Steric Effect in Kinetic Monte Carlo Simulation of Arm-First Synthesis of Star Polymers

Star polymers are attracting intense interest as functional materials because their three-dimensional topology affords tunable physicochemical properties. Yet the widely used arm-first route to star synthesis still struggles to precisely control arm number and its distribution. Simulation can reveal microscopic events that are inaccessible experimentally, but models often fail to reproduce measurements when key factors are neglected. Here, we investigate the macroinitiator-based, arm-first synthesis of polyethylene glycol star polymers by activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP). We develop a kinetic Monte Carlo model that explicitly incorporates steric congestion between growing arms─an effect frequently overlooked in prior work. Using this framework, we systematically examine how cross-linker feeding profiles, catalyst-to-reducing-agent ratios, and initial concentrations govern the evolution of molar mass and composition (core proportion) distributions during star formation. In addition, we establish a procedure for quantitatively comparing simulations with experimental data acquired by size-exclusion chromatography coupled to multiangle light scattering (SEC-MALS). Together, these advances highlight the central role of steric effects in shaping star-formation kinetics and dispersity, as demonstrated by the good agreement to the experimental results, and provide practical tools for optimizing synthesis and rigorously benchmarking simulations against experiment in polymers with complex architectures.