Nanogels and microgels: The new polymeric materials playground

Abstract

Microgels, or intramolecularly crosslinked macromolecules (ICMs), have been known for a number of years. They are formed during the polymerization of polyfunctional precursors en-route, but at incomplete reaction, to the macrogelation predicted by the theories of gelation initially propounded by Carothers and Flory. These gelation theories did not predict that such microgels made in solution could be prepared from such polyfunctional reactive solutions at complete reaction and high concentrations, without gelation. Microgels have been successfully and usefully prepared, however, at complete conversion of their contained reactive groups by the use of either aqueous or non-aqueous dispersion polymerization in which surface active agents are utilized to contain the polymerization to isolated submicron-size domains. The progression towards macrogelation is constrained to the maximum size of the dispersed domains. This paper demonstrates the contrasting general observation that macrogelation in solvents of such polyfunctional reactive precursors cannot occur below a Critical Gelation Concentration (CGC) if a carefully selected match of solvent solubility parameters for the polymerization solvent and the formed polymer is made. In some cases the addition of only ca. ten percent w/w of solvent is required to completely prevent macrogelation at complete conversion. In addition polymerization the molecular weight of the produced ICMs may be varied in a simple controllable manner from the low thousands (nanogels) to many millions (microgels) simply by the choice of concentration at which they are prepared. The products are a distinct form of polymer which is quite different to the linear analogue and almost certainly these solution-prepared products are different to the microgels formed by aqueous dispersion polymerization techniques. This new solution technique provides a simple and general new method for the ready synthesis and design of an enormous range of bespoke globular polymers having both fundamental academic interest and potential commercial utility.