Figure shows the IDTBT16 monomer (where 16 is the number of carbon atoms in each sidechain) and polymer structures with 5, 10, and 20 repeating units. Note that the molecular weight of IDTBT16-20 (≈ 26,000 g/mol) is in the range of experimentally synthesised polymers . Accordingly, three simulation boxes consisting of 100 IDTBT16-5, 50 IDTBT16-10, and 25 IDTBT16-20 chains, were constructed (see SI section 1 for force field parameters and model details). Annealing MD simulations (based on a sub-Tg equilibration protocol, suitable for MD polymer equilibration ) were employed to equilibrate polymer melts (see Figure , and SI section 2 for details). The average end-to-end distance Le of polymers in simulation box was monitored during equilibration (Figure ). As shown, the required number of annealing cycles for equilibration of polymers considerably increases with chain length so that at least 60 annealing cycles (> 1500 ns simulation time) is needed to obtain an equilibrated IDTBT16-20 model. It should be noted that the average Le for equilibrated IDTBT16-20 is in quantitative agreement with predicted Le value (dashed line in Figure ) based on worm-like chain model and the simulation of shorter oligomers, confirming the good equilibration of the model (see SI section 2 for details of this analysis). Also, as indicated in Figure , IDTBT16-5 shows fully stretched chains in the equilibrated state (Le/Lc=1.00, where Lc is polymer contour length) while longer polymer chains (i.e., 10 and 20mer) are not fully stretched. torsional potential (see SI section 1) both at 500 K. The ΦIDT-BT distribution as obtained from our models show two maxima at 0 ° and 180 ° and a rather wide torsional distribution (≈ ±30 ° at half-height of both peaks) suggesting a non-planar and non-rigid backbone. It should be noted that a great match between (the peak position and the width of peaks of) Boltzmann distribution and the ones obtained from our MD models exist.