Degradation Kinetics of Model Hyperbranched Chains with Uniform Subchains and Controlled Locations of Cleavable Disulfide Linkages

Abstract

We developed a strategy to make model hyperbranched structure with uniform subchains and controlled locations of cleavable linkages. First, a novel seesaw-type tetrafunctional initiator with one alkyne, one disulfide linkage, and two bromine groups (≡–S–S–(Br)2) was prepared. Using such an initiator, an AB2-type macromonomer (azide∼∼alkyne∼∼azide) with one disulfide linkage at its center was prepared via successive atom transfer radical polymerization (ATRP) and azidation substitution reaction, where ∼∼ represents polystyrene chains. Further interchain “clicking” coupling between the azide and alkyne groups on the macromonomers led to model hyperbranched polystyrenes with uniform subchains and controllablly located cleavable disulfide linkages. The 1H nuclear magnetic resonance spectra, Fourier transform infrared spectroscopy, and size exclusion chromatography with a multiangle laser light scattering detector confirmed the designed degradable hyperbranched structure. Armed with this novel sample, we studied its dithiothreitol (DTT)-induced degradation in various organic solvents by a combination of static and dynamic LLS. We found that the cleavage of disulfide bonds contains a fast and a slow process. The fast one reflects the degradation of disulfide bonds on the chain periphery; while the slow one involves those inside. Both the fast and slow degradation reaction rate constants (Kfast and Kslow) are a linear function of the initial DTT concentration ([DTT]0), but the relative contribution of the two processes is mainly governed by the hyperbranched chain structure, nearly independent of [DTT]0.