To first summarize our overall method: Uniform seeds were prepared from polydisperse fibers of PCL 45 -b-PDMA 348 formed in ethanol (Fig. ), followed by sonication to produce short fibers with consistent size and length (mean length 24.6 ± 4.9 nm, Figs. &). Seeds were then attached to the coverslip via spin-coating, with a silicone spacer placed on top to form a reaction chamber. This sample was then mounted on a custom-built microscope, ready for imaging (Fig. ). Unimer solutions were then introduced into the reaction chamber, followed by immediate iSCAT imaging to monitor the CDSA process (Fig. ). An in-depth description of the experimental protocol and our instrumentation is provided in the Experimental Methods section and the Supplementary Information.