Ring opening polymerization of cyclic oligosiloxanes without producing cyclic oligomers
Résumé
A long-standing problem associated with silicone synthesis is contamination of the polymer products with 10 to 15% cyclic oligosiloxanes that result from backbiting reactions at the polymer chain ends. This process, in competition with chain propagation via ring-opening polymerization (ROP) of cyclic monomers, was thought to be unavoidable, and routinely leads to a thermodynamically controlled reaction mixture (polymer/cyclic oligosiloxanes = 85/15). Here we report that simple alcohol coordination to the anionic chain ends prevents the backbiting process and that a well-designed phosphonium cation acts as a self-quenching system in response to loss of coordinating alcohols to stop the reaction before the backbiting process begins. The combination of both effects allows a thermodynamically controlled ROP of the 8-membered siloxane ring D 4 without producing undesirable cyclic oligosiloxanes.
Domaines
Chimie
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