Macromolecules with branched architecture via radical polymerization: Insight from computer simulations

Abstract

Branched polymers offer highly tunable properties and functionality. However, obtaining soluble, sufficiently branched macromolecules within the desired molecular weight range by conventional radical polymerization (RP) is still a challenging task, as there is no systematic investigation of this problem. In this work, we develop a three-dimensional coarse-grained molecular-dynamics model of RP in the presence of a divinyl crosslinker (CL) and a chain-transfer agent (CTA). Simulations are based on the Kremer–Grest bead-spring framework with Langevin dynamics under good-solvent conditions and include stochastic reactions: initiation, propagation, crosslinking, chain transfer, and termination. Macromolecular architecture is quantified by graph-based decomposition into dangling ends, elastically active subchains, and cycles, and by extracting an effective fractal dimension from the scaling of the radius of gyration with molecular mass. Two distinct regimes emerge. At low CTA content, gelation occurs at relatively small conversion; a rapidly growing network quickly dominates the molecular-weight distribution, which broadens substantially, while the population of isolated branched macromolecules diminishes. Increasing the CTA content shifts gelation toward high conversion, enabling the formation of a stable sol fraction enriched in high-molecular-weight branched molecules. Phase diagrams over 2%–16% crosslinker and 0%–8% CTA identify a simple optimal condition: the gel-point conversion approaches unity along [CL] = 2[CTA], consistent with an average of two effective intermolecular attachment points per growing chain. Along this optimum line, higher crosslinker content produces more compact branched macromolecules, implying higher coil-overlap concentrations and lower intrinsic viscosities at fixed molecular weight. These results provide practical, quantitative guidance for selecting reagent ratios to synthesize soluble branched polymers via standard RP.