The CG representation of P(NDI2OD-T2) is given in Fig. , in which each spherical CG site is centred at the centre-of-mass of the atoms that comprise it. Each aromatic ring was assigned to a single CG site, and the side-chain sites composed of three (site type 5) or four (site type 4) CH n groups. This mapping groups atoms whose motion is expected to be strongly correlated into the same site. We note that while the entire NDI group is relatively rigid and could theoretically be coarse-grained into a single site, doing so with a spherical CG site would not capture its significantly anisotropic shape, which is expected to impact chain packing in polymer aggregates. Site masses were taken as the sum of the masses of atoms in the AA representation that composed each CG site. Where atoms were shared between sites, such as within the NDI group, the masses were split evenly over the sites (e.g. the carbon shared between site 2 and the two sites of type 3 in the CG representation contributed 1/3 of its mass to each of these sites; see ESI Table for a list of masses).