Efficient and economical synthesis of dendrimer-like polystyrene with long subchains through arm-first divergent strategy

Abstract

An efficient and economical route for synthesizing well-defined dendrimer-like polymers with long subchains has been developed based on the “arm-first” divergent approach. 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 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) the transformation of the terminal bromo group of the former generation (G₁) into azido group; (2) alkynyl–azido click reaction between the dendrimer-like polymer and the linear seesaw-type macromonomers; (3) the removal and recovery of excess macromonomers through precipitation in a selective non-solvent. Repeating these steps led to the formation of dendrimer-like PSt up to the fifth generation (G₅) with polydispersity index below 1.18. 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 entire 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. The thermal and solution properties of dendrimer-like PSts were investigated from the viewpoint of generation number, based on the measurements of glass transition temperature and intrinsic viscosity.