Predominant methyl radical initiation preceded by β-scission of alkoxy radicals in allyl polymerization with organic peroxide initiators at elevated temperatures

Abstract

Allyl monomers polymerize with difficulty and their polymerization yields polymers of medium molecular weight or oligomers. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry is a powerful tool for structural identification of oligomers; therefore, we aimed to use it to collect direct evidence for a full mechanistic discussion, including initiation and termination reactions. In the course of our investigation, which was concerned with the temperature dependence of the reinitiation reactivity of resonance-stabilized monomeric allyl radicals and was especially focused on polymerization behavior at elevated temperatures, we found by chance that any direct contribution of cumyloxy radicals to the initiation reaction was not observed in the polymerization of allyl benzoate with dicumyl peroxide (a typical peroxide initiator) at elevated temperatures. However, a methyl radical initiation preceded by b-scission of cumyloxy radicals predominantly occurred. This finding was extended to the generalization of an initiation reaction mechanism in allyl polymerization with organic peroxide initiators at elevated temperatures.

Peculiar initiation behavior of azo‐initiators in allyl polymerization

“…To reassess the allyl polymerization mechanism, the resulting oligomers were characterized by MALDI‐TOF‐MS spectrometry; we have proposed an improvement of the well‐known reaction scheme for the free‐radical polymerization of allyl monomers 10. On the course of a series of our studies concerned with the elucidation of allyl polymerization mechanism,10, 13–15 we used azo‐initiators such as AIBN and DMAIB in place of BPO as a most typical peroxide initiator in allyl polymerization. Incidentally, we found that the initial rate of polymerization initiated by DMAIB was more than two times higher than that by AIBN, although the decomposition rate constants of AIBN and DMAIB are almost same at 80 °C.…”

“…On the course of a series of our studies concerned with the elucidation of allyl polymerization mechanism,10, 13–15 we used azo‐initiators such as 2,2′‐azobis(isobutyronitrile) (AIBN) and dimethyl 2,2′‐azobis(isobutyrate) (DMAIB) (Fig. 1) in place of BPO as a most typical peroxide initiator.…”

Skewered reactions in free‐radical crosslinking (co)polymerizations of liquid polybutadiene as internal olefinic multivinyl crosslinker

“…The primary cumyloxy radical may add to CC double bond or abstract allylic hydrogen or undergo β‐scission to generate methyl radical and acetophenone via eq 2. Recently, we found by chance that any direct contribution of cumyloxy radical to the initiation was not observed in the bulk polymerization of ABz with DCPO at elevated temperatures 20. So, in the polymerization of LBR, we may omit the possibility of the initiation by the cumyloxy radical because the reactivity of internal olefinic CC double bond in the LBR backbone could be much lower compared with that of ABz containing α‐olefinic CC double bond.…”

Synthesis of novel copolymers obtained from acceptor/donor radical copolymerization of phosphonated allyl monomers and maleic anhydride

“…It is widely recognized that conventional radical homopolymerization of allyl monomers usually leads to low molecular weights oligomers (DP n less than 50 units); this behavior is generally ascribed to chain transfer process occurring onto the methylene group of allyl monomers, then limiting the propagation reaction 14, 15, 24. As already mentioned in the introduction part, allyl monomers are good electron‐donating monomers, and their radical copolymerizations with electron‐accepting monomers mainly afford alternated copolymers.…”

“…We also showed that, depending on both the reaction conditions and of the R substituent, the resulting polymeric species could have either hyperbranched (with high M w values) or linear structure (with low M w values). Indeed, it is well‐known that the radical homopolymerization of allyl monomers usually results in low M w oligomers, mainly due to degradative chain transfer 13–15. The weakness of the allyl CH bond arises from the high resonance stability of the allyl radical, which is too stable to reinitiate polymerization and undergoes termination by reaction with a propagating radical 11…”