Continuous Precipitation Polymerization of Acrylic Acid in Supercritical Carbon Dioxide

Abstract

The precipitation polymerization of acrylic acid in supercritical carbon dioxide (scCO2) was carried out in a continuous stirred tank reactor. The product polymer was a white, dry, fine powder that dissolved in water. A wide range of polymer molecular weights (5 to 200 kg/mol) was obtained. The effect of the operating variables on the polymerization rate and on the polymer molecular weight was evaluated. The polymerization showed distinct deviations from the normal solution polymerization. By assuming that chain initiation occurs in the solution phase, but chain propagation and chain termination occur in the polymer phase, a 'surface polymerization model' and a 'particle polymerization model' both described the polymerization well.

Scanning electron micrographs showed that three types of polymer particles were obtained: agglomerates of primary particles of about 100 nanometers in size, irregular particles of 5—20 micrometers, and spheres of 10—100 micrometers. It is speculated that the agglomerates were produced when the polymerization temperature (TP) was below the polymer glass transition temperature (Tg), the irregular particles were obtained when TP was close to Tg, and the spheres were prepared when TP was above Tg. The CO2 absorption into poly(acrylic acid) (PAA) was measured with a quartz crystal microbalance. The Tg depression by scCO2 was calculated with the Chow's equation. The calculated results lent strong support to the proposed particle formation mechanism.

Cross-linking polymerization of acrylic acid in scCO2 was studied in a batch reactor at 50°C and 207 bar. All products were white, dry, fine powders. By adjusting the cross-linker concentration, water-soluble and water-insoluble PAAs were synthesized. The water-insoluble PAA was neutralized by ammonia gas and sodium hydroxide alcohol solution to make superabsorbent polymers.