Abstract
This work reveals the silyl ketene acetal (SKA)/B(C6F5)3 Lewis pair-catalyzed room-temperature group transfer polymerization (GTP) of polar acrylic monomers, including methyl linear methacrylate (MMA), and the biorenewable cyclic monomers γ-methyl-α-methylene-γ-butyrolactone (MMBL) and α-methylene-γ-butyrolactone (MBL) as well. The in situ NMR monitored reaction of SKA with B(C6F5)3 indicated the formation of Frustrated Lewis Pairs (FLPs), although it is sluggish for MMA polymerization, such a FLP system exhibits highly activity and living GTP of MMBL and MBL. Detailed investigations, including the characterization of key reaction intermediates, polymerization kinetics and polymer structures have led to a polymerization mechanism, in which the polymerization is initiated with an intermolecular Michael addition of the ester enolate group of SKA to the vinyl group of B(C6F5)3-activated monomer, while the silyl group is transferred to the carbonyl group of the B(C6F5)3-activated monomer to generate the single-monomer-addition species or the active propagating species; the coordinated B(C6F5)3 is released to the incoming monomer, followed by repeated intermolecular Michael additions in the subsequent propagation cycle. Such neutral SKA analogues are the real active species for the polymerization and are retained in the whole process as confirmed by experimental data and the chain-end analysis by matrix-assisted laser desorption/ionization time of flight mass spectroscopy (MALDI-TOF MS). Moreover, using this method, we have successfully synthesized well-defined PMMBL-b-PMBL, PMMBL-b-PMBL-b-PMMBL and random copolymers with the predicated molecular weights (Mn) and narrow molecular weight distribution (MWD).
Highlights
Lewis pair (LP) polymerization has emerged and attracted intense investigations of the cooperative catalytic effects of Lewis acid (LA) and Lewis base (LB) pairs on the polymerization of conjugated polar alkenes [1,2,3,4,5,6,7,8], since the seminal works on Frustrated Lewis Pairs (FLPs) by Stephan and Erker in small molecule chemistry [9,10,11,12]
The cooperativity of the LA and LB sites of Lewis pairs is essential to achieve an effective polymerization system, which was demonstrated by the borane/phosphine LPs that showed that interacting LPs, and even classical Lewis adducts (CLAs), Molecules 2018, 23, 665; doi:10.3390/molecules23030665
S16) with that for B(C6F5)3·methyl methacrylate (MMA) adduct (Figure S8), we proposed the structure for corresponding dimeric silyl ketene acetal (SKA) analogues as shown in Scheme 2
Summary
Lewis pair (LP) polymerization has emerged and attracted intense investigations of the cooperative (or synergistic) catalytic effects of Lewis acid (LA) and Lewis base (LB) pairs on the polymerization of conjugated polar alkenes [1,2,3,4,5,6,7,8], since the seminal works on Frustrated Lewis Pairs (FLPs) by Stephan and Erker in small molecule chemistry [9,10,11,12]. LPs based on strongly acidic, bulky E(C6 F5 ) (E = B, Al) LAs and bulky LBs including phosphines and N-heterocyclic carbenes (NHCs) have been employed to initiate rapid addition polymerization of conjugated polar vinyl monomers such as methyl methacrylate (MMA), cyclic and the naturally renewable monomers α-methylene-γ-butyrolactone (MBL), and γ-methyl-α-methylene-γ-butyrolactone (MMBL) [13,14,15] and monomers bearing the C=C–C=N functionality such as 2-vinylpyridine and 2-isopropenyl-2-oxazoline as well [16,17] In such polymerizations, the cooperativity of the LA and LB sites of Lewis pairs is essential to achieve an effective polymerization system, which was demonstrated by the borane/phosphine LPs that showed that interacting LPs, and even classical Lewis adducts (CLAs), Molecules 2018, 23, 665; doi:10.3390/molecules23030665 www.mdpi.com/journal/molecules.
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