The assembly of nanoparticles holds potential for materials with yet unachieved optical, electronical and mechanical properties. This requires an access to nanoparticles with defined shapes and uniform sizes. Especially particle shapes beyond spheres are desirable as these anisotropic particles offer mutual directional interactions and properties, as studied in our Collaborative Research Center SFB1214. For metal or metal oxide nanoparticles a variety of different particle shapes is accessible, while for polymers, an important material class, shapes other than spheres are only rarely found.
As a theoretical consideration towards this aim, particles each composed of one polymer chain of identical length should be uniform in size. Single-chain particles are formed by post-polymerization collapse of a single polymer chain into one particle of uniform size (or, more precisely, volume). However, these methods often require post-polymerization steps and complex synthetic routes. In contrast, catalytic olefin polymerization with late transition metals is a straightforward and highly-productive method to form polymers with precise microstructure. With these two concepts in our minds, we found our major challenge: Can we design a direct polymerization to single-chain uniform-shape monodisperse nanoparticles based on a highly productive catalytic process? And can we demonstrate this for an important synthetic material like polyethylene?
Traditional insertion polymerization catalysts are extremely sensitive and not applicable under aqueous conditions, required for efficient compartmentalization and colloidal stabilization. This can be overcome by less oxophilic late-transition metal nickel catalysts, that can generate high-molecular-weight polyethylene in aqueous polymerizations. This is formed as nanocrystals with an unusually high degree of order that arises from the immediate deposition of the growing chain on the crystal growth front. Employing this unique process for our aims is however difficult, as two main features are required: On one hand, the chain formation process needs to be strictly living without any deactivation reactions and, on the other hand, these chains must be immediately folded into one crystal to ensure an undisturbed living particle growth (Fig. 1).
This requires polymerization methods, with an advanced catalyst system and a robust polymerization procedure. Based on our mechanistic understanding, we designed a new catalyst with long fluorocarbon groups in the ligand structure that directly influence the active center electronically to enable a highly controlled and living polymerization process. Additionally we performed a comprehensive polymerization study with a customized pressure reactor setup to gain insights into chain and particle growth and identify ideal polymerization conditions.
Remarkably, this aqueous catalytic polymerization is of a truly living nature (Fig. 2a) and produces well defined nanocrystals, that were found to grow larger uniformly together with increasing molecular weights (Fig. 2b).
A comparison of the average mass of a particle (size analysis from TEM and AFM data) with the molecular weight of the formed polymer shows the particles to be composed of a single chain. This means that one particle is grown by one active Ni(II) site and that the final particle size is determined by the time of growth (given by the duration of the polymerization experiment).
Preliminary findings show these anisotropic nanocrystals to assemble with aligned orientation of adjacent particles. This underlines their utility to create advanced polymer particle based materials.
Full article is available here.
(M. Schnitte, A. Staiger, L. A. Casper, S. Mecking, Nature Communications 2019, 2592.)