A comparative study of intrachain cyclization and solution properties of long-subchain hyperbranched polymers prepared via Y-type and V-type macromonomer approaches

Abstract

V-type (B∼∼A∼∼B) and Y-type (A∼∼∼∼B₂) macromonomer approaches are two of the most widely adopted synthetic methodologies for the preparation of long-subchain hyperbranched polymers (LHPs) with a controlled subchain length. However, little attention has been paid to the difference of the structural regularity of hyperbranched polymers prepared via the two approaches in previous related studies. This work aims to elucidate how these two synthetic approaches quantitatively influence the polymerization, branching pattern and solution properties of the prepared hyperbranched polymers. First, two sets of model hyperbranched polystyrenes (HPS_V and HPS_Y) were prepared from V-type and Y-type macromonomers by a combination of atom transfer radical polymerization (ATRP) and azide–alkyne cycloaddition coupling reaction, respectively. The intrachain cyclization study indicated that the macromonomer self-cyclization during the interchain coupling process was strongly affected by the chain length and the structure of the used macromonomer as well as the reaction concentration. The V-type macromonomer was found to be less inclined to undergo interchain coupling due to the stronger steric hindrance effect. Furthermore, via triple-detection size exclusion chromatography (TD-SEC) characterization, the intrinsic viscosity ([η]) and the radius of gyration (R_g) were found to be scaled to the molar mass of the macromonomer (M_m) and the molar mass of the entire chain (M_t) as [η] = K_ηM_t^νM_m^μ (μ ≃ 0.25 and ν ≃ 0.40) and R_g = H_RM_t^αM_m^β (α ≃ 0.47 and β ≃ 0.083) for both HPS_V and HPS_Y samples. These scaling relationships evidenced that both HPS_V and HPS_Y hyperbranched structures hold a similar statistically fractal feature. More importantly, the determined values of ratios K_η,Y/K_η,V (∼1.5) and H_R,Y/H_R,V (∼1.2) demonstrated, for the first time, that the average equivalent length of the branching subchain (L_s) for HPS_Y is much longer than the initial macromonomer chain length (L_m), i.e. L_s = 2.5L_m while L_s is exactly equal to 1/2L_m for HPS_V. This finding clearly clarified the difference between L_s and L_m, and thus the terms of L_s and L_m should not be misused in the future structure–property study. Finally, the value of the R_g/R_η ratio is determined to be ∼1.05 and ∼1.15 for HPS_V and HPS_Y samples (R_η: viscometric radius), respectively, which are much smaller than the typical values for linear chains in good solvents (1.20–1.40), revealing the less draining nature of these hyperbranched structures.