Preparation Of POSS-based Star Two Block Copolymers Via Click Chemistry

Polyhedral Oligomeric Silsesquioxane(POSS)is an organic/inorganic hybrid molecule with nano-scale hexahedral cage structure. Due to the multiple surface functional groups that are easily modified, the nano-biomaterials with special topology constructed from POSS have become the research hot topics in recent years. In this thesis, the POSS-OH monomer with eight hydroxyl groups was used to initiate the ring-opening polymerization of ε-caprolactone(CL) and form POSS-g-poly(ε-caprolactone)(POSS-g-PCL) polymer, which was further linked with poly(ethylene glycol) by click chemistry to synthesize amphiphilic star-like POSS-g-PCL-b-PEG block copolymer. The structures and molecular weights of the copolymer and precursors were characterized by nuclear magnetic resonance spectroscopy(1H-NMR), silicone nuclear magnetic resonance spectroscopy(29Si-NMR) gel permeation chromatography(GPC) and etal.Firstly, two POSS monomers with eight functional groups, octapoly(dimethylsiloxy) silsesquioxane(POSS-H) and octapoly[2-(2-hydroxyethoxy) butyldimethylsilyloxy] silsesquioxane(POSS-OH), were prepared by one-step hydrolysis condensation method. By modification of synthetic and purification procedures, the POSS-OH monomer with structural integrity and high purity was acquired and suitable for further treatment without possible cross-linking reaction occurred.Secondly, eight-arm star-like POSS-g-PCL polymer was synthesized by melt ring-opening polymerization. After conversion of the terminal hydroxyl group of POSS-g-PCL and methyl poly(ethylene glycol)(MPEG) to double bond(-C═CH2)and thiol(-SH), respectively, the star-like two-block POSS-g-PCL-b-PEG copolymer was synthesized by click reaction between the terminals of POSS-g-PCL-CH═CH2 and MPEG-SH prepolymers. It was observed that both the POSS-g-PCL-CH═CH2 and MPEG-SH prepolymers have high purity and high conversion of terminal groups. The possible coupling reaction between-SH groups due to oxidation for MPEG-SH was fundamentally suppressed. The POSS-g-PCL-b-PEG copolymer has a much higher molecular weight than that for POSS-g-PCL, and has a very narrow molecular weight distribution. These facts indicate a high coupling efficacy for the click reaction that isbeneficial to obtain the predominant POSS-g-PCL-b-PEG product with eight arms.Thirdly, by conversion of the terminal hydroxyl group of POSS-g-PCL and MPEG to azido(-N3) and alkynyl(-C≡CH), respectively, the star-like POSS-g-PCL-b-PEG block copolymer was synthesized via click reaction using Cu(I) as the catalyst between the terminals of POSS-g-PCL-N3 and MPEG-C≡CH prepolymers. Both the POSS-g-PCL-N3 and MPEG-C≡CH prepolymers have expected terminal structures with high purity and high conversion of terminal groups. A marginal degradation took place for the POSS-g-PCL-N3 prepolymer under the reaction conditions. The click reaction between the azido and alkynyl groups is highly controllable, which gave rise to high purity and narrow molecular weight distribution of POSS-g-PCL-b-PEG copolymer.In summary, the star-like POSS-g-PCL-b-PEG block copolymer was successfully acquired via two types of click reactions. The amphiphilic and highly-branched POSS-g-PCL-b-PEG molecule with globular structure, has potential application to develop nano-scale drug delivery micelles with controllable morphology, size and function.