Monodisperse Conjugated Polymer Particles via Heck Coupling-A Kinetic Study to Unravel Particle Formation in Step-Growth Dispersion Polymerization

Palladium-catalyzed cross-coupling dispersion polymerizations follow a step-growth mechanism and deliver highly monodisperse particles. This is surprising, considering the high polydispersity usually inherent to classical polycondensations at high conversion. Here we present a novel Heck-type dispersion polymerization yielding fluorescent submicrometer particles with an extremely narrow size distribution. We investigate the kinetics of the dispersion polymerization to unravel the mechanism behind the formation of monodisperse particles. We find that, at medium conversion, only one type of oligomer forms nuclei for the particles. These seed particles phase separate from solution at a critical molecular weight. Oligomers, reaching the critical molecular weight for dissolution after this initial nucleation event, condensate onto the existing nuclei, leading to uniform growth of the particles. Higher molecular weight material is obtained toward high conversion by coupling reactions inside of the particles. The comprehension of the mechanism behind particle formation will enable adaptation of other reactions to the dispersion polymerization protocol. The monodisperse polymer particles produced here are ideal materials for self-assembled organic photonics, optoelectronics, or fluorescent biomedical probes and markers.