Controlled two-step synthesis of n-type conjugated ladder polymers using a flow reactor

The coplanar backbone of conjugated ladder polymers (cLPs) enhances electron transfer and π-π interactions, making cLPs promising materials for various applications in organic electronics and optoelectronics. Nevertheless, the limited solubility and intricate synthetic processes for cLPs make it challenging to control their molecular weight and corresponding properties, and pose as significant obstacles to their widespread application. Here, an n-type cLP, PNDI-2BocL, was synthesized using a flow reactor for the first time, with controlled molecular weights. The polymerization reaction conditions to synthesize a precursor polymer, PNDI-2Boc, were first optimized in flow, followed by sequential ladderization in batch through the injection of an acid solution. The reaction time of polymerization was varied from 7.5 to 15 min, yielding soluble PNDI-2BocL with a number-averaged molecular weight (M_n) ranging from 9.8 to 54.0 kg/mol with a deviation of 11 %. Additionally, 15 min of polymerization and ladderization were successfully conducted in a single flow system sequentially for the first time. In the molecular weight-dependent studies, higher optical absorption at longer wavelengths, increased degrees of crystallinity, and higher electron mobilities in organic field-effect transistors (OFETs) were observed for higher molecular weight PNDI-2BocL. Our results highlight the importance of controlling the molecular weights of cLPs and demonstrate that the flow synthesis technique provides a viable solution for synthesizing soluble cLPs in a controlled, reproducible, and rapid manner.