Insertion Ring-Opening Polymerization (iROP): An Extended Zwitterionic ROP Applicable for the Efficient Synthesis of Cyclic-Oligomer-Free Silicones

Abstract

Zwitterionic ring-opening polymerization (zROP) is a polymerization method in which a zwitterionic intermediate, generated by the addition of a neutral organic base to a cyclic monomer, initiates a ring-opening polymerization process (ROP). This process often ends with the recombination of the oppositely charged chain ends to form a cyclic polymer, which opened the way for the development of metal-free cyclic polymer synthesis. Here, we present an alternative pathway of zROP, insertion ROP (iROP), in which propagation proceeds through successive insertions of the zwitterionic intermediate into the middle of a polymer chain, thus enabling the extension of linear polymers without active chain ends. This iROP process was demonstrated by the polymerization of octamethylcyclotetrasiloxane (D4), catalyzed by N-heterocyclic olefins (NHO), allowing the direct synthesis of cyclic-oligomer-free polydimethylsiloxanes (>99.9%). Experimental and theoretical mechanistic studies indicate that the polymerization proceeds via a hybrid process that starts with conventional ROP, followed by iROP, without the involvement of SiOH chain ends and, thus, without backbiting reactions. The transition of ROP/iROP states is controlled by polycondensation reactions that exclude terminal SiOH groups from the growing polymers. In the case of the hexamethylated NHO catalyst, the complete ROP of D4 was achieved with an extremely low catalyst loading of 50 ppm.