Xiaosong Cao

Synthesis of acid-degradable hyperbranched polymers by chain-growth CuAAC polymerization of an AB3 monomer

A tetrafunctional AB3 monomer that was composed of one alkynyl group, three azido groups and one acetal linker was used in the one-pot copper-catalyzed azide–alkyne cycloaddition (CuAAC) polymerization for producing acid-degradable hyperbranched polymers (HBPs). In various feed ratios of the AB3 monomer to a B3 core, the polymerizations demonstrated a chain-growth mechanism with a linear increase of molecular weight versus conversion, low polydispersity and a high degree of branching (DB). The large amount of terminal azido groups on the HBPs periphery were further modified via reaction with an alkynyl-terminated poly(ethylene glycol) (PEG) to produce water-soluble PEGylated HBPs. Under acidic conditions, both the HBPs and the PEGylated HBPs exhibited clean and fast degradation into low-molecular weight compounds, confirming the labile acetal linkers in the backbone of HBPs.

Highly Branched Polymers with Layered Structures that Mimic Light-Harvesting Processes

Hyperbranched polymers (HBPs) with decorated donor and acceptor chromophores in different domains were constructed to demonstrate the function of light harvesting in a polymeric nanostructure. Taking advantage of our recently developed chain-growth copper-catalyzed azide-alkyne cycloaddition polymerization, two structural parameters in the HBPs, for example, the molar ratio of the acceptor Coumarin 343 in the core to the donor Coumarin 2 on the periphery, and the average distance between these two layers, could be independently varied in a one-pot synthesis. The results demonstrated an efficient energy transfer from the excited Coumarin 2 to the ground-state Coumarin 343 in the core, with the efficiency of the energy transfer reaching as high as 98 %. The excited Coumarin 343, after receiving energy from donor Coumarin 2 emitted higher fluorescence intensity than when directly excited, indicating an observed light concentration effect from the periphery dye to the central dye in one polymer structure.

Preparation of water-soluble hyperbranched polymers with tunable thermosensitivity using chain-growth CuAAC copolymerization

Thermoresponsive hyperbranched polymers with a dangling oligo(ethylene oxide) (EOx, x as the averaged number of EO units) chain on every monomer unit were constructed using the chain-growth copper-catalyzed azide–alkyne cycloaddition (CuAAC) copolymerization of two AB2-F monomers with EO3 and EO7.5 as F groups. The one-pot polymerization produced hyperbranched polymers with a linearly increased molecular weight versus conversion and narrow molecular weight distribution (Mw/Mn ∼ 1.1). The composition of hyperbranched copolymers was governed by the monomer feed ratio and directly determined the cloud temperature of the polymers in water. The cloud temperature could be further tuned via periphery modification of the hyperbranched polymers using alkynyl-terminated oligomeric EO3 chains, in the range of 17 to 60 °C within our investigation.

Tandem Functionalization in a Highly Branched Polymer with Layered Structure

A hyperbranched polymer with multilayer structure was developed to demonstrate the possibility of highly efficient tandem functionalization reactions at different domains within one nanostructured platform. The polymer scaffold was constructed by chain-growth copper-catalyzed azide-alkyne cycloaddition polymerization of three functional monomers with sequential monomer addition in one pot. Subsequent reactions with different monomer units resulted in efficient functionalization of each segment with construction of a highly sophisticated polymer structure by a robust procedure. As a proof of concept, the ability of this polymer structure to quantitatively load six species of guest molecules through three different types of conjugation reactions was demonstrated.

Tunable Fluorescence from a Responsive Hyperbranched Polymer with Spatially Arranged Fluorophore Arrays

In the present study, a water-soluble hyperbranched polymer platform that contained a Förster resonance energy transfer (FRET) array and exhibited varied fluorescence in response to solvent, light, and CN- anion stimuli was constructed. The use of chain-growth copper-catalyzed azide-alkyne cycloaddition polymerization (CuAACP) enabled accurate control of the ratio and distance of three incorporated fluorophores, coumarin (Cou), nitrobenzoxadiazole (NBD), and photoswitchable spiropyran (SP), that could be reversibly transformed into the red-emitting merocyanine (MC) state. Within the FRET system, the energy flow from Cou to MC was significantly enhanced by the introduction of NBD as a central fluorophore relay. Moreover, the energy-transfer efficiency was increased by changing the solvent from tetrahydrofuran to more polar water; this was accompanied by a clear color change and fluorescence behavior. These correlations of polymer composition and solvent polarity to the FRET efficiency were finally applied to the effective detection of CN- anion; thus demonstrating a function of this polymer as a CN- sensor.