Synthesis And Characterization Of Cyclic Copolymers With Complex Architectures

Since the polymer’s properties are inherently dependent on its architecture, tailored control of polymer architecture has always attracted much attention from polymeric chemists. Because of their unique "endless" topology, cyclic polymers exhibit significant different physical properties in both solution and bulk compared with the linear counterparts, such as higher glass-transition temperature, lower hydrodynamic volume, reduced intrinsic viscosity, higher refractive index. Additionally, cyclic topology is also one of the basic architectures widely spread in the nature. The research in biological chemistry and molecular biology discovered that there were some large cyclic structures in the living body, such as cyclic DNA, cyclic peptides and cyclic oligosaccharides and polysaccharides. Therefore, the study of cyclic polymers not only has important values on thoeiy, but also is helpful for exploring the secret of life. However, compared with the significant development on the study of other complex architectural polymers, cyclic polymer has been less studied due to the synthetic difficulties and lack of the research models.In this thesis, by combination of several "living"/controlled polymerization techniques with some high efficient cyclization methods, such as "Click" chemistry and "Glaser" coupling, a series of amphiphilic cyclic copolymers with novel architectures have been designed and synthesized. The main works finished in this thesis show as follow: 1. A novel amphiphilic tadpole-shaped copolymer [linear-poly(s-caprolactone)]-b-[cyclic-poly(ethylene oxide)]-b-[linear-poly(ε-caprolactone)][(l-PCL)-b-(c-PEO)-b-(l-PCL)] was synthesized by combination of "Glaser" cyclization with ring-opening polymerization (ROP) mechanism. Firstly, a linear PEO precursor with two terminal alkyne groups and two interior active hydroxyl groups was prepared via successive ROP of EO monomers and a series of functional group transformations. Then, a cyclic PEO precursor with two active hydroxyl groups at the opposite sites was obtained by "Glaser" cyclization in dilute conditions. Finally, the target tadpole-shaped copolymer (l-PCL)-b-(c-PEO)-b-(l-PCL) was obtained by ROP of ε-Cl monomers directly from the opposite active hydroxyl groups on cyclic PEO. The self-assembling behaviours of (l-PCL)-b-(c-PEO)-b-(l-PCL) and their π-shaped analogs of (PEO/PCL)-b-PEO-b-(PCL/PEO) with comparable molecular weight in water were preliminarily investigated. The results showed that the tadpole-shaped copolymers formed intertwined fibril-like micells, however, the π-shaped copolymers formed spherical micells.2. The tadpole-shaped copolymers [cyclic-poly(ethylene oxide)(PEO)]-b-[linear poly(ε-caprolactone)(PCL)]2[(c-PEO)-b-PCL2] with one PEO ring and two PCL tails were synthesized by combination of "Glaser" coupling with ROP. First, a linear PEO precursor with two terminal alkyne groups and two hydroxyl groups at the chain middle was prepared by ROP of EO monomer and the following transformation of functional groups. Then, cyclic PEO with two hydroxyl groups at the same site was obtained by the "Glaser" cyclization. Finally, the hydroxyl groups on cyclic PEO directly initiated the ROP of ε-CL monomer to produce the target copolymers (c-PEO)-b-PCL2.3. An amphiphilic hetero eight-shaped copolymer cyclic-[poly (ethylene oxide)-b-polystyrene]2[c-(PEO-b-PS)]2composed of hydrophilic PEO and hydrophobic PS segments was synthesized by combination of "Click" chemistry with anionic ROP and ATRP mechanisms. According to "core-first" strategy, the A2B2star-shaped precursor was obtained by successive ROP of EO, ATRP of styrene and modification of functional groups. Under high dilution condition, the intramolecular cyclization by "Click" chemistry produced the amphiphilic hetero eight-shaped copolymer [c-(PEO-b-PS)]2. The self-assembling behaviors of the obtained eight-shaped copolymers [c-(PEO-b-PS)]2and their four-arm star-shaped precursors (PEO-Alkyne)2-(PS-N3)2were compared. The results revealed that although both of them formed spherical micells in aqueous solution, the sizes of micells increased significantly with the topologies of the copolymers changing from star to cycle.4. The "sun-shaped" copolymer c-PHEMA-g-(PS-b-PEO) consisting of macrocyclic poly(2-hydroxylethyl methacrylate)(c-PHEMA) as backbone and polystyrene-b-poly(ethylene oxide)(PS-b-PEO) amphiphilic block copolymers as side chains was synthesized by combination of ATRP,"Click" chemistry, and single-electron transfer nitroxide radical coupling (SET-NRC). First, a linear a-alkyne-co-azido heterodifunctional PHEMA was prepared by ATRP of HEMA using3-(trimethylsilyl) propargyl2-bromoisobutyrate as initiator, and then chlorine end groups were transformed to-N3group by nucleophilic substitution reaction in DMF in the presence of an excess of NaN3. The3-trimethylsilyl protective groups could be removed in the presence of tetrabutylammonium fluoride, and the obtained product l-(HC=C-PHEMA-N3) was cyclized by "Click" chemistry in high dilution conditions. The hydroxyl groups on c-PHEMA were transferred into bromine groups by esterification with2-bromoisobutyryl bromide and then initiated the ATRP of styrene to afford the macrocyclic molecular brushes c-PHEMA-g-PS. The PS side chains on c-PHEMA-g-PS were coupled with the TEMPO-PEO to afford the target macrocyclic molecular brushes c-PHEMA-g-(PS-b-PEO) by SET-NRC.All of the intermerdiates and target copolymers have been characterized in detail by1H NMR, GPC, FT-IR and MALDI-TOF MS. It is expected that all of these amphiphilic cyclic copolymers can be used as novel models to explore the relationship between the polymeric architectures and properties.