The CG bonded parameters were parameterised based on AA simulations of P(NDI2OD-T2) trimers in DCB, so as to include the bonds between monomers, at the same concentration as the monomer simulations (8 trimers, 3229 DCB molecules). Note that this concentration is substantially higher than both those in experiments of P(NDI2OD-T2) aggregation and used later in the CG simulations (5-10 g/L). This higher concentration was used to obtain good statistics for the configurational distributions needed for the coarse-graining procedure at a reasonable computational expense. The parameters described above for P(NDI2OD-T2) were used, along with unmodified OPLS parameters for the solvent (see ESI, Section S13). DCB was chosen as the solvent based on solution-phase UV-visible spectroscopy data of P(NDI2OD-T2), which indicate little aggregation, guaranteeing a homogeneous system as required by the coarse-graining process.