UV-Vis spectroscopy provides further validation for the observed sensitivity of aggregation on molecular weight. Normalized absorption spectra of the low and high MW PII-2T solutions in CB and Dec are shown in Figure . The major two peaks at 710 nm and 655 nm correspond to the 0-0 and 0-1 transitions, respectively, while the peak at 420 nm corresponds to localized transitions from within the isoindigo moiety . While the spectral shape can be predominantly captured by the vibronic progression of a single polymer chain , the absorption peaks ratio A0-0/A0-1 is modified by J-and H-aggregation character . Comparing the low and high MW PII-2T solutions for either CB or Dec solvent, the A0-0/A0-1 ratio is larger for the high MW cases in both solvents indicating increased J-aggregation in agreement with enhanced fibril aggregation. Furthermore, comparing the A0-0/A0-1 ratio between CB and Dec solutions for both MWs shows Dec to have larger ratios indicating even further J-aggregation. Likewise, the red shifted 0-0 peak for high MW (compared to low MW) and for Dec solvent (compared to CB solvent) supports the presence of enhanced aggregation.  The degree of aggregation and agglomeration is highly sensitive to the solvent but more interestingly to the molecular weight distribution of the polymer and its polydispersity.