Silyl Ketene Acetals/B(C6F5)3 Lewis Pair-Catalyzed Living Group Transfer Polymerization of Renewable Cyclic Acrylic Monomers

Lu Hu, Wuchao Zhao, Jianghua He, Yuetao Zhang
2018 Molecules  
This work reveals the silyl ketene acetal (SKA)/B(C 6 F 5 ) 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(C 6 F 5 ) 3 indicated the formation of Frustrated Lewis Pairs (FLPs), although it is sluggish for MMA
more » ... r 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(C 6 F 5 ) 3 -activated monomer, while the silyl group is transferred to the carbonyl group of the B(C 6 F 5 ) 3 -activated monomer to generate the single-monomer-addition species or the active propagating species; the coordinated B(C 6 F 5 ) 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 (M n ) and narrow molecular weight distribution (MWD). can be highly active for the polymerization [18] . This was further nicely demonstrated by Rieger and co-workers, showing the high activity and high degree of control over the polymerization of Michael-type and extended Michael monomer systems by the highly interacting organoaluminum and phosphine LPs [19] . Extending beyond the commonly employed NHC and phosphine LBs, in 2014, Lu and co-workers first reported the N-heterocyclic olefin-based LPs for the polymerization of acrylamides, (meth)acrylates and disymmetric divinyl polar monomers as well [20, 21] . Although FLPs or CLAs exhibited high activity for polymerization of conjugated polar alkenes, the application of such polymerization is hampered by both low initiation efficiencies and chain-termination side reactions [18, 21] , evidenced by the much higher observed M n than the calculated M n and broad MWD of the resulting polymers (high Ð values), thus giving rise to low initiation efficiencies (I*) and rendering the inability to produce well-defined block copolymers. Based on the above facts, it is really difficult to achieve LP-catalyzed living/controlled polymerization of polar acrylic monomers. More recently, Taton and co-workers reported the direct employment of organic LPs based on phosphine and Me 3 SiNTf 2 for living polymerization of MMA, which proceeds through a similar mechanism compared to the group transfer polymerization (GTP) of MMA [22] . GTP is a successful, commercialized strategy for living polymerization of polar vinyl monomers proceeding through Mukaiyama-Michael reactions [23] [24] [25] . Such GTP process could be catalyzed by nucleophilic anionic bases, such as SiMe 3 F 2 − [23,26,27], HF 2 − [23,26-28], F − [27-29], CN − [23,27-29],
doi:10.3390/molecules23030665 pmid:29543743 fatcat:ulvixgpmave57bnutmg5qpc6na