Perfectly alternating linear-dendritic polymers via dithiol–yne click chemistry

Abstract

A perfectly hybrid alternating linear-dendritic (ALD) copolymer with linear dithiol spacers and 2,2′-bis(hydroxymethyl)propionic acid (bis-MPA) dendrons was synthesized through thiol–yne “click” chemistry. This new ALD polymer has a dendritic macromonomer with an alkyne focal point (A₂), that can react with a dithiol linker (B₂) to undergo a step-growth polymerization. The dendron generation (G1–G4) and the alkyl length of the linear dithiol were evaluated to determine the highest increase in M_n with a maximum dispersity of 2.0. Size exclusion chromatography (SEC) analysis showed that the polymerization of propargyl[G4]Ac with alkyl dithiols primarily produced oligomers (dimers and trimers), while propargyl[G1]Ac monomers resulted in alternating copolymers with a higher number of repeat units (approximately 7 repeating units). The size of a linear molecule also influenced reaction kinetics, with 1,4-butanedithiol leading to ALD copolymers of lower number of repeat units compared to 1,6-hexanedi and 1,10-decanedithiol in a 30-minute reaction time under UV. In addition, both cyclic and linear structures were observed with matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. A library of hybrid ALD copolymers was synthesized, and all products were further characterized through MALDI-TOF MS, SEC, and nuclear magnetic resonance (NMR).