Preparation And Characterization Of POSS-Based Star-Shaped Polymers Via Living Anionic Polymerization

Star-shaped polymers are defined as polymers having more than three linear polymeric chains of comparable lengths radiating from a core. Generally speaking, these polymers could be synthesized by “arm-first”, “core-first” and “coupling-onto” methodologies. Polyhedral oligomeric silsesquioxane(POSS) has attracted considerable interests in materials science due to its well-defined nano-scale organic-inorganic structure, which makes it an ideal building block for constructing nano-structured hybrid materials and nanocomposites. It is found that the addition of a small amount of POSS can significantly improve the properties of polymer matrix, such as thermal performance, oxidation resistance, mechanical properties, surface properties and biocompatibility. As a representative living polymerization method, living anionic polymerization has been widely used for the preparation of star-shaped polymers with well-defined structure, high molecular weight and narrow polydispersity indices(PDI). However, the preparation of star-shaped polymers by using the reaction of living polymer chains and POSS is rarely reported. This thesis mainly focuses on the synthesis of star-shaped polymers based on POSS through a combination of living anionic polymerization, CuAAC “Click” chemistry and hydrosilylation. Part I: Synthesis and characterization of eight-arm hetro-arm star-shaped polymers(Polystyrene)lithium(PS-Li) was firstly prepared by living anionic polymerization. The mono-hydroxyl functionalized seven-arm polystyrene(7PS-POSS-OH) was then prepared via the direct addition reaction between PS-Li and VPOSS-OH with one hydroxyl and seven vinyl groups. Subsequently, the hydroxyl group of 7PS-POSS-OH was changed to azide group to obtain the 7PS-POSS-N3. At the same time, two kinds of homopolymers having alkynyl group(Propargyl-PCL and Propargyl-PEG) were synthesized. The CuAAC “Click” reactions between 7PS-POSS-N3 and Propargyl-PCL or Propargyl-PEG were conducted to obtain the eight-arm star-shaped polymers 7PS-POSS-PCL and 7PS-POSS-PEG. The chemical structures, molecular weights and PDIs of these polymers were characterized by 1H-NMR, 13C-NMR, GPC, FT-IR and MALDI-TOF MS. The glass transition temperature(Tg) and thermal decomposition temperature were measured by differential scanning calorimetry(DSC) and thermal gravimetric analysis(TGA), respectively. The Tg of 7PS-POSS-PCL and 7PS-POSS-PEG were located between the Tg of 7PS-POSS-N3 and the base PS, and the melting peaks of PCL or PEG in the star-shaped polymer were barely visible. It was also found that about 6% residue was left at 600 °C, which may be due to the incorporation of POSS segment.Using the similar method, two kinds of star-shaped polymers [7PI-POSS-OH and 7(PS-PI)-POSS-OH] were prepared by living anionic polymerization. Unfortunately, the subsequent CuAAC “Click” reactions between 7PI-POSS-N3 or 7(PS-PI)-POSS-N3 with Propargyl-PCL(or Propargyl-PEG) were unsuccessful revealed by GPC analysis, which may be due to the entrapped azide group derived from the entanglement of more flexible PI chains compared with PS chains. Part II: Synthesis and characterization of amphiphilic eight-arm star-graft copolymer 8[(PS-co-PVPDMS)-g-PEG]-POSSFrom the study of the first part, it is found that the modification of star-shaped polymer can be achieved by the use of functional POSS derivative. However, this approach is also somewhat limited since this modification is sensitive to the steric hindrance of szide group. Therefore, we try to functionalize the star-shaped polymer by increasing the number of functional group in the following work. Herein, sec-butyllithium(s-Bu Li) was first used to initiate the living anionic copolymerization of dimethyl(4-vinylphenyl)silane(VPDMS) and styrene(St) to yield two living copolymeric chain(PS-co-PVPDMS)-Li with different Si-H contents. Subsequently, the eight-arm star-shaped polymers were prepared by the direct addition reaction of(PS-co-PVPDMS)-Li and octavinylPOSS(OVPOSS) with eight vinyl groups. A purified 8(PS-co-PVPDMS)-POSS product containing several Si-H groups was then obtained after fractionation precipitation. It was indicated that the resulting star-shaped copolymer possess well-defined structure and narrow PDI, and the Si-H groups can keep stable during polymerization. Finally, the amphiphilic eight-arm star-graft copolymer 8[(PS-co-PVPDMS)-g-PEG]-POSS was synthesized by the hydrosilation reaction between 8(PS-co-PVPDMS)-POSS and Vinyl-PEG with one vinyl group at the end.