IDENTIFICATION OF ESR-SPECTRA OF THE PROPAGATING RADICAL FOR POLYSTYRENE AND THEIR CHANGES WITH MEDIUM ENVIRONMENT

Reported here is the study on various types of ESR spectra of the prop agating radical in polystyrene. Spectrum simulation, analysis of the h yperfine coupling constant, and calculations of the conformation energ y show that the propagating radical of styrene has two stable inner-ro tation conformations R1 and R2. R1 corresponds to a B-line spectrum wh ich, in some special conditions, can appear as a 4-line or 3-line spec trum. R2 corresponds to an 8-line spectrum which, in some special cond itions, appears as a 6-line spectrum. All the ESR spectra observed in the polymerization process can be regarded as the superposition of som e of these spectra. The concentration ratio of R1 to R2 is affected by rotational interchange between R1 and R2, as well as the propagating reaction. The rotational interchange between R1 and R2 favors the form ation of R1, and the propagating reaction favors the formation of R2. At the beginning stage of polymerization (low monomer inversion stage) , the rotational interchange between R1 and R2 is dominated and the va lue of the concentration ratio of R1 to R2 is consistent with the one given by the Boltzmann distribution. However, in the middle to end sta ge of the reaction (high monomer inversion stage), the propagating rea ction dominates. Therefore, R2 is the major conformer at last. Further more, when the radical is in the solid matrix in which the monomer has almost been exhausted and the propagating reaction is close to stoppi ng, the rotational interchange between R1 and R2 dominates again. Ther efore R1 returns as the major conformer in the solid matrix.