A new class of organic nanoparticles through hyperbranching and crosslinking

Abstract

Brush-modified nanoparticles, created by grafting functional polymers from nanoparticle surfaces, have demonstrated their versatility in various applications.^[1] Traditionally, these nanoparticle brushes are formed by tethering polymer chains onto surface-modified “inorganic” particles. However, customizing the internal molecular structure of these nanoparticles is quite limited. In contrast, “organic” nanoparticles (oNPs) offer significant potential for incorporating small functional molecules, enabling applications in areas such as luminescence, sensing, and drug delivery.^[2] Additionally, materials composed of oNPs are less dense, making them suitable for lightweight equipment, for example, for soft robotics. Despite these advantages, realizing oNPs remains a challenge. Most synthetic approaches rely on “bottom-up” methods such as self-assembly. However, in these systems labile interactions can undermine performance and long-term stability under harsh conditions.

In a recent breakthrough, Matyjaszewski, Bockstaller, and their teams have developed a new class of oNPs by leveraging macromolecular architecture through concurrent hyperbranching and crosslinking, utilizing atom transfer radical polymerization (ATRP) in microemulsion (Figure 1A).^[3] Unlike previous work on hyperbranched homopolymers such as poly[2-(2-bromoisobutyryloxy)ethyl methacrylate] (PBiBEM),^[4] the new method introduces copolymerization with ethylene glycol dimethacrylate (EGDMA) to enhance internal rigidity. Within a confined space, these inimer-based oNPs had tunable sizes and mechanical properties through varying crosslinking densities. With BiBEM composition varying from 60 to 90 mol%, the resulting oNPs are spherical macromolecules (M_n,MALS ranged from 2 to 6 million Daltons), with nanometer-scale diameters, uniform distribution, and minimal aggregation after purification. Also, the increasing crosslinker content yielded smaller oNPs (diameters from 17 to 36 nm) with lower dispersity (M_w/M_n), and greater rigidity (Figure 1B). Furthermore, the addition of sodium dodecyl sulfate (SDS) as an anionic cosurfactant improved uniformity, achieving size dispersity below 3% (Figure 1C).

Due to the high concentration of alkyl bromide ATRP functionality from the inimer BiBEM and the retention of polymer chain-end functionality via ATRP, these oNPs are well suited as effective macroinitiators for surface-initiated ATRP (Figure 1D). Surface grafting of poly(methyl methacrylate) brushes from oNPs demonstrated high grafting density (σ ∼ 0.5 chains nm⁻²), indicating the potential for direct assembly of brush-modified oNPs or their integration into matrix materials. As crosslinking density increased, each oNP was densely decorated with approximately 2000 chains with near uniform M_n ∼ 40 kDa, yielding individual oNP brush macromolecules with a total molecular weight up to approximately 30–100 million Daltons. Atomic force microscopy revealed a distinct “core and shell” structure in the most rigid oNPs grafting relatively soft poly(methyl acrylate) (with a T_g of approximately 10°C) brushes at room temperature (Figure 1E).

This research provides a foundational comprehension of the synthesis and properties of this new class of oNPs. The corresponding brush-tethered oNPs hold promise for innovative applications across various nanomaterial technologies and aggregate science.^[5] Future research should focus on exploring functionalization opportunities, such as fluorescence, and evaluating the performance of these oNPs in practical applications, building on the robust groundwork established by this study.

The authors declare no conflict of interest.