Tianxiang Tong

The development of hyperbranched polymers (HBPs) with controlled structures remains a significant challenge in polymer chemistry. A one‐step synthesis of HBPs has been previously reported with controlled molecular weight and low dispersity by organotellurium‐mediated radical polymerization (TERP) in the presence of an “evolmer,” which induces controlled branch structure. Although indirect evidence supports the structural control of these HBPs, direct visualization of their branch architecture has been lacking. In this study, the use of atomic force microscopy (AFM) is demonstrated to directly visualize individual HBP molecules synthesized by this method using octadecyl acrylate (ODA) as an acrylate monomer. Two HBP samples with different branch molecular weights with a theoretical dendritic generation ( G _n ) of 4 are synthesized and analyzed on highly oriented pyrolytic graphite (HOPG) substrates. High‐resolution AFM phase imaging successfully reveals dendritic branch structures with 16 branches, closely matching the theoretical values of 17 for a G _n = 4. These results provide direct evidence of the controlled dendritic architecture of HBPs synthesized by this method. This study thus represents a critical step toward advancing structurally defined HBPs for materials applications.

Dendritic hyperbranched poly(methyl acrylate)s (HBPMAs) with controlled molecular weight and low dispersity (Đ ≤ 2) were synthesized by radical copolymerization of MA and a branch-inducing monomer, evolmer 2b, in the presence of organotellurium chain transfer agent (CTA) 1, by targeting theoretical generation (Gn) of 3-5 and branch molecular weight (MBL) of 0.25Me - 2.0Me, with Me (= 11,000) being the entanglement molecular weight of PMA. The control of the self-similar branch layer structure in HBPMAs thus synthesized was examined by measuring their linear viscoelastic moduli (G* = G' + iG"). For all Gns having MBL = 0.25 Me, the storage modulus G' was lower than the loss modulus G" at all ω examined. This unentangled feature was observed even when the portion of the chain spanning the two outmost ends of the branches had the molecular weight of 2.0Me, possibly because of a lack of deep penetration among those Gn chains. In contrast, for MBL ≥ Me, G' was higher than G" in a given range of ω, and this range became wider and the relaxation mode distribution therein became broader on an increase of MBL, which is characteristic of entangled branch polymers. For the entangled HBPMA chains having the same MBL but different Gn, hierarchically self-similar relaxation based on the branch retraction mechanism was confirmed for the G' and G" data: The data for higher Gn agreed with the data for lower Gn shifted according to this self-similarity. For the unentangled BPMA chains of different Gn, the G' and G" data obeyed the Rouse-type relationship based on the self-similarity but without the hierarchy. These results support good control of the dendritic and branch structure of the HBPMAs synthesized with the current method.