Hyperbranched poly(phenylquinoxaline-ether-ketone) synthesis in poly(phosphoric acid)/P2O5 medium: Optimization and some interesting observations

Abstract

A new self-polymerizable AB 2 monomer, 2,3-bis(4-phenoxyphenyl) quinoxaline-6-carboxylic acid, was synthesized from the double condensation reaction of 4,4'-diphenoxybenzil and 3,4-diaminobenzoic acid. It was polymerized to afford the corresponding hyperbranched poly(phenylquinoxaline) (PPQ) via a diaryl ketone formation from the Friedel-Crafts reaction of the carboxylic acid group (A) and the phenoxy ring (B) in polyphosphoric acid (PPA)/P 2O 5 medium. The polymerization was conducted at (i) 130 and 160 °C and (ii) with and without additional amounts of P 2O 5 and monitored by analyzing the polymerization mixture sampled at certain reaction temperature and time intervals with Fourier transform infrared (FT-IR) spectroscopy and gelpermeation chromatography (GPC). Both FT-IR and GPC results corroborated our previously established optimal conditions: (i) PPA:P 2O 5 ratio of 4:1 and (ii) efficient polymerization temperature ∼130 °C. Furthermore, the series of 10 aliquots taken from the mixture prepared under the optimal conditions, precipitated into water, and stored in the vials for a month showed visually a consistent trend and a transition in the polymer densities and collectively allowed the visualization of a changeover in the macromolecular architecture from a "fanlike" conformation at the early stage of polymerization to a denser, globular conformation at higher molecular weights. The resultant hyperbranched PPQ was thermally stable with 5% weight loss at temperatures over 500 °C in both air and helium atmospheres. It showed UV absorption maxima at 365-370 nm and emission maxima at 433-446 nm. The subsequent chain-end functionality conversion for the parent hyperbranched polymer could be performed in either a one-pot fashion or a two-staged process. Finally, some anomalous dilute-solution behavior (negative or "inverse" polyelectrolyte effect) was observed for the chain-end-modified hyperbranched polymers that appeared to be significantly influenced by the nature of solvents (acidic vs basic) and chain-end groups (polarity).