Organocatalytic Ring-Opening Polymerization Of Cyclic Ester Or Epoxide

Aliphatic polyesters and polyethers are the most popular oxygen-containing polymers that have been widely investigated and used for their special and desirable properties originating from the ester-and ether-bonds in their main chain structures.Living ring-opening polymerization(ROP)of cyclic ester and epoxide provides a simple,convenient,highly efficient,and atom-economic approach to well-defined aliphatic polyesters and polyethers with controlled macromolecular characteristics.In addition to their homopolymerization,cyclic esters and epoxides are easily copolymerized with other homogeneous and heterogeneous(structurally distinct)compounds,which is a frequently employed strategy to enrich the structures and functionalities of heterochain polymers.Recently,the development of organocatalytic/metal-free ROP methods have been greatly spurred on by the rapidly expanding applications of polyesters and polyethers in various high-value added fields such as biomedicine and microelectronics.Except for the inherit merit that the product is free of metallic residues,organic/metal-free catalysts frequently rivals or even outperforms organometallic catalysts in terms of catalytic efficiency and selectivity,sophisticated design,and convenient synthesis of tailored polymers.This thesis mainly focuses on developing new organocatalytic/metal-free methods for the advancement of homopolymerization of cyclic esters and epoxides or their copolymerization with heterogeneous monomers.The effects of structure,composition,and loading of the catalysts as well as the catalytic mechanisms on the polymerization efficiency,selectivity and controllability have been explored in detail.Accordingly,strategies for one-pot or one-step tandem synthesis of block copolymers from heterogeneous monomers have been actualized.The main contents are summarized below.(1)Tuning the crystallinity and degradability of poly(?-caprolactone)(P?CL)by organic acid-catalyzed copolymerization.The copolymerization of?-caprolactone(?CL)with its structural isomer,?-hexalactone(?HL),or trimethylene carbonate(TMC)has been investigated with the catalysis of diphenyl phosphate.Random copolymers with molar fraction of?CL units ranging from 54%to 95%were obtained at high temperature by varying the initial feed ratios of monomer to initiator and?CL to the comonomer.The crystallinity and enzymatic degradability of the copolymers were analyzed with differential scanning calorimetry and quartz crystal microbalance with dissipation,respectively.The melting point of the copolymers decreased with the enhanced incorporated amount of comonomers,which indicated the reduced crystallinity of the resultant products.In addition,poly(?CL-co-?HL)showed good thermostability and enzymatic degradability with?HL content being as low as 5 mol%,outperforming the poly(?CL-co-TMC)copolymer with the same comonomer content.(2)Tandem ring-opening block copolymerization of?-butyrolactone(?BL)and epoxide catalyzed by combined phosphazene base and triethylborane(Et₃B)accomplishing one-pot synthesis of poly(?-butyrolactone)(P?BL)-based ester-ether type block copolymers.Typically,?BL was first polymerized at low temperature in the presence of a phosphazene base.Then Et₃B was added,which effectively inhibited the depolymerization of P?BL when the mixture was brought back to room temperature and constituted the two-component catalyst needed for the ROP of epoxide.Therefore,polyester-b-polyether copolymers were obtained in one-pot through ROP of the epoxides initiated by the precursor P?BL.This in-situ catalyst switch strategy endows the P?BL macroinitiator with indispensable room-temperature stability and subsequently allowed chemoselective and controlled ROP of epoxides free of transesterification.Several substituted epoxides have also been copolymerized with the biobased renewable?BL,delivering a series of new class of polyester-b-polyether copolymers.(3)Controlled ROP of propylene oxide(PO)catalyzed by combined N-heterocyclic carbene(NHC)and triisobutylaluminum(ⁱBu₃Al).Compared with ROP of PO catalyzed by NHC alone which needs 3 days at 50°C in bulk,ROP by this two-component catalyst was considerably faster under milder conditions(25°C,in 2-Me THF or bulk),producing poly(propylene oxide)s(PPOs)with narrow molar mass distribution and predictable molar mass up to 60 kg mol^-1.The well-defined polymer structures were confirmed by the ¹H NMR spectrum and MALDI-TOF MS analyses,which also verified the absence of chain transfer to monomer.Such results indicated that ⁱBu₃Al took effects in this two-component catalyst by forming an initiating/propagating species of moderate basicity/nucleophilicity by complexing with alkoxide and by coordinating and activating PO monomer towards reaction with the initiating/propagating complex.Therefore,the ROP could proceed in a controlled and efficient manner to generate well-defined PPO under mild conditions.(4)Controlled ROP of propylene oxide(PO)catalyzed by the combined NHC and Et₃B.ⁱBu₃Al has been the most frequently used Lewis acid which activates the monomer and coordinates with the alkoxide species for controlled/living ROP of epoxides.But the Al-based catalyst residues would affect the environmental friendliness of the synthetic method and the biomedical applications of the polyether.Interestingly,metal-free Lewis acid Et₃B could also play the two roles and afford controlled ROP of PO at room temperature in THF or bulk,delivering well-defined PPOs with predictable molar mass and narrow distribution((?)<1.1).Compared with NHC alone,the two-component catalyst comprising NHC and Et₃B afforded significantly enhanced catalytic efficiency and molar mass of PPO.Thanks to the stability of Et₃B towards water,?,?-dihydroxyl telechelic PPOs could be readily synthesized by using water as a difunctional initiator,which was unattainable with the water-sensitive ⁱBu₃Al.(5)One-step sequence-selective block copolymerization of ethylene oxide(EO)and N-sulfonyl-2-methyl aziridine(Ts MAz)catalyzed by combined organic base and Et₃B.The conditions for controlled ROP of Ts MAz from alcoholic initiators was first screened.The combination of phosphazene base and Et₃B showed to be best choice in terms of activity and polymerization control.Then,copolymerization of EO and Ts MAz was carried out with this two-component catalyst.It was found that the polymerization efficiency of EO could be enhanced by varying the ratio of the catalytic components,and the hydroxy end group of PEO could continue to initiate the polymerization of Ts MAz.The strict sequence selectivity of the copolymerization was thus guaranteed,delivering tailored PEO-b-PTs MAz block copolymer in one synthetic step.After the full consumption of both monomers,the chain end of the block copolymer was still active,where the two monomers could be one-step block copolymerized again in the same sequence.Therefore,one-pot synthesis of multiblock copolymers was fulfilled through batch addition of mixed monomers.