Initiation Mechanism of Alternating Copolymerization of Styrene with Some Electron-Accepting Monomers in the Presence of Zinc Chloride by Means of Spin Trapping Technique

Abstract

The initiation mechanism of spontaneous alternating copolymerizations of styrene (St) and some electron-accepting monomers such as methyl methacrylate (MMA), methyl acrylate (MA), methacrylonitrile (MAN), and acrylonitrile (AN) in the presence of ZnCl₂ was studied by the spin trapping technique, in which 2-methyl-2-nitrosopropane (BNO) was used as a spin trapping reagent. When this technique was applied to the alternating copolymerization systems of St-MMA-ZnCl₂, St-MA-ZnCl₂, and St-MAN-ZnCl₂, the 2-phenylvinyl radical (·CHdbnd]CHsbnd]C₆H₅) was trapped as nitroxide. The structure of this nitroxide, which showed a large coupling constant (19～20 G) by β-hydrogen, was confirmed by comparison with the result of authentic experiment Accordingly it was concluded that this nitroxide was formed through proton migration from the St cation radical to the acceptor monomer anion radical in the charge- or electron-transfer complex, followed by reaction with BNO.

In the St-AN-ZnCl₂ system, however, a nitroxide derived from a cyclic radical was observed together with the nitroxide from 2-phenylvinyl radical. This cyclic radical seemed to be produced via the Diels-Alder adduct between St and AN.