Comprehensive Summary Borane catalysis, pioneered by BEt₃, is a powerful method for the ring‐opening polymerization (ROP) of epoxides and (co)polymerization of epoxides to generate diverse kinds of polymers. Among these systems, pre‐organized dinuclear boranes are highly effective due to cooperative effects. However, current examples are limited to ammonium, phosphonium, and silicon‐based structures, restricting the exploration of their structure‐performance relationships. Herein, malonate esters were used as precursors to synthesize preorganized dinuclear boron catalysts QC‐B1 and QC‐B2 . These catalysts were combined with tetrabutylammonium carbonate, and the resulting binary systems were investigated for their application in the ROP of propylene oxide (PO). The QC‐B1 /TBAC exhibited living and controllable characteristics in the ROP of PO and showed good tolerance toward chain transfer agents (CTAs), with one boron center tolerating up to 50 equivalents of CTA. Kinetic studies show the polymerization mechanism depends on the initiator: a mono‐initiator leads to intramolecular chain growth, while a dual‐initiator favors an intermolecular pathway.
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