Allyl peroxides chain transfer

In the case of allyl peroxides (12 X= CH2, A=CH2, BO),1 1 1 intramolecular homolytic substitution on the 0-0 bond gives an epoxy end group as shown in Scheme 6.18 (1,3-Sn/ mechanism). The peroxides 52-59 are thermally stable under the conditions used to determine their chain transfer activity (Table 6.10). The transfer constants are more than two orders of magnitude higher than those for dialkyi peroxides such as di-f-butyl peroxide (Q=0.00023-0.0013) or di-isopropyl peroxide (C =0.0003) which are believed to give chain transfer by direct attack on the 0-0 bond.49 This is circumstantial evidence in favor of the addition-fragmentation mechanism. 

Isopropenyl acetate and allyl chloride behave similarly. In the polymerization of the latter monomer degradative chain transfer occurs more readily by removal of the chlorine atom to yield the unsubstituted allyl radical CH2—CH—CH2, which manages to add monomer occasionally. This is indicated by the formation of about three polymer molecules, having an average degree of polymerization of six units, for each molecule of benzoyl peroxide decomposing. 

That peroxy radicals were important and common radical intermediates in autoxidation became clearer from the work of Criegee et al. [4J. These workers showed that the peroxide resulting from UV-initiated autoxidation of cyclohexene was a hydroperoxide formed by removal of a reactive allylic hydrogen followed by addition of oxygen and chain transfer between cyclohexenyl peroxy radical and cyclohexene, viz. 

Since such radicals react less rapidly than radicals not stabilized by resonance, the observable polymerization rate decreases. For this reason, the process is termed degradative chain-transfer. In degradative chain-transfer, growing, polymeric free radicals collide with a monomer molecule to form a new, stable free radical which propagates only with difficulty. The allylic radical may terminate growing radicals, dimerize, cause the decomposition of peroxidic initiators, or initiate the formation of new polymerizing species. The first of these possible processes has been termed cross termination [14]. 

The first of these is an example of induced decomposition of benzoyl peroxide (see Section 1.3.1). The second shows chain transfer to toluene, which is a solvent commonly used for solution polymerization. The last reaction is characteristic of allylic monomers and is prevalent to the extent that homopolymerizations of allylic monomers yield only chains with degrees of polymerization below about 20 (i.e. oligomeric chains). The high incidence of chain transfer to allylic monomers results from the high resonance stability of the allylic radical produced and the reaction is often referred to as autoinhibition. 

Also, from UT to 58, sponsored by DOD, an extended study on polymerization kinetics was undertaken, particularly of allylic compounds, in cooperation with N. Gaylord, N. Field, H. Starkweather, A. Adicoff and M. Litt, rounding out our knowledge of degradative free radical chain transfer and retarded polymerization. This work led to studies of the mechanism of peroxide decomposition as a function of environments, especially of the cage effect, with W. 

Scheme 14 Mechanism of addition-fragmentation chain transfer with an allyl peroxide transfer agent. Reproduced from Moad, G. Rizzardo, E. Thang, S. H., Radical addition-fragmentation chemistry in polymer synthesis. Polymer 2008, 49,1079-1131." ...

As exemplified in Figure 2, Type 1 mechanism, electron transfer from L to sens yields two radicals, the substrate radical, L", and the sensitizer radical anion (sens ). In the next step, the lipid radical may induce a chain peroxidation cascade involving propagation reactions -The sensitizer radical anion may also start a sequential one-electron reduction of 2 generating HO in the presence of reduced transition metals. As a result, this may lead to abstraction of a lipid allylic hydrogen with subsequent generation of a carbon-centered lipid radical, L, that is rapidly oxidized to a peroxyl radical (vide supra). 

Besides vinyl acetate monomer, three other components are neeessary to earry out an emulsion polymerization water, an emulsifier and/or a proteetive eolloid, and a water-soluble initiator. Most commonly, anionic long-chain alkyl sulfonates are used as surfactants in amounts up to 6%. Studies have shown that the rate of polymerization is dependent on the amoimt of emulsifier present, with the rates inereasing as the amoimt of emulsifier is increased up to a certain point and then falling olF as free-radieal ehain transfer to the surfaetant beeomes a serious competing side reaetion [240]. In general, surfactants are used in eombination with a protective colloid. Especially useful as protective colloids are poly(vinyl alcohol), hydroxyethyl cellulose, alkyl vinyl ether-maleic anhydride and styrene-allyl alcohol copolymers, and gum arable. Water-soluble initiators, particularly potassium persulfate, alkali peroxydisulfates, hydrogen peroxide, and various redox systems, are most commonly used. 

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