Synthesis Of Nonlinear Polymers By Arm-first Strategy

There exists a close relationship between the structure and properties of polymer. According to the difference of their topology structure, un-crosslinked polymer can be divided into linear polymer and nonlinear polymer. Compared with the linear polymer, nonlinear polymer has many excellent properties due to the particularity of their structure. In recent years, the development of living/controlled radical polymerization and click chemistry makes the synthesis of various nonlinear polymers with complex structure much easier. Combination of these frontier synthetic methods in nonlinear polymers has become an important subject of polymer science, prompting the synthesis of complex nonlinear polymer, the investigation of their self-assembly behavior and the development of new materials from them. This master thesis summarizes the progress in synthesis and application of some relevant nonlinear polymers. As well, we prepared dendrimer-like polystyrene (PSt) with long subchains through arm-first divergent strategy and studied the dependences of intrinsic viscosity and glass transition temperature on the generation number. We also obtained a star-shaped polymer with porphyrin core by ATRP and click chemistry, and studied it’s self-assembly behavior in aqueous solution with temperature. The specific research contents and main results are as follows:1. An efficient and economical route to synthesize well-defined dendrimer-like polymers with long subchains has been developed based on the "arm-first" divergent approach. At first, a linear seesaw-type polystyrene (PSt) macromonomer with one alkynyl group at the center and one bromo group at each end was prepared as the constituent unit, and a tri-arm star PSt with three terminal bromo groups was prepared as the first generation of dendrimer-like PSt (G1). Then, the second generation of dendrimer-like PSt (G2) carrying six terminal bromo groups was obtained through:(1) transformation of the terminal bromo group of the former generation (G1) into azido group;(2) alkynyl-azido click reaction between dendrimer-like polymer and linear seesaw-type macromonomers;(3) removal and recovery of excess macromonomers through precipitation in a selective non-solvent. Repeating those steps led to the achievement of dendrimer-like PSt up to the fifth generation (G5) with polydispersity index below1.18. Interestingly, excess PSt macromonomers were separated from the mixture of click reaction by selective precipitation instead of time-consuming fractionation and re-used in the following generation growth, promoting the efficiency and economy of the overall synthesis. The whole generation growth was monitored by nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy and gel permeation chromatograph with a multi-angle laser light scattering detector. Thermal and solution properties of dendrimer-like PSt were investigated in view of generation number, based on the measurements of glass transition temperature and intrinsic viscosity.2. A thermo-sensitive star-shaped polymer with prophyrin core and poly(N-isopropylacrylamide)(PNIPAM) arms was synthesized by the click reaction between alkynyl-modified porphyrin and block copolymer of PEG45-b-PNIPAM55[PEG:poly(ethylene glycol)] having one azido group anchored at the chain centre. Its chemical structure was confirmed by1H NMR spectroscopy, FTIR spectroscopy and GPC, etc. The star-shaped polymer with prophyrin core was water-soluble and showed a strong fluorescence in near infrared region, which enabled the noninvasive fluorescence monitoring of drug delivery in vivo. Because of the presence of PNIPAM, the star-shaped polymer can easily self-assemble into vesicles in aqueous solution with the lower critical solution temperature (LCST) at32℃. The formation of vesicles aggregates were confirmed by dynamic light scattering (DLS) and scanning electron microscope (SEM). Fluorescence and thermosensitive properties of this star-shaped polymer suggest its potential application in fluorescence monitoring drug delivery in vivo.