Abstract: Using a TM₁₁₀ cavity resonator and a 2mm sample tube,ESR spectra of PMMA propagating radical in bulk polymerization process were successfully recorded at room temperature (17℃). As soon as dimethyl aniline was mixed with MMA monomer and benzoyl peroxide, 0.17 ml of the mixture was quickly transferred into the 2mm sample tube with a syringe and the spectra were recorded every 2 minutes. The 13(8 + 5) line spectrum exhibited at the beginning stage gradually changed to 9 (4 + 5) line spectrum as the polymerization proceeded. These spectra are explained by hindered oscillation model, and/or overlapping conformation model. The former seems to be more reasonable.
Our ESR data show that the concentration of the propagating radical keeps almost constant at the beginning stage and increases abruptly at a certain time in the middle stage. This characteristic time exactly corresponds to the onset of the autoacceleration process and th variation of the radical concentration is synchronous with the variation of the monomer conversion. Finally, the weak satellite lines due to concurrent flips of an electron and a nearby nucleus were observed by differential saturation technique. It is verified that the dominant electron spin-lattice relaxation mechanism arises from the angular modulation of the dipolar coupling to lattice protons via torsional motion of the free radical around its equilibrium position.

Key words: PMMA, Polymerization Process, Propagating Radical, ESR