Synthesis, Characterization, and Ring-Opening Polymerization of the Cyclic Oligomers of Poly(Oxy-1,3-phenylenecarbonyl-1,4-phenylene)

Abstract

The cyclic oligomers (cyclomers) of the title composition were prepared from 4-fluoro-3‘-hydroxybenzophenone by nucleophilic aromatic substitution. Cyclomers prepared in dimethyl sulfoxide and toluene at 150−160 °C using pseudo-high dilution contained a high fraction of cyclic dimer, which was isolated by vacuum sublimation. Cyclomers prepared in o-dichlorobenzene at 180 °C using catalytic 18-crown-6 were suitable for polymerization. High-performance liquid chromatography was used to quantify the mixtures. The ring-opening polymerization of the cyclic dimer was studied using nucleophilic initiators. The conversions and relative molecular weight distributions were measured using gel permeation chromatography. Absolute molecular weights were estimated on the basis of laser light-scattering measurements using cyclomer-free polymer samples. Cesium fluoride and alkali-metal carbonates gave incomplete conversion of the cyclic dimer to very high molecular weight polymers. Potassium and cesium 4-benzoylphenolate gave essentially complete conversion and their stoichiometry controlled the molecular weights. The amorphous polymers possessed Tg = 132 °C and Mw = 60 000−180 000. The initiators are differentiated by the competition between the chain extension of the linear species generated by initiation and propagation of the ring-opening polymerization. The reactivity of the cyclomers was correlated with their 1H and 13C NMR chemical shifts. The cyclic dimer is less reactive than the higher cyclomers due to its structure. X-ray crystallography showed that the p-phenylene rings of the cyclic dimer are partially rotated out of conjugation with the carbonyls due to steric interactions with the m-phenylene rings. The steric and electronic effects, and the chemistry of the cyclomers in general, are due to the 3,4‘-catenation of the repeat unit.