Synthesis of structurally controlled hyperbranched polymers using a monomer having hierarchical reactivity

Abstract

Hyperbranched polymers (HBPs) have attracted significant attention because of their characteristic topological structure associated with their unique physical properties compared with those of the corresponding linear polymers. Dendrimers are the most structurally controlled HBPs, but the necessity of a stepwise synthesis significantly limits their applications in materials science. Several methods have been developed to synthesize HBPs by a one-step procedure, as exemplified by the use of AB₂ monomers and AB' inimers under condensation and self-condensing vinyl polymerization conditions. However, none of these methods provides structurally controlled HBPs over the three-dimensional (3D) structure, i.e., molecular weight, dispersity, number of branching points, branching density, and chain-end functionalities, except under special conditions. Here, we introduce a monomer design concept involving two functional groups with hierarchical reactivity and demonstrate the controlled synthesis of dendritic HBPs over the 3D structure by the copolymerization of the designed monomer and acrylates under living radical polymerization conditions.

Fig. 1

Synthetic strategy of hyperbranched polymers (HBPs). a The AB* monomer method (self-condensing vinyl polymerization (SCVP)) and b a method using an AB* monomer with hierarchical reactivity. The olefin acts as the A functional group, and the bonds originated from the olefin in each step are indicated in red for a, b. c Vinyl telluride design. Carbon–tellurium bond dissociation energy (BDE) in kJ mol⁻¹ obtained by density functional theory calculations at the (U)B3LYP/6-31 G(d,p)(C,H) + LANL2DZ(Te) level

Fig. 2

Synthesis and characterization of dendritic hyperbranched polymers (HBPs). a Formation of the HBPs by the copolymerization of 6 and methyl acrylate (MA) in the presence of an organotellurium chain-transfer agent 9. b Schematic structures of ideal polymer products produced at [6]/[9] ratios of 3, 7, 15, 31, and 63, corresponding to dendritic generations N of 2, 3, 4, 5, and 6, respectively. c Time evolution of the consumption of 6a and MA determined by ¹H NMR analysis for the synthesis of the sixth generation (Table 1, run 5). Additional AIBN (0.2 equiv.) was added after 84 h. d Correlation among the monomer conversion, number average molecular weight, and PDI for the synthesis of the sixth generation (Table 1, run 5). e Time evolution of the SEC traces from 9 to 120 h. f SEC traces observed by a refractive index (RI) detector. g Corrected SEC traces with the weight average molecular weight determined by MALLS (M _w[MALLS]) and peak intensity determined by an RI detector. h Mark–Houwink–Kuhn–Sakurada plot for linear PMA (dendritic generation N = 0) and copolymers with N = 2, 3, 4, 5, and 6 (Table 1, runs 1–6)

Fig. 3

Structure and microscopy images of the seventh generation dendritic hyperbranched polymer. a Schematic structures of the ideal polymer product, b height image (for the magnified image, see inset). Scale bar: 200 and 20 nm (for the magnified image), and c cross-sectional profile obtained by AFM for a sample prepared by spin-casting onto a freshly cleaved mica surface with a solution of 10d (0.001 mg/ml in CHCl₃) prepared at a [6]/[9] ratio of 127, which corresponds to a dendritic generation N of 7 (Table 1, run 10)

Fig. 4

Structural analyses of branching by deuterium and ¹³C-labeling experiments. a Illustration of the linear and branched structures of 10. b ²H NMR of 10d-D. c ¹³C NMR and DEPT 135° of 10d and 10d*. Newly observed signals are highlighted as red circles. d The major and minor branched structures and their formation mechanism

Fig. 5

Synthesis of hyperbranched PMA with different molecular structures. a Linear-block-hyperbranched PMA, b dumbbell-shaped PMA, and c clover-shaped PMA