Living Group Transfer Polymerization of Renewable α-Methylene-γ-butyrolactones Using Al(C6F5)3 Catalyst
2018
Hu, Lu | He, Jianghua | Zhang, Yuetao | Chen, Eugene Y.-X.
Here we report the room-temperature group transfer polymerization of conjugated polar alkenes, including linear methyl methacrylate (MMA) as well as biorenewable, cyclic γ-methyl-α-methylene-γ-butyrolactone (MMBL) and α-methylene-γ-butyrolactone (MBL), by the silyl ketene acetal (SKA)/Al(C₆F₅)₃ ([Al]) system and the detailed study of its polymerization mechanism. The polymerization of MMA by SKA/[Al] was uncontrolled, while the MMBL polymerization by the bulky SKA (ⁱᴮᵘSKA)/[Al] system is living and thus produces well-defined PMMBL with a predicted molecular weight (Mₙ up to 179 kg mol–¹), a narrow molecular weight distribution (Đ as low as 1.02), and a high initiation efficiency (I* ≥ 97%). The living polymerization of MMBL was established through five lines of evidence, including predictable polymer Mₙ and low Đ values, a linear increase of polymer Mₙ vs monomer conversion, a linear increase of polymer Mₙ vs monomer-to-initiator ratio, chain extension experiments, and synthesis of well-defined random, diblock, and triblock copolymers. A combined mechanistic study through isolation and characterization of single-monomer-addition intermediates that simulate the active propagating species, polymerization kinetics, and characterization of polymer chain ends has led to a polymerization mechanism. The polymerization is initiated via intermolecular Michael addition of the SKA enolate group to the vinyl group of the [Al]-activated monomer, while the silyl group is transferred to the carbonyl group of the monomer and [Al] to the oxygen atom of SKA; the coordinated [Al] is released to the incoming monomer, followed by repeated intermolecular Michael additions in the subsequent propagation cycle.
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