An Associative Pathway Enables Highly Controlled Ring‐Opening Polymerization of Cyclosiloxane with ppm‐Level Organocatalyst

ABSTRACT Precise control over molar mass and structure of polysiloxane is critical for its applications. Here, we report ring‐opening polymerization (ROP) of cyclotrisiloxane using anionized 1,3‐diphenylurea (UA1) as the catalyst. With a bulky tetra‐ n ‐butylammonium counterion, UA1 shows significantly higher activity than the one with an alkali metal counterion. Almost 100% monomer conversion can be reached in minutes at catalyst loadings of 5–100 ppm (TOF up to 52,000 h −1 for D 3 ). The obtained polydimethylsiloxane has well‐defined structure, controlled molar mass (2 to 140 kg mol −1 ), and low dispersity (1.06–1.18). Calculations reveal that both silanol species and the monomer are hydrogen‐bonded with the catalyst. The dominance of this associative pathway facilitates ring opening and prevents the formation of overly active (dissociated) silanolate anion, thus ensuring high efficiency and selectivity. The catalyst is also applicable to the ROP of non‐strained cyclotetrasiloxane (D 4 ), with molar mass reaching as high as 1,660 kg mol −1 . This study paves the route for efficient and controlled synthesis of polysiloxane materials using minimum amount of simple organocatalyst.