We initially applied iSCAT microscopy to monitor seeded-growth of 1D fibers: 50 µL of 2.51 nM (0.1 µg/mL) PCL 45 -b-PDMA 348 seed solution was spin-coated onto the coverslip. PCL 73b-PDMA 204 unimer was added into methanol to reach a final concentration of 0.06 µM (1.67 µg/mL) before being introduced into the reaction chamber and imaged using iSCAT. Fig. depicts a representative time series of iSCAT images collected during the formation of 1D fibers (637 nm, 4 µW µm -2 , 400 µs exposure time, 3 s -1 per frame time-lapse). Upon unimer addition, uniform fiber elongation can be monitored as the reaction proceeds (the size of PCL 45 -b-PDMA 348 seeds is below the detection limit of our setup, thus they are not observable; Movie S1). Following image segmentation (see SI, Data Analysis), the length evolution of individual fibers can be determined as shown in Fig. . Applying a previously established kinetic model for 1D fiber growth, 30 rate constants were extracted by fitting to the trajectories of length vs. time (5.1 × 10 -3 s -1 , Fig. , Supplementary Methods section). Comparing these data to previous work on poly(ferrocenyldimethylsilane)-b-polydimethylsiloxane (PFS 63 -b-PDMS 513 ) 1D fibers (10 to 30 mg/mL unimer and 0.1 mg/mL seed), we observe faster kinetics in our PCL-based system (5.1×10 -3 s -1 compared to 1.8×10 -4 to 2.13×10 -4 s -1 ) despite lower unimer and seed concentration.