Microgels .2. Solution polymerization using a urethane stepgrowth mechanism

Microgels are defined as intramolecularly crosslinked macromolecules. They have in common with crosslinked polymers a three-dimensional structure and fixed surfaces but are still soluble in certain solvents like most linear or branched polymers. A general hypothesis by which microgels would be expected to be formed from polyfunctional polymerizable monomers in good solvents, and at complete conversion to polymer bur without macrogelation, has been previously proposed and demonstrated for radical addition polymerization (Graham and Hayes, Macromolecular Symposium, Polymer Networks 94, 93 (1995) 293). This paper extends the generality of the practical demonstration of this hypothesis by experimental extension to stepgrowth polymerization. The polyurethane microgels of this study were synthesized using stoichiometric amounts of poly(ethylene glycol) of molecular weights ranging from 3800 to 8000, a trifunctional hydroxylic crosslinking agent, trimethylolpropane, and a diisocyanate, diphenylmethane-4,4'-diisocyanate (MDI). The polymerization was done in various solvents. It was found that in every solvent/monomer system there is a critical monomer concentration below which no macrogelation occurred but microgels were formed. The monomer concentration and the solubility parameter of the solvent were found to be the two most important factors influencing the formation of microgel or macrogel. Auto-steric stabilization, is believed to have played an important role in the formation of small microgel particles and in preventing macrogelation.