A Paradigm for BF3 co-initiated Cationic Polymerization with Low Dispersity: Introducing Acetonitrile and Lowering the Reaction Temperature

Non-metallic cationic polymerization of low-reactivity styrene (St) derivatives remains challenging due to broad molecular weight distributions and rapid deactivation of active species. Here we show that adding only a minor amount of CH 3 CN ( V/V , 3/97) to the conventional BF 3 ·OEt 2 /COH/CH 2 Cl 2 /St system affords polystyrene with narrow dispersity ( M w / M n , Đ ≈ 1.3). Notably, the M n increases linearly with conversion and closely matches the M n.calcd . 1 H-nuclear magnetic resonance ( 1 H-NMR) analysis and comparative experiments with the BF 3 ·CH 3 CN co-initiation system indicate CH 3 CN does not coordinate strongly with BF 3 compared to diethyl ether (OEt 2 ); however, it stabilizes the propagating carbocation and decreases the the propagation rate constant ( k p ), which leads to a narrower dispersity in the product. Lowering the temperature to −25 °C further extends the active-chain lifetime. This strategy is also effective for other low-reactivity styrene monomers, including p -chlorostyrene ( p ClSt) and p -chloromethylstyrene ( p CMSt), and is even applicable to the relatively more reactive p -methylstyrene ( p MeSt), offering a versatile metal-free route to the synthesis of styrenic polymers with a narrow dispersity.