MICROGELS .4. THE PREPARATION OF NOVEL MICROGELS AND THEIR APPLICATIONS

This paper describes studies on the formation of microgels from high f unctionality mixtures of monomers which are able to polymerise by a st ep-growth process to polyurethanes in the specially selected solvent b utan-2-one. Using the reactants poly(ethylene glycol) 6000, 1,2,6-hexa netriol and biscyclohexylmethane-4,4'-diisocyanate in formulations mad e to comprise exactly one isocyanate group per hydroxyl, it was found that microgels of number average molecular weight, as determined by Ge l Permeation Chromatography,of less than 20,000 were obtained up to th e Critical Gelation Concentration which ranged from w/w reactant to so lvent concentrations of from similar to 0.24 to 0.74 at PEG/HT ratios of 1:4 to 4:4 respectively. These polymerisations surprisingly, went t o complete conversion without gelation and produced 95 - 99% yields of microgel product after precipitation into a non-solvent. Intrinsic vi scosity determinations on the microgels dissolved in chloroform were d etermined and used to obtain estimates of the swollen coil densities a t infinite dilution from which the critical packing concentration of t hese swollen coils in the solvent used for the solvent could be estima ted. The critical packing concentration was in all cases well below th e Critical Celation Concentration, It was thus clearly demonstrated th at the formation of ''soluble'' microgels is progressing in spite of t he system being critically packed. This is a surprising observation an d especially so when it is combined with the very low measured weight average molecular weights and high conversions of the products in the critically packed region. The GPC molecular weights decreased at the h igher concentrations which is contrary to expectations. The ability to form microgels above critical packing confirms the results obtained a nd reported previously by us, that a similar phenomenom occurs in the case of the polymerisation of polyfunctional free-radically polymerise d monomer systems. These observations will need to be taken into accou nt when formulating mathematical analyses of the gelation process. Thi s class of products are able to be crystallised from organic solvents and are soluble in water. They are potentially useful as components of pharmaceutical formulations for the controlled release of drugs, high molecular weight active materials and living microorganisms.