How does the precursor backbone length affect the grafting efficiency and kinetics for bottlebrush polystyrenes prepared by graft-onto Strategy?

Abstract

The clarification of the potential dependence of grafting efficiency on the backbone length during the graft-onto reaction process is extremely important for subsequent structure–property study. In this work, by using alkyne-functional polypropargyne acrylate (PPANb, Mw/Mn < 1.10 and Nb = 960, 193, 35) and azide-functional polystyrene (PSNs, Mw/Mn < 1.15 and Ns = 54, 38, 22) as the backbone and sidechain precursors, we have investigated the Nb-dependence of the grafting reaction during the graft-onto click process. Specifically, the influence of solution concentration and feed ratio are investigated, and a combination of SEC characterization, Gaussian fitting and theoretical analysis is used to estimate the Nb-dependent grafting reaction for the bottlebrush product (PPANb-g-PSNs). The main findings include: 1) when the reaction time (t) is sufficient long (t = 24 h), the grafting efficiency is found to be nearly irrelevant with Nb, even if we consider 2 ∼ 5 times of difference in the diffusion coefficient (D) based the well-known scaling relation of D ∼ Nb1/2 in the semidilute reaction solution; 2) the time-dependent kinetics investigation further shows abnormal and different pathways for different backbone lengths when 0 min < t < 10 min, but the final grafting efficiency determined at t = 300 min shows no much difference, going back to the preliminary result of equilibrium state at t = 24 h; 3) the origin of abnormal pathways may be attributed to the relatively poor signal-to-noise ratio and potential dependence of dn/dC of PPANb-g-PSNs product on the composition at low grafting density range, which needs clarification in future. For the first time, our result has unambiguously demonstrated that, for a mixed solution of reaction system composed of backbone precursors with their lengths differing by hundreds of times, the polydispersity effect of the backbone length does not affect the final grafting efficiency during the process, no matter for long or short sidechain grafts. The result implies that it is the local dynamics of chain segments, instead of the overall translational diffusion dominates the graft-onto process in the concentration-regime studied.