Radical polymerization from a single initiator molecule in a microenvironment is a nearly ideal system in which bimolecular termination, solution concentration, and viscosity changes could be neglected. In this study, we provide two facile methods of preparing polymers via atom-transfer radical polymerization (ATRP) under single-initiator conditions: tether initiators on planar substrates at superlow density through mixed self-assembled monolayers (SAMs) and encapsulated single initiators in microfluidic droplets. The molecular weight (MW) of the resultant polymers characterized by atomic force microscope-based single-molecule force spectroscopy (AFM-based SMFS) showed that the single-chain ATRP had an extraordinarily faster chain propagation rate (2 unit/s) on planar substrates and gave polymers with much higher MWs (10⁵–10⁶ g/mol) than those obtained from traditional ATRP (10³–10⁵ g/mol). The former method offered a general platform for single-chain polymer synthesis and investigation, and the latter could be amplified to obtain abundant single-chain polymers with ultrahigh molecular weight (UHMW) for commercial applications.