Taking a Step Back: An Attempt at Rationalizing Anionic and Cationic Ring-Opening Polymerization of Cyclosiloxanes in Miniemulsion

The ionic polymerization of cyclosiloxanes in aqueous media has long been described, first by industry and, more recently, by academic researchers worldwide. The simplest way to perform such polymerization is to use a surfactant that can bring the initiator to the interface. The present article aims to close the loop opened 30 years ago on the mechanism of ionic polymerization of D4 (and, by extension, other functional cyclosiloxanes) in miniemulsion, independent of the catalyst used (cationic or anionic). To do so, I first present experimental works on the processes of D4 cationic polymerization and D5 anionic polymerization in miniemulsion, focusing on the evolution of molar masses and the formation of cyclic species as a function of conversion at different temperatures. In the following discussion, I complement the mechanism previously described for the anionic polymerization of cyclosiloxanes in miniemulsion in light of these additional results. Both anionic and cationic processes exhibit a similar reactive pattern when the polycondensation/hydrolysis equilibrium is considered, which is primarily determined by the water content inside the droplets. Open questions, such as the competition between propagation and backbiting reactions, or the possibility of reactions in water, are also addressed. These findings pave the way to better-controlled ROP in aqueous emulsions, particularly in more complex conditions, such as in emulsions or starved-feed processes.