| This thesis is mainly focused on two parts: the first part is synthesis,characterization and their potential applications as drug delivery carriers ofbiocompatible and biodegradable polyphosphoester-based hydrogels with pHresponsivity; and the second one is preparation and characterization of functionalpolymers by oxyanion-initiated polymerization (OIP), high-vacuum living anionicpolymerization, ring-opening polymerization (ROP) and "Click" chemistry. With thefinancial support from China Scholarship Council (CSC), the author has spent fourteenmonths studying in Prof. Stephen Z. D. Cheng's group in the College of PolymerScience and Polymer Engineering in the University of Akron, where his research wasmainly focused on the synthesis and characterization of novel shape amphiphiles andwell-defined star polymers via a combination of living anionic polymerization andhighly efficient organic synthesis methodologies including "Click" chemistry andhydrosilylation. The present work can be summarized as follows:(1) The polyphosphoester macrocrosslinker bearing two vinyl groups(HEMA-PEOP-Ac) was first synthesized by ROP and esterification, which was thencopolymerized with acrylic acid in the presence of ammonium persulfate (APS)/N, N, N′,N′-tetramethylethylenediamine (TEMED) catalysis system via conventional free radicalcopolymerization, resulting in the formation of pH-responsive anionic hydrogels. Thepolymer structure was characterized by FT-IR,1H NMR,31P NMR, and gel permeationchromatography (GPC) analyses. In addition, the pH responsive swelling/deswellingbehaviors, rheological properties, in vitro cytotoxicity and in vitro drug loading andrelease with bovine serum albumin (BSA) as a model drug were also investigated.(2) On the basis of the previous work, it was found that robust hydrogels wereformed under mild conditions upon mixing aqueous solutions of three componentsincluding the HEMA-PEOP-Ac macrocrosslinker,2-(dimethylamino)ethyl methacrylate (DMAEMA) monomer and APS initiator. The sol-gel transition could be completedvery fast ranging from several to tens of minutes with varying concentrations ofcomponents at room temperature without the aid of any additional accelerator anddeoxygenating operations. The rapid in situ gel formation was affirmed by dynamictime sweep rheological measurement, and the SEM analysis of crosslinked hydrogelshowed that the network was formed with porous structure. The in vitro drug release ofDOX from such a polyphosphoester-based hydrogel showed a long-term and sustainedrelease profile, and the release rate could be further regulated depending upon thecrosslinking density. Combined with favorable cytotoxicity, this kind of pH-sensitiveand rapidly in situ forming hydrogel system with adjustable gelation time has shownpotential for use as injectable drug delivery carriers.(3) Using the difunctional potassium alcoholate of F127, K+O--F127-O--K+, as themacroinitiator, the POFPMA-F127-POFPMA pentablock copolymers were synthesizedvia oxyanion-initiated polymerization. The chain length of POFPMA can be controlledby the original molar ratio of macroinitiator to OFPMA monomer, i.e. F-monomer. Thecomposition and chemical structure of POFPMA-F127-POFPMA pentablockcopolymers have been characterized by FT-IR,1H NMR and19F NMR spectroscopy,and GPC techniques. The solution behavior of POFPMA-F127-POFPMA pentablockcopolymers was investigated by the methods of surface tension, cloud point, andtransmission electron microscopy (TEM). The results indicated that these PluronicF127-based block copolymers modified with fluorinated segments possess relativelyhigh surface activity and low cloud points, depending on various factors, such as thelength of fluorinated block, the concentration of the copolymers in aqueous solution andthe adscititious inorganic salt. TEM measurements showed that the pentablockcopolymers could self-assemble in aqueous solution to form various micellarmorphologies.(4) In this part, we report on the model synthesis of functionalized polystyreneswith well-defined structure and different functional groups prepared by combing anionicpolymerization, copper-catalyzed alkyne-azide cycloaddition (CuAAC) and thiol-ene"Click" reaction. It was found that the reaction sequence of "Click" chemistry makelittle influence on the resultant products. The obtained polymers have been fullycharacterized using FT-IR,1H NMR,13C NMR, MALDI-TOF mass spectrometry, andGPC analyses. Furthermore, the polystyrenes with functional groups in both chain ends and in the middle of polymer chain were also synthesized and characterized. Thesynthesis provides an alternative way for polymer functionalization and this methodcould be easily extended to other polymers with various functional groups forfine-tuning of the interaction among different components in polymer materials.(5) The design and synthesis of fluoroalkyl-functionalized polyhedral oligomericsilsesquioxane (FPOSS)-based shape amphiphiles with two distinct topologies wererepoted:(i) from a monohydroxyl-functionalized heptavinyl POSS, a PCL chain wasgrown via ring opening polymerization (ROP) of ε-caprolactone; subsequent thiol-ene"click" chemistry with1H,1H,2H,2H-perfluoro-1-decanethiol allows the facileintroduction of seven perfluorinated alkyl chains onto the POSS head, resulting in themono-tethered FPOSS-poly(ε-caprolactone)(PCL) and (ii) FPOSS tethered with twopolymer chains possessing different compositions, namely, polystyrene (PS) and PCL,denoted as PS-(FPOSS)-PCL. It was synthesized from a PS precursor bearing bothhydroxyl group and heptavinyl POSS at the ω-end, which was prepared by livinganionic polymerization and hydrosilylation. The compounds have been fullycharacterized by FT-IR,1H NMR,13C NMR spectroscopy, MALDI-TOF massspectrometry, and GPC techniques. The introduction of perfluorinated molecular clusterinto polymers is expected to make them surface-active while the interplay betweencrystallization and fluorophobic/fluorophilic bulk phase separation in these shapeamphiphiles shall lead to intriguing self-assembly behavior and novel hierarchicalstructures. This study has demonstrated FPOSS as a versatile building block in theconstruction of shape amphiphiles and established a general and efficient method tointroduce such fluorous molecular clusters into polymers.(6) A rapid, stoichiometry-insensitive, molecular weight (MW)-insensitiveapproach was developed for the preparation of well-defined, eight-arm star polymers viathe addition of living anionic (polystyryl)lithium (PSLi) to octavinylPOSS in benzene.The reaction proceeds rapidly (within5min for MW of each arm up to33.6k) tocompletion, forming predominantly eight-arm star polystyrene which was purified byfractionation and fully characterized by FT-IR,1H NMR,13C NMR,29Si NMR,MALDI-TOF mass spectrometry, and GPC techniques. The process was found to beless insensitive to stoichiometry of the reactants and molecular weight. Subsequently, itwas found that (polyisoprenyl)lithium (PILi) was also applicable to this addition reaction. Therefore, three star polymers with distinct architectures were prepared andcharacterized, that is eight-arm star polymer with one polystyrene chain and sevenpolyisoprene chains (PS-POSS-7PI), and eight-arm star block copolymers with reverseblock sequence8(PSPI)-POSS and8(PIPS)-POSS. The method has general implicationsin the understanding of reactive nature of the living anionic polymers and may findpractical application in the synthesis of diverse functional star polymers of distinctcomposition and architecture. |
