A robust procedure for large scale synthesis of a high molar mass, unsubstituted poly(m,p-phenylene)

Due to the absence of functional groups and the relative inertness of phenylene units, polyphenylenes are an interesting research target for materials such as films and fibers, combining some π–π conjugation with chemical (e.g. hydrolytic) and thermal robustness. However, unsubstituted polyphenylenes practically cannot be obtained at high molar masses due to their already poor solubility at an early stage of growth. Herein, we report a synthetic route which nevertheless provides such polymers in a two-step process recently promoted by Sakamoto et al. First, a soluble precursor poly(m,p-phenylene) is obtained by AA/BB-type Suzuki polycondensation (SPC) which is then subjected to a virtually quantitative removal of its solubilizing side chains. SPC was carried out on a multigram scale using highly pure monomers which resulted in a fully reproducible and scalable route that provided up to 14 g of a substituted poly(m,p-phenylene) with a molar mass of 150 kg mol−1 (300 kg mol−1 after the removal of the common cyclic side products). This molar mass is amongst the highest ever reported in the open literature for SPC. The precursor polymer was compression moulded into films which were then subjected to side chain cleaving conditions by simple immersion into acidic solutions. Gratifyingly, the side chains could almost completely be removed with preservation of the overall film shape. We consider this work a step towards novel stiff and strong fluorescing films and fibers resistant to hydrolytic conditions.