Efficient tin-free route to a donor-acceptor semiconducting copolymer with variable molecular weights

For the fabrication of efficient photovoltaic devices and thin-film transistors, π-conjugated polymers with high molecular weight are desirable as they frequently show superior charge transport, morphological, and film-forming properties. Herein, we present an extremely fast tin-free method to polymerize a naphthalene diimide-dithiophene-based anion-radical monomer in the presence of Pd catalyst having bulky and electron-rich tritert-butylphosphine ligands (Pd/P^tBu₃). With this method, the corresponding semiconducting polymer, PNDIT2 (also known as P(NDI2OD-T2 or N2200) with a molecular weight in excess of 1000 kg/mol can be obtained quickly at room temperature and at rather low catalyst concentrations. In general, molecular weights of resulting polymer can be regulated by reaction conditions (e.g., catalyst concentration and reaction time). Besides high molecular weight PNDIT2 (e.g., with M_N ∼ 350 kg/mol, _M =2.9), PNDIT2 with moderate molecular weight (e.g., M_N ∼ 110 kg/mol, _M = 2.3) and low molecular weight (e.g., M_W ∼ 12 kg/mol, _M = 1.9), can also be obtained. It was found that thus-prepared PNDIT2 exhibits field-effect electron mobilities of up to ∼0.31 cm²/(V s), similar to the Stille-derived N2200 control polymer (up to ∼0.33 cm ²/(V s)). Preliminary studies demonstrated that Pd/P ^tBu₃ catalyst is remarkably efficient in polymerizing of other anion-radical monomers, such as isoindigo-, and diketopyrrolopyrrole-based ones, although conventional Ni and Pd catalysts (e.g., Ni(dppp)Cl₂, Ni(dppp)Cl₂, Pd(PPh₃)₄) failed to polymerize these monomers.