... 

Azonitriles are not susceptible to radical-induced decompositions (56) and their decomposition rates are not usually affected by other components of the environment. Cage recombination of the alkyl radicals occurs when azo initiators are used, and results in the formation of toxic tetrasubstituted succinonitrile derivatives (56). This can be a significant drawback to the use of azo initiators. In contrast to some organic peroxides, azonitrile decomposition rates show only minor solvent effects (54—56) and are not affected by transition metals, acids, bases, and many other contaminants. Thus azonitrile decomposition rates are predictable. Azonitriles can be used as thermal initiators for curing resins that contain a variety of extraneous materials since cure rates are not affected. In addition to curing of resins, azonitriles are used for polymerization of commercial vinyl monomers. 

The azonitrile (AZDN) and ammonium persulphate were stored together as polymerisation initiators. The AZDN was warmed by a supposedly blanked-off steam pipe... 

The rates of initiation depend on the type of activation ch.osen. In photochemical initiation, Vs = 2 0/, where I is the absorbed light intensity and 0 = the efficiency coefficient. With an average intensity, rates of initiation of approximately 10"7 mole l 1 s 1 are attained. In thermal activation, autoinitiation by interaction between oxygen and aldehyde gives low values of approximately 10 9 mole l-1 s"1 under standard laboratory conditions. When azonitrile is used, since the thermal decomposition rate of this product is approximately first order [62], Vi is given by... 

V>. [Wako Cbem. USA] Azonitrile conqKis. polymerization initiatOR. 

A number of reportsindicate that primary radical termination can be important during polymerizations initiated by azonitriles. However, for the case of S polymerization initiated by AIBN, NMR end group determination shows that primary radical termination is of little importance except when very high rates of initiation are employed (e.g. with high initiator concentrations at high temperatures). Cyanoalkyl radicals give a mixture of combination and disproportionation in their reactions with other radicals (see also Sections 2.5,... 

Another common class of initiators are symmetric azonitriles which also decompose at 60 to 70°C to form radicals having structures like the one indicated below ... 

The azonitriles may also be used as initiators in photopolymerizations, in which case free radicals are formed at temperatures in the neighborhood of O C upon irradiation with rays of short wavelength in the visible or neaiv ultraviolet region. 

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