Structural characterization of quinoxaline homopolymers and quinoxaline/ether sulfone copolymers by matrix assisted laser desorption ionization mass spectrometry

Polyphenylquinoxalines (PPQs) are prepared from self-polymerizable quinoxal ine monomers that carry fluorine, hydroxyaryl (ArOH), and phenyl substituen ts. In basic media, these monomers self-polymerize via a series of nucleoph ilic aromatic substitution reactions (SNAr) in which aromatic enolates (ArO - nucleophiles) attack the electrophilic carbons bearing F leaving groups t o effect fluoride displacement. Polyphenylquinoxaline/polyethersulfone (PPQ /PES) capolymers are synthesized similarly by combining self-polymerizable quinoxaline monomers with a 1:1 molar mixture of 4,4'-dichlorodiphenyl sulf one and bisphenol A. The MALDI mass spectra of the polymers reveal that the major products up to similar to 15 000 Da molecular mass are homo- or copo lymeric macrocycles. Linear byproducts are also observed, arising from nucl eophilic ring opening of already formed macrocycles. Oligomers containing a t least one PPQ unit readily protonate upon MALDI, whereas PES homopolymers require alkali metal ion addition to became detectable, Molecular orbital calculations point out that the nucleophilic and electrophilic reactivities of the PPB monomer and the PPQ growing chains generated during propagation are comparable, allowing for continued condensations via SN. Ar, until cyc lization terminates this process. The calculations also predict a significa ntly lower electrophilic reactivity for carbons substituted by chlorine ins tead of fluorine, justifying the discrimination against incorparation of PE S units observed for the copolymers. The computationally optimized structur es of PPQ and PPQ/ PES macrocycles show a diverse array of cavity sizes and geometries which depend on the size of the macrocycle, the sequence of the repeat units, and the position of the substituents in the quinoxaline ring ; quinoxaline pendants (phenyl groups) are found to favor helical arrangeme nts in the prepared macrocycles.