Organocatalytic Strategies For Tandom And Simultaneous Block Copolymerization Of Heterogenous Monomers

Block copolymers are composed of two or more polymeric segments with chain ends linked together through covalent bonds.Owing to the different physical/chemical properties of the blocky components,block copolymers usually exhibit properties that are unachievable from the corresponding homopolymers and polymer blends,such as microphase separation and self-assembly in bulk and selective solvents,which have led to wide applications in the areas of thermoplastic elastomers,surfactants,compatibilizers,micro-and nanostructured materials,etc.The block distinctions and nanostructuring properties may become much more prominent when the block copolymers are derived from heterogenous monomers?monomers carrying different polymerizing groups?owing to the different main-chain functionalities of the constituent repeat units.However,the polymerization of heterogenous monomers usually follow dissimilar mechanisms and/or are suited to different optimal conditions.As a result,synthesis of such block copolymers often suffers from poor compatibility and discontinuity of the two polymerization reactions,and requires additional separation,purification,and/or end-group functionalization steps.Therefore,the development of facile and effective synthetic strategies for block copolymers of heterogenous monomers holds great importance for enriching the structures and properties of polymers as well as advancing theories and applications of block copolymers.In recent decades,organocatalytic ring-opening polymerization?ROP?has been a fast-growing area of research,showing unique advantages of organocatalysts for the simplicity,controllability,selectivity,and sustainability of polymer synthesis.It has also remarkably promoted the innovation and improvement of synthetic strategies for complex macromolecular structures.In this thesis,several novel strategies for tandem or simultaneous block polymerization of heterogenous monomers have been formulated based on organocatalytic ROP of epoxides and/or cyclic esters for facile and controlled synthesis of well-defined block copolymers,including one-pot tandem ROP using a single catalyst,one-pot tandem ROP via catalyst switch,one-pot tandem performance of anionic vinyl-addition polymerization and ROP,one-pot tandem or simultaneous performance of photoactivated controlled radical polymerization and ROP.The main contents are summarized below.1)One-pot synthesis of eight-arm star-shaped polyether-b-polyester block copolymer using a single organocatalyst.Ethylene oxide?EO?was polymerized using a phosphazene base as the catalyst and sucrose as the initiator.By varying the feed ratio of the monomer to initiator,eight-arm star-shaped poly?ethylene oxide?s?PEO?with molar masses from 4 to 40 kg mol^-1and dispersities below 1.1 were synthesized.Sucrose was insoluble in the reaction media?a mixture of tetrahydrofuran and EO?,complete dissolution was observed soon after the growth of PEO,which helped gaining the solubility.All of the hydroxyl groups have initiated ROP without being affected by the insolubility in the initial stage.The cytotoxicity of the catalyst residues?phosphazenium salts?was studied through the MTT assay.The impacts of chemical structures and counterions on the cytotoxicity has also been revealed.EO and?-caprolactone?CL?were then copolymerized in a tandem manner by the catalysis of a phosphazene base with lower basicity and residual toxicity,which resulted in well-defined amphiphilic eight-arm star-shaped polyether-b-polyester block copolymer.2)One-pot synthesis of poly?ethylene oxide?-b-polyester/polycarbonate block copolymers via a“base-to-base”catalyst switch strategy.Ethylene oxide was first polymerized from a diol in the presence of a phosphazene superbase,then a thiourea was added to be deprotonated by the strongly basic alkoxide,which attenuated the basicity of the catalytic system and thus allowed for controlled polymerization of the subsequently added cyclic ester from the polyether chain end.This strategy shows advantages over the previously established“base-to-acid”catalyst switch strategy including better monomer/solvent suitability and lower catalyst loading.It was then used for one-pot tandem block copolymerization of lactide,?-caprolactone or trimethylene carbonate with EO to prepare well-defined block polymers.The combination of ROP and step-growth polymerization could also be realized using the catalyst switch strategy to synthesize polyurethane with PEO as the soft segment.Impact of the N-substituent,i.e.,p K_a of the thiourea,on the catalytic activity of the deprotonated thiourea has also been preliminarily revealed.3)One-pot synthesis of block copolymers by tandemly performed anionic vinyl-addition polymerization and ROP.The alkyllithium-initiated anionic vinyl-addition polymerization of isoprene was conducted first and followed by end capping with EO.Then a N,N-disubstituted urea was added leading to proton abstraction of the urea and in-situ generation of a urea anion which then catalyzed the subsequent ROP of CL by activating both the monomer and chain-end hydroxyl group.The ROP of CL showed high efficiency and selectivity against macromolecular transesterification because of the alleviated basicity and the bifunctional synergetic effect.Therefore,polyisoprene-b-poly??-caprolactone?block copolymers with controlled molar mass and composition could be tandemly synthesized with the dispersity staying around 1.2 even after full conversion of CL is reached.The impact of p K_a value and structural symmetry of urea on the catalytic activity was revealed by DFT calculations.4)One-pot synthesis of block copolymers by tandemly or simultaneously performed photoactivated controlled radical polymerization and ROP.A hydroxyl-functionalized trithiocarbonate was used as a dual initiator where ROP was initiated at the hydroxyl-containing R-terminus and the photoactive trithiocarbonate moiety acted as the initiator,chain transfer agent and reversible terminator for the photoiniferter polymerization.The initiator was mixed with an acrylamide and irradiated with UV light to initiate the controlled radical polymerization.Then an epoxide and a Lewis acid-base type two-component organocatalyst were added to start the ROP,which eventually affords a well-defined polyacrylamide-b-polyether block copolymer.Chain-extension experiments confirmed good retention of the trithiocarbonate functionality during the ROP in the presence of the organocatalyst.The two polymerization reactions are highly controlled and compatible,and can also be performed in the reverse order or simultaneously.The customization of the relative block lengths was achieved with dynamic light control in real time without adjusting the monomer feed ratio,while the ROP was unaffected.This method could be used to copolymerize a variety of acrylamides/acrylates and epoxides.This new type of block copolymer exhibited distinctly inherent immiscibility of the blocky components and self-assembling properties in a selective solvent.