Core-shell type hyperbranched grafting copolymers: Preparation, characterization and investigation on their intrinsic fluorescence properties

Hyperbranched polyethylenimine (HPEI) was modified with glycidol, resulting in hydroxyl terminated HPEI (HPEI-OH). Subsequently, HPEI-OH was used as the macroinitiator to initiate the anionic ring-opening polymerization of glycidol, realizing the hyperbranched grafting copolymer with HPEI as core and hyperbranched polyglycerol (HPG) as shell (HPEI-g-HPG) through the 'grafting from' method. The diagrams from gel permeation chromatography (GPC) showed that the products were a mixture of copolymers and homopolymers. A two-step purification method was developed to separate HPEI-g-HPGs from the mixture, which was much more effective than the normally-adopted dialysis method. The hyperbranched grafting polymerization condition was optimized and the maximal grafting efficiency was found to be ca. 30%. Under the optimal grafting polymerization condition, we prepared a series of HPEI-g-HPG copolymers that have the same HPEI core, but have a different number of glycidol units in the HPG shell. The detailed structural information of these copolymers was deduced from ¹H NMR, inverse gated decoupled ¹³C NMR and GPC measurements. Although no traditional fluorophores existed in HPEI-g-HPGs, HPEI-g-HPGs could emit blue fluorescence centered at ca. 470 nm. The fluorescence intensity was influenced pronouncedly by the thickness of HPG shell, pH, oxidation time, the amount of THF in the solvent mixture. The detailed characterizations and analyses supported that the luminogen of HPEI-g-HPGs was tertiary amine oxide.