Metal‐Dependent Kinetic Control in Cationic‐Anionic Synchronous Ring‐Opening Polymerization

ABSTRACT Synchronous polymerization that combines mechanistically orthogonal pathways within a single catalytic system offers an attractive route to well‐defined block copolymers, yet remains challenging because it requires balancing electronically opposing reactions. Here, we systematically investigate how p ‐block metal chlorides regulate cationic–anionic synchronous ring‐opening polymerization (CAP) of 2‐oxazolines and cyclic esters. By comparing GaCl 3 , InCl 3 , SnCl 4 , SbCl 3 , and BiCl 3 , we found that synchronous copolymerization is broadly accessible across this series, demonstrating the generality and robustness of the CAP framework. In contrast to the overall feasibility of copolymer formation, the rate of oxazoline polymerization is highly sensitive to the identity of the metal center, leading to pronounced differences in propagation kinetics. Kinetic analyses, Lewis acidity measurements, and density functional theory calculations collectively indicate that metal‐dependent electronic interactions at the propagating chain end modulate oxazoline activation, whereas cyclic ester polymerization is comparatively less affected. These findings reveal that p ‐block metal chlorides primarily act as kinetic regulators in synchronous CAP systems. By decoupling polymerization feasibility from rate control, this work clarifies the role of metal identity in multi‐mechanistic polymerizations and provides a general strategy for tuning polymer growth without compromising architectural precision.