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How Does the Branching Effect of Macromonomer Influence the Polymerization, Structural Features, and Solution Properties of Long-Subchain Hyperbranched Polymers?

Hao, Nairong, Duan, Xiaozheng, Yang, Hongjun, Umair, Ahmad, Zhu, Mo, Zaheer, Muhammad, Yang, Jinxian, Li, Lianwei
Macromolecules 2019 v.52 no.3 pp. 1065-1082
dendrimers, fractal dimensions, molecular weight, polymerization, synergism, viscosity
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,AB₃MₕyₚₑᵣᵅMₘₐcᵣₒᵝ (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.