Synthesis And Self-assembly Of Cholesterol-based Functional Small Molecules And Polymers

Cholesterol,a natural sterol,owns good biocompatibility and plays a critical role in the formation of bile acid,as well as the construction of cytomembrane and hormone.Due to its unique rigid skeleton,multiple chiral carbon centers,hydroxyl modifiable site and liquid-crystal property,cholesterol is considered as an ideal building block in designing various supramolecular self-assemblies.By chemically modifying cholesterol unit,versatile functional molecules can be synthesized to exert its merits of unique rigid structure,and good biocompatibility,and further illustrate the mechanisms during the self-assembly process,which is of great significance to exploit the applications of natural products in the fields of materials preparation and biomedicine transportation.In this thesis,small molecules based on chol and tetraphenylethylene(TPE)skeletons were synthesized by well ultilizing its facile gelling capacity and good anisotropic feature of chol,and taking its rigid skeleton and excellent aggregation induced emission(AIE)character of TPE,and then studied the effects of linker length on molecular liquid crystal property,AIE property,gelling property and self-assmbled morphology.Besides,metallic catalysts and organic catalyst were designed and synthesized,and their properties in the ring opening polymerization(ROP)of chol-based cyslic carbonates to prepare amphiphilic copolymers were further compared.Finally,bio-compatible and biodegradable amphiphilic block copolymers were obtained through ROP of cyclic carbonate modified on cholesterol skeleton;Dynamic self-assmbly behavior of these copolymers and mechanism of the smectic vesicle formation were studied and elucidated,respectively.Above studies may pave the way for the exploration of gel constructions,self-assembly formations and their applications in materials and biology.Work and results are as follows:1.Small molecules with AIE property,TPE-Cn-Chol(n=1,4,5,6),were synthesized by bringing in TPE group.The results showed that chol itself possesses good liquid crystal property,but small molecules TPE-Cn-Chol did not have when linked to non-rod-like TPE skeleton.In addition,linker length influenced on both gelling abilities and gel morphology.Besides,gelation process could enhance fluorescence intensity of these small molecules immensely;finally,these four molecules all possessed good thermos-,mechano-and vapochromic properties.These AIE gels represent potential value in the fluorescence imaging and fluorescence detection fields.2.Metallic catalysts with different metallic centers were prepared via metathesis reactions of metal halides with sodium bis(trimethylsilyl)amide,and some ligands with different electronic effects and steric effects were synthesized,organometallic catalyst was then obtained by the complexation reaction of ligand with metal-coordinative catalyst.The results showed that metallic catalyst Ph3C(N(SiMe3)2)could be stabilized in the absence of complexation with solvents(the product was solid),but metallic catalysts using Ca、Mg、Ge and Zn as metallic centers must be complexed with solvents to form stable structures like Ca[N(SiMe3)2]·2THF、Mg[N(SiMe3)2]·2THF、Ge[N(SiMe3)4]·2THF and Zn[N(SiMe3)2]·2THF(the former three catalysts were solid,but the latter one was liquid).Meanwhile,thiourea-based organic catalyst was also prepared.Although metallic catalysts differed from organic catalyst in electronic effects and steric effects,which further caused distinguishing catalytic activities,they were still turned up to be efficient catalysts for ROP of cyclic esters.3.Chol-based amphiphilic block copolymers(mPEG45-b-PMCC)were prepared by ROP of six-membered cyclic carbonate modified on cholesterol skeleton.The results revealed that metal-coordinative catalysts showed better polymeric controllability towards ROP of cyclic carbonates compared with organic catalysts.Furthermore,self-assembly behavior of liquid crystalline(LC)block copolymers in dioxane/H2O,transition from nanofiber-to-rod-lamellae-to-vesicle by a liquid crystallization-driven process,was observed for the first time in situ by cryo-transmission electron microscope(cryo-TEM),and these self assemblies all exhibited thermotropic smectic structures.Anisotropic property of chol led to disparate degrees of orientation in different directions,and futher made the smectic layer spacing unequal on both sides of the membrane,which could explain why the smectic vesicles formed were always ellipsodidal but not spherical.The direction of the cholesterol mesogen and the mormal of smectic layer were parallel to the long axis of polymeric vesicles.Above biocompatible and biodegradable polymeric vesicles may present great potential in the high loading efficiency for hydrophilic molecules.