Application of spin trapping technique to radical polymerization, 14. Initiation reaction of vinyl monomers with the tert-butoxyl radical and evaluation of monomer reactivities†

The initiation of various vinyl monomers with tert-butoxyl radicals derived from the decomposition of di-tert-butyl peroxalate (DBPOX) was investigated by using the spin trapping technique with 2-methyl-2-nitrosopropane (BNO), to detect and identify the intermediate radical species, and to evaluate the monomer reactivities toward the tert-butoxyl radical. When the reaction of DBPOX with vinyl monomers, which have no α-methyl group (CH2CH(X)), was carried out in the presence of BNO at room temperature, the propagating radical 1 ((H3C)3COCH2CHX) was trapped as its nitroxyl radical. However, in the cases of using α-methylvinyl monomers (CH2CH(X)CH3), such as methyl methacrylate and methacrylonitrile, two kinds of radicals, 6 ((H3C)3COCH2C(X)(CH3), similar to 1) and 8 (CH2CH(X)CH2), were trapped as corresponding nitroxyl radicals. The coupling constants of the nitroxyls obtained from 1 and 6 were determined and discussed on the basis of their structure. When these reactions were performed in a reference solvent, p-xylene, the above radicals, 1 (or 6) and/or 8, together with the p-tolylmethyl radical (2), were trapped as their nitroxyls. From the ESR peak areas the relative reactivities of vinyl monomers toward the tert-butoxyl radical were evaluated and found to depend not only on the polar nature, but also on the resonance effect of the substituents. In α-methylvinyl monomers such as methyl methacrylate, the hydrogen-abstraction from the α-methyl group by the tert-butoxyl radical was important.