Polymerization in the crystalline state. I. Acrylamide

Abstract

AbstractCrystalline acrylamide was irradiated at −78°C. with γ‐rays and the postpolymerization of the irradiated crystals was studied in the temperature range 0–60°C. The polymerization rate decays rapidly at low conversion, and the polymer yield is, for long polymerization times, linear in the logarithm of time. The activation energy for the polymerization is 25 kcal./mole. The molecular weight of the polymer depends at a given polymerization temperature on the polymerization time, but remains unchanged over a sixteenfold variation of the irradiation dose. The number of polymer chains, as deduced from the ratio of polymer yield and molecular weight, increases only slightly with conversion, probably due to chain transfer to monomer. The number of chains calculated in this way is in satisfactory agreement with the number of radicals estimated from electron spin resonance. The ESR signal changes very little during the postpolymerization. The presence of oxygen has little effect on the polymerization. Molten and shock‐cooled monomer yields about twice as much polymer as large slowly grown crystals, but the molecular weight of the product is the same, implying that crystal imperfections increase slightly the chain initiation efficiency, but are without effect on chain propagation. Propionamide is isomorphous with acrylamide and forms with it solid solutions over the entire concentration range. The polymerization of acrylamide in solid solutions containing up to 10% propionamide proceeds at a rate indistinguishable from that for pure acrylamide; at higher propionamide concentrations the rate falls off less rapidly than the concentration of the monomer. The molecular weights of polymers obtained from solid solutions approach a limiting value for long polymerization times which is determined by chain transfer to propionamide. Up to 10% propionamide concentration, the limiting polymer chain length is inversely proportional to the propionamide concentration and corresponds to a chain transfer coefficient of 0.022 at 25°C. and 0.0064 at 60°C. The high value of the chain transfer coefficient at 25°C. is ascribed to the directing influence of the crystal lattice which makes it difficult for a chain to “grow around” a site occupied by a propionamide molecule. The gradual loosening of the lattice restraints at higher temperatures explains the decrease in the chain transfer at higher temperatures. Acetamide is not isomorphous with acrylamide, but a limited amount of acetamide can be incorporated into the acrylamide lattice; the polymerization of such solid solutions exhibits, however, no chain transfer. X‐ray diffraction studies indicate that partially polymerized acrylamide contains separate monomer and polymer phases. Since the chain transfer behavior of propionamide and other evidence points to the importance of lattice control in solid‐state polymerization, it is suggested that the polymer chains grow at a monomer—polymer interface with the growing end anchored in the crystal lattice of the monomer.