Localization of Critical Molar Ratio Intervals for Highly Branched Step-Growth Michael Addition Networks

Experimental stoichiometric gelation boundaries of a set of branched thiol to acrylate Michael addition polymerization systems were investigated, and the observed limits were compared to theoretical critical molar ratio (CMR) intervals. These CMR intervals were calculated via the Flory–Stockmayer equation (for the upper limit) and via a formula derived from the Carothers equation (for the lower limit). The method provides a simple procedure to verify the existence of a step-growth mechanism in cross-linked polymerizations, and it is also applicable to monomers with functionality distributions. It is shown that the investigated Michael addition systems have gelation boundary values which are consistent with a predominant step-growth mechanism.