Diffusion-ordered NMR spectroscopy of conjugated polymers: a complementary method for determining polymer size

Size exclusion chromatography (SEC), the standard way to characterize the molecular weight (MW) and distribution of polymers, is less suited for characterizing conjugated polymers that strongly absorb at the detection wavelengths. We demonstrate diffusion-ordered NMR spectroscopy (DOSY) as a complementary method for characterizing the size and size distribution of conjugated polymers. Starting with four distinct batches of poly(3-hexyl thiophene), or P3HT, whose MW distributions had been fully characterized by a combination of SEC and NMR end-group analysis, we establish a power-law relationship between the weight-average MW and the diffusion coefficient measured through DOSY. Quantification of the distribution of diffusion coefficients with the power-law relationship yields MW distributions that are qualitatively similar to those extracted from SEC measurements. We extended this approach to characterizing poly[4-(4,4-dihexadecyl-4H- cyclopenta[1,2-b:5,4-b′]dithiophen-2-yl)-alt-[1,2,5]thiadiazolo- [3,4-c]pyridine], or PCDTPT, with absorption in the visible that precludes SEC characterization of its MW. Applying the same power law on the diffusion coefficients obtained by DOSY measurements has yielded P3HT-equivalent MW and MW distribution of PCDTPT.