Janus Polymerization-Ring Opening Polymerization Of Cyclic Monomers Catalyzed By Rare Earth Triflates

Janus polymerization initiates cationic and anionic polymerizations simultaneously at two ends of a single polymer chain,followed by self-triggered chain-extensions via coupling reactions between the cationic and anionic chain ends,which generates topological polymers in one step directly from monomers,such multiblock copolymers（MBCPs）,star polymers,cylindrical polymer brush and branched polymers.Janus polymerization not only combines cationic and anionic polymerizations and broadens the ranges and catagories of comonomers,but also simplifies synthetic process,for instance,the one-step and one-pot synthesis of MBCPs.MBCP consists of multiple,chemically independent and versatile blocks in backbone via covalent bonds.The properties and performances of MBCPs are modulated by their constitutions to meet modern industrial demands.The syntheses of well-defined MBCPs include sequential addition of comonomers,polycondensation,chain-extension,post-modification,etc.Most of these methods incorporate some problems,such as multistep reactions,side reactions,protection/deprotection procedures,poor control of molecular weights,polymer constitutions and sequences,etc.While Janus polymerization provides efficient and facile approaches to synthesize MBCPs.The ever reported Janus polymerization employs lutetium triflates（Lu（OTf）₃）/propylene epoxide（PO）as the catalytic system,tetrahydrofuran（THF）andε-caprolactone（CL）as the comonomers to synthesize thermoplastic elastomers with excellent mechanical properties.Based on the advantages of Janus polymerization,more monomers can be incorporated and the universality of Janus polymerization is deserved to be investigated.This dissertation applies 1,3-dioxolane（DO）,3,3-bis（chloromethyl）oxacyclobutane（CO）,γ-butyrolactone（BL）as well as the classical THF and CL as comonomers to synthesize diversified,specific and functional materials.We employ rare earth metal triflates（RE（OTf）₃,RE=Sc,Y,Gd,Tm and Lu）as catalysts for the controlled homopolymerization of DO.Poly（1,3-dioxolane）（PDO）consists of alternating oxyethylene and oxymethylene repeating units along the backbone.The hydrogen bonding between ethylene glycol ether with water molecules leads to stable hydration layers and thus uniform spherical nanoparticles in aqueous solution.The hydrodynamic diameters of PDO nanoparticles（NPs）depend on temperature and concentration.Their morphologies are modulated by thermal treatment and various metal ions,generating rod-like,sandwich-like and worm-like aggregates.PDO NPs are proved to absorb metal ions and to degrade at a weakly acidic solution releasing reductive agent in situ.PDO acts as a template to fabricate metal NPs both as a surfactant and a reductant.For instance,a hybrid material Cu/Cu SO₄/PDO is prepared from Cu SO₄ aqueous solution.The synthesis and solution properties of PDO are investigated to support the application for DO in Janus polymerization.Tm（OTf）₃/PO and Er（OTf）₃/PO catalyzes Janus polymerization of DO with CL producing MBCPs（[PCL-b-P（DO-co-CL）]_m）while RE（OTf）₃ with other rare earth metals produces amorphous cationic copolymers（P（DO-r-CL））via cationic mechanism.[PCL-b-P（DO-co-CL）]_m consists of long poly（ε-caprolactone）（PCL）sequences and PDO-rich segments of DO with CL（P（DO-co-CL））as repeating blocks.They exhibit multiple melting temperatures and shape memory properties indicating the promising applications in biomedical devices.This work employs RE（OTf）₃/PO as the catalytic system to synthesize the block copolymer of CO with CL via Janus polymerization.The compositions of copolymers are modulated by the catalysts among which Sc（OTf）₃ yields the copolymers with the highest CO fraction up to 21 wt%.The anionic site generates long PCL blocks and the cationic site generates random blocks of CO with CL.CO repeating units provide the modification sites on the backbone.The chlorine atoms in CO repeating units are transferred into azides reactive with hydrophilic m PEG-alkyne and fluorescent fluorene-alkyne.The combination of Janus polymerization and click reaction provides an efficient and facile approach to prepare functional PCL.Sc（OTf）₃ is applied to catalyze the cationic zwitterionic copolymerization of CO with BL.The copolymerization demonstrates good controllability,zeroth-order kinetics and the tendency of alternating sequence(r _CO=0.30,r _BL=0.11).Linear and cyclic copolymers are synthesized by inter-molecular and intra-molecular coupling reactions between cationic and anionic chain ends,respectively.Without terminal groups,cyclic copolymers exhibit lower hydrodynamic volume,higher decomposition temperature,higher glass transition temperature than the corresponding linear copolymers with the same molecular weight.Isopropanol and tributylphosphine are employed to capture cationic active centers followed by a cationic zwitterionic mechanism.The incorporation of CO not only promotes the insertion of BL in the cationic copolymerization,but also provides modification sites to further react with imidazoles via quaternization generating amphiphilic polyether-esters.In a zwitterionic polymerization,one ion initiates polymerizaiton and the other does not,suggesting that zwitterionic polymerization can be considered as a special Janus polymerization.Classical Lu（OTf）₃/PO-catalyzed Janus polymerization of THF with CL is also investigated yielding the MBCPs of[PCL-b-P（THF-co-CL）]_m with different weight fractions of CL according to the various feed ratios.The microphase separation behaviors and mechanical properties are modulated by polymer constitutions and thermal treatment.The MBCP J1（CL,53 wt%）exhibits higher plasticity and elongation at break（2610%）than J2(（CL,88 wt%）.The mechanical properties of J2are improved after quenching at 70?C for 30 min.Elongation is increased from 1020%to 1942%and strength at break from 17.1 MPa to 45.4 MPa.Cell viability tests prove that[PCL-b-P（THF-co-CL）]_m is biocompatible since J1 exhibits the comparable cell dispersity and viability with tissue culture polystyrenes indicating the potential applications in biomedical engineering fields and self-sealing films.