High swelling and active sites: unlocking the hidden challenges in amine-polymer design for aldol reactions

Polymer materials hold strong potential for aldol reactions (AR) in water-rich media (WRM) due to their structural behaviors and stability. However, proposed approaches often fall into the trap of maximizing swelling and active site density without knowing the effect of the local microenvironment. Herein, we explore these behaviors by investigating the cooperative roles of polymer rigidity and functional group density using hydroxyethyl methacrylate (HEMA) and glycerol diglycerolate diacrylate (GDGDA) functionalized with ethylenediamine (EDA). Elemental analysis reveals a significantly higher nitrogen content in GDGDA-EDA (10.2 ± 0.09 wt%) compared to HEMA-EDA (2.8 ± 0.01 wt%). GDGDA beads exhibit low swelling (2.64 ± 0.11 g g −1 ) compared to HEMA beads (5.67 ± 0.14 g g −1 ). Despite its lower swelling, GDGDA-EDA shows higher activity, achieving a turnover frequency of 6.32 ± 0.49 s −1 compared to HEMA-EDA (2.67 ± 0.08 s −1 ) under the same amine site (0.075 mmol g −1 ) for AR. Spectroscopic methods and density functional theory (DFT) analyses suggest that the rigid, dense distribution of OH groups and active site configuration within the GDGDA-EDA provides a favorable local environment for substrate pre-activation. In these polymer-substrate interactions, GDGDA-EDA exhibits a substantially lower Gibbs free energy change ( Δ G 298.15 K = +5.45 kJ mol −1 ) compared to HEMA-EDA ( Δ G 298.15 K = +31.18 kJ mol −1 ) for the polymer-acetone interaction, indicating a potentially important cooperative contribution. This work demonstrates that high swelling capacity or active-site density alone does not necessarily determine catalytic performance, highlighting the importance of a rigid polymer framework and supportive active-site cooperativity in enhancing catalyst performance in WRM.