Anomalous solubility of polyacrylamide prepared by dispersion (precipitation) polymerization in aqueous tert-butyl alcohol

Polyacrylamide prepared by dispersion (precipitation) polymerization in an aqueous t-butyl alcohol (TBA) medium is only partially soluble when the TBA concentrations in the polymerization media are in the range 82 Vol % < TBA < 95 vol %. Independent experiments with a soluble (linear) sample of polyacrylamide show that the polymer swells sufficiently in the aforementioned media to lower the glass-transition temperature of the polymer below the polymerization temperature (50 degreesC). The anomalous solubility has been attributed to the crosslinking of polymer chains that occurs during the solid-phase polymerization of acrylamide in the swollen polymer particles. It is postulated that some of the radical centers shift from the chain end to the chain backbone during solid-phase polymerization by chain transfer to neighboring polymer molecules, and when pairs of such radicals come into close vicinity, crosslinking occurs. However, dispersion (precipitation) polymerization in other media such as aqueous methanol and aqueous acetone yields polymers that are soluble. This result has been attributed to the fact that the polymer radical undergoes a chain-transfer reaction with these solvents at a much faster rate than with TBA, which overcomes the effect of the polymer-transfer reaction. Even the addition of as little as 5% methanol to a TBA-water mixture (TBA:water = 85:10) gives rise to a soluble polymer. The chain-transfer constants for acetone, methanol, and TBA have been determined to be 9.0 X 10(-6), 6.9 X 10(-6), and 1.48 X 10(-6), respectively, at 50 degreesC. (C) 2001 John Wiley & Sons, Inc.