Polymerization in the crystalline state. II. Alkali acrylates and methacrylates

Abstract

Abstract Alkali acrylates and methacrylates were irradiated with gamma rays from a Co 60 source at −78°C. and their subsequent polymerization was studied in vacuo in the temperature range 0 to 155°C. Potassium acrylate proved to be the most rapidly polymerizing solid monomer studied to date; it reacted at 0°C. faster than the lithium salt at 101°C. or the sodium salt at 120°C. The reaction rate of the acrylates decayed rapidly at low polymer yields which were, for long polymerization times, linear in the logarithm of time. The activation energy for the polymerization of potassium acrylate was 16.7 kcal./mole. The chain length of the polymer was independent of the polymer yields in all kinetic runs except one and was an order of magnitude higher for potassium acrylate than for the sodium or the lithium salt. Sodium methacrylate was more reactive than potassium methacrylate; the lithium salt of methacrylic acid was entirely inert. The sodium methacrylate polymerization had an initial accelerating phase. Studies of x‐ray diffraction of the polymerizing potassium acrylate showed only minor changes in the diffraction pattern of the monomer, the size of the unit cell remaining almost unchanged. Poly(potassium acrylate) produced in the solid state showed four diffuse diffraction rings which were not observed in samples prepared from poly(acrylic acid) obtained by polymerizing a benzene solution of the monomer. While the x‐ray data indicated the existence of a separate polymer phase in the polymerizing salts, the pronounced difference in the characteristics of the polymerization process of different salts of the same monomeric anion pointed to a control of the reaction by the geometry of the crystal lattice. The evidence suggests that the polymer lies in an amorphous phase but that the reactive ends of growing chains are anchored in the monomer lattice. Electron spin resonance spectra are similar for irradiated potassium acrylate and sodium methacrylate, but the potassium acrylate spectrum changes gradually from a three‐peak to a single‐peak structure when the sample is heated to 100°C., while the other two salts have spectra which are not affected by a similar heating cycle. The absence of the characteristic ESR spectrum of polymethacrylate radicals in irradiated sodium methacrylate suggests that under the experimental conditions used few of the primary radicals were able to initiate chains. A five‐peak ESR spectrum was observed in irradiated sodium acrylate.