Effect of aggregation on backbone stiffness P(NDI2OD-T2) consists of a fused-ring NDI system connected through a bTh group. Flexibility of the backbone therefore comes largely from the rotatable Th-Th and Th-NDI bonds. As aggregation occurs in a manner in which both the NDI and Th groups π stack, aggregation has the effect of reducing the flexibility of the chain. The average Kuhn length b of each chain in a pair of aggregated (fully overlapping, N pair /N = 1) 30mers from a simulation in the poor solvent (20.0 monomers, assuming a monomer length of 1.4 nm) was found to be three times that of a single 30mer in the same solvent (6.58 monomers), corresponding to a substantial increase in bending rigidity. This increased backbone stiffness means that folding of sections of the polymer where aggregation has occurred becomes highly unlikely. Details of measurements of the Kuhn length for these and other systems are given in the ESI, Section S12.1.