Synthesis And Phase Transition Of Stimuli-responsive Core Crosslinked Star Polymers

Star Polymers have complex macromolecular architectures with at least three linear chains (arms) radiating from one central core. They have been widely studied because of their unique topological structures and attractive physical and chemical properties. For instance, Compared with linear analogs with the same molecular weight, star polymers exhibit lower solution viscosity in dilute solutions. This study aim at synthesize and properties of stimuli-responsive star polymers via RAFT polymerization. The main contents are listed as follows:(1) Thermosensitive core crosslinked star polymer was synthesized via two steps. Firstly, linear thermosensitive copolymer P(OEGA-co-DEGA) was prepared by copolymerization of oligo(ethylene oxide) acrylate (OEGA) and di(ethylene oxide) ethyl ether acrylate (DEGA) using RAFT polymerization. Then linear P(OEGA-co-DEGA) was used as macroRAFT agent and star polymer with crosslinked core was prepared via crosslinking of the preformed linear macro-RAFT agent using a divinyl compound N,N’-double acryloyl cystamine(BAC). The result show that obtain star polymer exhibit reversible therm-responsive and the LCST can be affected by polymer concentration, NaCl concentration, and pH values and so on.(2) A series of well-defined multi-responsive mictoarm core crosslinked star polymers (CCS), in which poly(oligo(ethylene oxide) acrylate-co-di(ethylene oxide) ethyl ether acrylate) P(OEGA-co-DEGA) and poly(2-dimethylaminoethyl acrylate) (PDMAEA) served as the thermosensitive and pH-responsive building blocks, have been successively prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization using the "arm-first" approach. Firstly, P(OEGA-co-DEGA) and PDMAEA were prepared via RAFT technique to give well-defined linear trithiocarbonate terminated macro-RAFT agents. Then the two preformed linear macro-RAFT agents were cross-linked by a disulfide crosslinker N, N’-double acryloyl cystamine (BAC) to give the desired multi-responsive (temperature, pH and redox) star polymers. The structure and composition of CCS were characterrized by the FT-IR and 1H-NMR, the molecule weight and molecular weight distribution of the copolymers were obtained by GPC. The results show that this method can obtain narrow polydispersity star polymers (PDI< 1.3) and the molar ratio of the two arms in the CCS follows the feeding composition due to their similar conversion rate. DLS and UV-Vis were used to investigate the lower critical solution temperature (LCST) of the resultant polymers in aqueous solution. The polymers exhibit reversible thermal-induced volume phase transition and pH responsive. The results show that the LCST of star polymers in the aqueous solution could be tuned by changing the feed ratio of the arms or the pH values. For example, the LCST of the star copolymer was determined to be 38.2℃ when the molar feed ratio of P(OEGA-co-DEGA) and PDMAEA was 50:50 and polymer concentration was 1 mg/mL...