No abstract

For a deeper understanding of allyl polymerization mechanism, the reinitiation efficiency of resonance-stabilized monomeric allyl radical was pursued because in allyl polymerization it is commonly conceived that the monomeric allyl radical generated via the allylic hydrogen abstraction of growing polymer radical from monomer, i.e., ''degradative monomer chain transfer,'' has much less tendency to initiate a new polymer chain and, therefore, this monomer chain transfer is essentially a termination reaction. Based on the renewed allyl polymerization mechanism in our preceding article, the monomer chain transfer constant in the polymerization of allyl benzoate was estimated to be 2.7 Â 10 À2 at 80 C under the polymerization condition, where the coupling termination reaction of growing polymer radical with allyl radical was negligible and, concurrently, the reinitiation reaction of allyl radical was enhanced significantly. The reinitiation efficiencies of monomeric allyl radical were pursued by the dead-end polymerizations of allyl benzoate at 80, 105, and 130 C using a small amount of initiators; they increased remarkably with raised temperature. Thus, the enhanced reinitiation reactivity of allyl radical at an elevated temperature could bias the wellknown degradative monomer chain transfer characteristic of allyl polymerization toward the chain transfer in common vinyl polymerization.

Butyl vinyl ether (BVE) was polymerized radically, and the resultant oligomeric poly(BVE)s were characterized mainly using MALDI-TOF-MS spectrometry. For a further structural identification, the poly(BVE) obtained in bulk was subsequently subjected to 1 H NMR measurement. Notably, an intensive peak assignable to -CH 2 C(dO)CH 2protons was observed unexpectedly although a peak assignable to -CHO proton as the terminal end-group generated via β-scission of growing polymer radical as a presumed chain-end forming reaction appeared only weakly. Thus, a plausible chain-end forming reaction would be the intramolecular (backbiting) hydrogen abstraction to form mid-chain radical which subsequently undergoes β-scission resulting in oligomeric poly(BVE) with carbonyl group in the polymer backbone. Then, the specific polymerization mechanism involving β-scission of mid-chain radical yielding oligomers is discussed. The hydrogen abstraction ability of growing poly(BVE) radical was checked by using toluene as a solvent having labile hydrogen. The dependence of the degree of polymerization on initiator concentration was examined because the occurrence of primary radical termination was remarkable, probably leading to reduced chain length. Finally, the reinitiation reaction by butyl group generated via β-scission of mid-chain radical was pursued.

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.

On the course of a series of studies concerned with the elucidation of the allyl polymerization mechanism, the bulk polymerization of allyl benzoate was carried out using a large amount of 2,2′‐azobis(isobutyronitrile) (AIBN) and dimethyl 2,2′‐azobis (isobutyrate) (DMAIB). Incidentally, 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. This peculiar initiation behavior of azo‐initiators is discussed to clarify the initiation mechanism in allyl polymerization, especially focused on the competitive contribution to the initiation and termination reactions by primary radicals.