Ring-opening polymerization (ROP) is a versatile approach to well-defined polymers. Achieving both high efficiency and precise control remains a major challenge. We herein report room-temperature living ROP of epoxides catalyzed by combined organobase and triethylborane (Et 3 B) with water or alcohols as initiators. Extremely high catalytic efficiency (turnover frequency up to 6000 h −1 ) is exhibited despite the relatively mild Lewis basicity and acidity, and the catalyst loading can be minimized to 44 ppm. Poly(ethylene oxide) products are noncytotoxic toward a variety of cell lines even without purification. Poly(propylene oxide) with molar mass as high as 210 kg mol −1 and low dispersity (Đ M < 1.1) can be achieved owing to the absence of detrimental transfer reactions. The chemoselectivity allows the catalyst to be readily applied to living ROP of glycidyl ethers, tailored synthesis of diverse functional/block/nonlinear (co)polyether structures, and one-pot one-catalyst synthesis of polyether-based polyurethane. Our study reveals the effective cooperation modes of the Lewis pair and hydroxyl, including the reversible formation of threecomponent initiation/propagation center with appropriate basicity, activation of epoxide by uncomplexed Et 3 B, and fast activity exchange between dormant and active hydroxy species.

Polymerizing epoxides after cyclic esters remains a major challenge, though their block copolymers have been extensively studied and used for decades. Reported here is a simple catalytic approach based on a metal‐free Lewis pair that addresses the challenge. When the Lewis acid is used in excess of a base, selective (transesterification‐free) polymerization of epoxides occurs in the presence of esters, while selectivity toward cyclic esters is achieved by an oppositely biased catalyst. Hence, one‐pot block copolymerization can be performed in both ester‐first and ether‐first orders with selectivity being switchable at any stage, yielding ether‐ester‐type block copolymers with unlimited ordering of sequences as well as widely variable compositions and architectures. The selectivity can also be switched back and forth several times to generate a multiblock copolymer. Experimental and calculational results indicate that the selectivity originates mainly from the state of catalyst‐activated hydroxy species.