Comparison of Direct Arylation and Zinc-Mediated Polymerizations for Naphthalenediimide–Bithiazole Polymers with Focus on Efficiency, Structure and Green Metrics

The synthesis of n-type semiconductors traditionally relies on the environmentally harmful Stille organotin polymerization. Here, we report the optimized direct arylation polymerization (DARP) of an electron-transporting polymer, poly[2,7-bis(2-decyltetradecyl)-4-methyl-9-(2′-methyl-[5,5′-bithiazol]-2-yl)benzo[lmn]-[3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone], P(NDI2DT-Tz2). A series of PNDITZ-Mn polymers with molecular masses (Mn) from 6.5 to 32.9 kg mol–1 was synthesized and investigated for charge transport and thermoelectric properties. Optimized DARP conditions yielded PNDITZ-19 K (Mn/D̵ = 18.8 kg mol–1/2.8), comparable to Zn-PNDITZ prepared via the anionic zinc protocol (16.4 kg mol–1/2.39). Thermal, UV/vis, and VT-NMR analyses confirmed that PNDITZ-19 K displays photophysical properties and structural integrity indistinguishable from those of Zn-PNDITZ. Charge transport and thermoelectric measurements gave σ ≈ 2 × 10–2 S cm–1 and a power factor near 1 μW m–1 K–2. GIWAXS analysis revealed that polymer crystallinity correlates directly with Mn/D̵, with PNDITZ-19 K showing the best performance across the series and approaching that of Zn-derived materials. Notably, DARP-synthesized P(NDI2DT-Tz2) achieved a 15-fold reduction in the E-factor (7.23 vs 108.65), highlighting its much lower environmental footprint. These results establish DARP as a sustainable and efficient method for producing high-quality n-type semiconducting polymers.