How Does the Branching Effect of Macromonomer Influence the Polymerization, Structural Features, and Solution Properties of Long-Subchain Hyperbranched Polymers?
This work aims to elucidate how the branching effect of macromonomer influences the polymerization, structural features, and solution properties of ABn long-subchain hyperbranched polymers (LHPs). Our result reveals that compared with linear AB2 macromonomers, star AB3 macromonomers result in the suppression of chain extension, and the enhancement of macromonomer self-cyclization during the preparation of LHPs by “click” polymerization, due to the branching-enhanced steric hindrance effect. The combined triple-detection SEC and stand-alone LLS studies of unfractionated and fractionated AB3 LHPs unambiguously demonstrate their statistically fractal nature. Namely, the intrinsic viscosity ([η]) and radius of gyration (Rg) are scaled to the macromonomer molar mass (Mmacro) and the total molar mass (Mhyper) as [η] = Kη,AB3MhyperνMmacroμ (ν ≃ 0.39, μ ≃ 0, and Kη,AB3 ≃ 0.29 mL/g) and Rg = HR,AB3MhyperαMmacroβ (α ≃ 0.47, β ≃ 0, and HR,AB3 ≃ 3.6 × 10–2 nm). Surprisingly, [η] and Rg are both almost independent of Mmacro (μ ≃ 0 ≃ β), indicating a similar draining property and local segment density for LHPs with different subchain lengths, which is different from the classic AB2 systems (μ ≃ 0.3 and β ≃ 0.1). A comparison of results for ABn LHPs (n = 2, 3) and short-subchain hyperbranched systems indicates that the fractal dimensions (f) for LHPs are generally smaller than short-subchain systems, whereas f is not sensitive to the local segment density or branching pattern. A combination of experimental observation and Langevin dynamics simulation of ABn dendrimers and LHPs further reveals (i) the segment back-folding phenomenon is prominent only for ABn (n ≥ 3) LHPs systems because it is mainly dominated by the macromonomer branching effect, rather than the internal subchain length, and (ii) the trend for segment interpenetration increases remarkably as Mmacro increases for both dendrimers and LHPs. The result also indicates that the unique synergistic effect of segment back-folding and segment interpenetration in AB3 system is the most probable reason for the observed Mmacro independent solution properties. Specifically, because of the unique synergistic effect, small macromonomer/oligomer chains can interpenetrate more easily into hyperbranched oligomer chains composed of longer subchains and subsequently “click” couple with the back-folded segments in the interior space of LHPs, which eventually could lead to a similar draining property and local segment density for AB3 LHPs with different subchain lengths.
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