Chemoselective And Controlled Synthesis Of Ether-ester Type(Co) Polymers

Ether and carboxylic ester linkage are the most occurring functional groups in oxygenated heterochain polymers.Their presence entitles many useful properties for the corresponding polymer materials.For instance,aliphatic polyesters represented by poly(lactide)(PLA)and poly(?-caprolactone)(PCL)exhibit good biodegradability,biocompatibility and mechanical strength due to a large number of carboxylic ester bonds existed in the main chain.The unique(CH₂CH₂O)_n backbone leads to the high flexibility,water-solubility and biocompatibility of poly(ethylene oxide)(PEO).Incorporating both ether and ester linkage into one polymeric molecule provides an opportunity to integrate the characteristics of the two kinds of bonds,thereby to enhance and enrich the properties of oxygenated polymer.Specifically,introducing ether bond(or polyether)into polyester main chain can adjust the crystallinity,hydrophilicity and biocompatibility of polyesters,as well as afford cleavable or modifiable sites.And the degradability and mechanical properties of polyethers can be boosted by anchoring ester bond(or polyester)into polyether chain.However,the design and synthesis of ether-ester coexisting polymers are generally limited by the chemoselectivity of each polymerization reactions.This thesis aims at developing new organocatalysis system and polymer synthetic strategy to overcome the restrictions of conventional methods in chemical selectivity and polymer structure control,so as to obtain a new type of ether-ester(co)polymers and discuss their properties/applications.(1)A bicomponent“catalyst switch”strategy for cascade and sequences-controlled synthesis of ether-ester block copolymers.Polymerizing epoxide after cyclic ester remains a major challenge,though their block copolymers have been extensively studied and used for decades in virtue of the biocompatibility,biodegradability,amphiphilicity and crystallinity,derived from the two types of blocky components.We report here a bicomponent catalytic system composed of metal-free LA and LB that achieves the monomer-selective and sequence-controlled synthesis of ether-ester block copolymers.When LA is in excess of LB,the catalyst(ALP)allows selective ring-opening polymerization(ROP)of epoxide in the presence of cyclic and/or acyclic(poly)esters.When the catalyst is LB alone or with LB in excess of LA(BLP),ROP of cyclic ester occurs selectively in the presence of epoxide.During the polymerization of one monomer,selectivity can be switched toward the other monomer promptly at any conversion,by simply adding an adequate amount of LA or LB to render the Lewis pair oppositely“biased”.Hence,mixed epoxide and cyclic ester can be readily block-copolymerized with either being the first monomer,leading to well-defined block copolymers with controlled and unlimited block sequences.The selectivity can be switched back and forth several times to afford a multiblock copolymer.Commodity epoxides and cyclic esters that are commonly used for aliphatic polyethers and polyesters,as well as mono-(functional),di-,and multi-hydroxy(small-molecule and macro-)initiators,can be freely combined,to construct an unprecedentedly rich diversity of ether-ester type block copolymers.Besides,the origin of monomer-selectivity is investigated and interpreted by both experiment and DFT calculation.Particularly,alkoxide capped by two equivalent LA is found to be the actual active center that selectively reacts with epoxides and leaves cyclic and acyclic(poly)esters intact,which provides an alternative mechanistic insight other than the previously reckoned activated monomer mechanism.The novel block copolymers achieved by this approach could be of broad fundamental interests for the structure-related properties,and will largely enrich the catalog of functional biocompatible and biodegradable polymeric materials.The concept of using the ratio between LA and LB to modulate the catalytic activity and selectivity could be translated into a general strategy for a wide range of reactions involving both LA-and LB-sensitive substances that are to be selectively and/or tandemly reacted.(2)Chemoselective ROP of epoxides initiated by hydroxycarboxylic esters for simple and controlled synthesis of?-carboxyl-?-hydroxyl polyethers.A commercially available ethyl6-hydroxyhexanoate and one-pot pre-synthesized oligo(?-caprolactone),are employed as initiators for the ROP of ethylene oxide(EO)catalyzed by a metal-free Lewis pair.Well-defined PEOs with controlled molar masses and high end-group fidelities are afforded owing to the chemoselectivity of the catalyst which prevents end-group transesterification.The?-carboxylic ester-?-hydroxyl PEOs are readily subject to quantitative in situ hydrolysis to form?-carboxyl-?-hydroxyl PEOs which are usually synthesized by much more tedious and costly routes.Extensions of the method for?-carboxyl-?-hydroxyl poly(propylene oxide)and?-dicarboxyl-?-hydroxyl PEO,using a hydroxydicarboxylic ester as the initiator,are also demonstrated.A convenient and cost-effective pathway is thus paved toward?,?-heterobifunctional polyethers which hold great values for biomedical applications.(3)Linear polyglycerol is highly valued for its excellent hydrophilicity and biocompatibility as well as its multihydroxy nature.We report here a convenient route for controlled synthesis of polyglycerol through ROP of commercialized glycidyl butyrate(GB).Starting from enantiopure GBs,well-defined poly(glycidyl ester)s with controlled molar mass and stereoregularity are achieved thanks to the chemoselectivity of the bicomponent metal-free catalyst that prevents both transesterification and epimerization.The pendent butyrate groups are readily cleaved by organobase-catalyzed methanolysis,yielding linear polyglycerols that inherit the isotacticity and regioregularity from the parent polymers.Copolymerization of SGB and propylene oxide occurs in a random manner resulting in a series of narrowly distributed copolyethers with precisely regulated number of pendent hydroxyl groups to afford tunable aqueous thermosensitivity.The method is further extended to construct polyglycerol-based amphiphilic and double-hydrophilic copolyethers by block copolymerization of RGB with tert-butyl glycidyl ether and EO,respectively.