Lewis Acid Catalyzed Group Transfer Polymerization Of Polar Vinyl Monomers Using Silyl Ketene Acetal As Initiator

Group transfer polymerization?GTP?,as an effective living/controlled polymerization,has attracted increasing attention,and researchers have been exploring better catalysts and initiator systems to achieve higher activity and control.In recent years,Lewis based/acid-based catalyst systems have evolved considerably because of their unique high activity and near-100%initiation efficiency.Vinyl polar monomer is a polymer monomer with a wide range of market applications and scientific research needs because of its high transparency,good physical strength and corrosion resistance,but the synthesis of a well-defined polymer?molecular weight close to theory Molecular weight and molecular weight distribution close to 1.0?was still a challenge.In this paper,we discuss the Lewis acid-catalyzed group transfer polymerization of vinyl polar monomers from the following two aspects.In the first part,we studied the room-temperaturegroup transfer polymerization of conjugated polar alkenes,including linear methyl methacrylate?MMA?,as well as biorenewable,cyclic?-methyl-?-methylene-?-butyrolactone?MMBL?and?-methylene-?-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,whilethe MMBL polymerization by the bulky SKA(^iBuSKA)/[Al]system is living and thus produces well-defined PMMBL with a predicted molecular weight(M_nup to 179 kg·mol^-1),a narrow molecular weight distribution?Das low as 1.02?,and ahigh initiation efficiency?I*?97%?.The living polymerization of MMBL was established through five lines of evidence,including predictable polymer M_n and lowDvalues,a linear increase of polymer M_n 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.Acombinedmechanisticstudythroughisolationandcharacterizationof 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 viaintermolecular Michael addition of the SKAenolate 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 oxygenatom of SKA;the coordinated[Al]is released to the incoming monomer,followed by repeated intermolecular Michael additions in the subsequent propagation cycle.In the second part,we studied 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?-methyl-?-methylene-?-butyrolactone?MMBL?and?-methylene-?-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 activityand living GTP of MMBL and MBL.Detailed investigations,including the characterization of key reaction intermediates,polymerization kinetics and polymer structureshave led to a polymerization mechanism,in which the polymerizationis 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 NMR 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?.In summary,we conducted group transfer polymerization of vinyl polar monomers by using different Lewis acids as catalysts,wherein the polymerization of MMBL and MBL was proved to be living/controlled catalyzed by SKA/E?C₆F₅?₃?E=Al,B?system.We have proposed and verified the mechanism of polymerization through the characterization of intermediates,polymerization kinetics and characterization of polymer chain ends.Using such a living polymerization system,the copolymer of MMBL and MBL was successfully synthesized.