Synthesis And Properties Of Block, Star And Hyperbranched Polymers

Recently, block copolymers have attracted considerable attention because of their unique behaviors and potential applications, such as thermoplastics, surfactants, modifiers, dispersants, and tackifier etc. With the development of living radical polymerizations, such as SFRP, ATRP, RAFT and Iniferter, a large number of novel well-defined block copolymers with controlled molecular weights and narrow molecular weight distributions were prepared.Polymer micelles, formed by self-assembly of block copolymers in selective solvent, have been extensively investigated for their potential applications in drug delivery, separation, catalysis and nanomaterials fields. For example, Hydrophobic drugs loaded by polymer micelles with physically entrapment or chemically conjugation are improved their water solubility and bioavailability. The drug-loaded micelles prolong their circulation in body and release selectively their payload in tumor tissue or within cells. However, the "smart" stimuli-responsive linkages, which are used to link drug and micelle in chemically conjugation method, are still limited.The applications of polymer micelles are related to their morphologies. Beside frequently observed spherical core-shell micelles, Many other morphologies have been obtained, such as rod, worm-like, vesicle, lamellae and other complicated aggregates. These morphologies were usually prepared from crew-cut micelles by controlling the preparation conditions until now. There are few reports on the multiple morphologies of star micelles.Polyion complex micelle, a novel micelle with its unique behaviors, is formed by a pair of opposite charged polymers in aqueous solution. The charged polymers used are usually homo-polyelectrolytes and double hydrophilic block copolymers consist of a polyelectrolyte block and a nonionic hydrophilic block such as PEO. The self-assemly of the mixture, which is composed of a pair of amphiphilic block copolymers with opposite charge and equal length, has not been researched.Star polymer is a class of simplest branched polymer, which is usually prepared by "arm-first" and "core first" approach. No matter which approach is used in the literature, two steps are needed to get a star polymer. Either the core or the arms should be synthesized in a separate step; at least chemicals were added step by step. Therefore, a simple way of synthesis is highly appreciated.Electrospinning is a very suitable method to obtained nanofibers. Many kinds of polymers, such as homopolymers, block copolymers and polymer blends, had been used to prepare nanofibers by electrospinning. In addition, inorganic nanofibers were fabricated by this method in recent years. The research of electrospinning of functional hyperbranched polymer is still an outgoing challenge.All these facts are the origin and impetus of this thesis. The main results obtained in this thesis are as follows:1) A new type of double hydrophilic block copolymer, poly(ethylene oxide) (PEO)-block-poly(glycerol monoacrylate) (PGA) have been synthesized via atom transfer radical polymerization of solketal acrylate (SA) using PEO-Br as macro-initiator, and subsequent hydrolysis of the acetal-protecting group. A hydrophobic fluorescent compound, 1-pyrenecarboxaldehyde, was used as a model drug, which was covalently bound to the PEO-b-PGA block copolymer via a pH-sensitive acetal linkage. The kinetics of the pyrene release was studied in THF/aqueous buffers at pH 1.0, pH 5.0 (close to pH in endosomes) and 7.4 (pH of blood plasma) by fluorescent spectroscopy. The results demonstrate that the double hydrophilic block copolymer PEO-b-PGA is a promising candidate for potential application as pH-sensitive carrier for carbonyl-containing hydrophobic drugs.2) Novel amphiphilic block copolymers, poly(ethylene oxide)-b-poly(p-nitrophenyl methacrylate) (PEO-b-PNPMA) with controlled molecular weights and narrow molecular weight distributions were successively synthesized by ATRP of NPMA, using PEO-Br as macro-initiator. Self-assembling of the diblock copolymer PEO₁₁₃-b-PNPMA28 in the different solvent mixtures yielded various morphologies of star micelle-like aggregates, such as spheres, vesicles, cauliflower-like aggregates and rod-like aggregates, which were determined by the nature of the common solvents and the selective solvents. Thus the critical selective solvent contents and the solvent contents in PNPMA-rich phase were measured, and they have the following orders, ethanol>methanol>water, and THF>CH₃NO₂>DMSO. Based on this, the probable self-assembling mechanism was discussed. This method is convenient for preparation of multiple morphological star micelle-like aggregates in solution, especially from amphiphilic block copolymers with long shell block.3) Triblock copolymers PNPMA-b-PS-b-PNPMA with 146 St units and 8, 20 and 36 NPMA units were prepared by ATRP of St using dibromoxylene as initiator, and following ATRP of NPMA. The anionic copolymer PMAA-b-PS-b-PMAA was synthesized by the hydrolysis of the PNPMA-b-PS-b-PNPMA, and the cationic copolymer PNPMAAm-b-PS-b-PNPMAAm was prepared by the amino substitution of the same PNPMA-b-PS-b-PNPMA. Equal mole of PMAA-b-PS-b-PMAA and PNPMAAm-b-PS-b-PNPMAAm both having the same chain lengths were mixed and dissolved in common solvent THF with 20% deionized water, and the solution was dialyzed against water to induce the copolymers self-assembly to form aggregates. Polydisperse spheres were observed containing an asymmetrically placed single void space, which has broken through the surface (so-called bowl-shaped aggregates).The possible mechanism was discussed. These bowl-shaped aggregates are promising candidates for nano reactor or drug carrier.4) 2,7-dimethylene-1,4,6-trioxaspiro[4, 4]nonane with two vinyls was prepared to be used as electron-donor and crosslinker. The ATRP of this crosslinker and excess methylacrylate (used as electron-accpetor) was proved to mainly form homopolymers. With the chain-transfer agent S-1-Dodecyl-S'-(α,α'-dimethyl-α"-acetic acid)trithiocarbonate, the copolymerization of the spiro hydrocarbon, maleic anhydride (Mah) and MA by RAFT method produced star PMA. The molecular weights of obtained polymers measured by GPC were much higher than that of the linear polymers with the same monomer conversion. Samples taken with different time intervals in the polymerization initial stage were analyzed by ¹H NMR spectroscopy. The results demonstrated the possible mechanism. Charge transfer complexes of spiro hydrocarbon and Mah were formed, and first consumed in the initial stage, which produce a core with many chain-transfer groups on its surface. Then MA monomers were polymerized onto the core to form star polymers. This method can be expanded to other monomers. It is a simply way to prepare star polymers by one-pot.5) Hyperbranched Poly(amidoamine) was obtained by the Michael addition polymerization of N,N'-cystaminebisacrylamide (CBA) and 1-(2-aminoethyl) piperazine (AEPZ). The electrospinning of hyperbranched Poly(amidoamine)s in methanol solution was performed. The effects of solution concentration, added voltage and accepting distance on the morphology of the obtained fibers were discussed. These polymer fibers could emit blue, green or red photoluminescence when excited by lights of different wavelength. The colorful photoluminescent fibers have potential applications in display field.