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

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)/P2O5 medium. The polymerization was conducted at (i) 130 and 160 degrees C and (ii) with and without additional amounts of P2O5 and monitored by analyzing the polymerization mixture sampled at certain reaction temperature and time inter-vats with Fourier transform infrared (FT-IR) spectroscopy and gel-permeation chromatography (GPC). Both FT-IR and GPC results corroborated our previously established optimal conditions: (i) PPA:P2O5 ratio of 4:1 and (ii) efficient polymerization temperature similar to 130 degrees 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 degrees C in both air and helium atmospheres. It showed UV absorption maxima at 365-370 mn and emission maxima at 433446 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).