Determination of Propagation Rate Coefficients of 2-Hydroxyethyl Methacrylate in Aqueous Solution

Mechanistic understanding of free radical polymerization (FRP) in aqueous solution is hindered by the lack of kinetic coefficient data. Here, we investigated the propagation rate coefficient (k_p) for FRP of HEMA in aqueous solution. The k_p at 80 °C increased by 6 times as the monomer weight fraction in aqueous system decreases from 100 wt.% to 6 wt.%, that is., from 6.0 × 10³ to 3.3 × 10⁴ L·moL⁻¹·s⁻¹. This increase in k_p is associated with the increase in pre-exponential factor (A), which suggests an increase in entropy with increasing water molecules in the polymerization system. To investigate the effect of solvent on k_p, pulse-laser polymerization in solvents of different hydrogen bonding affinity, that is, 50 wt.% butyl propionate (BP) and dimethyl formamide (DMF), are conducted. The k_p obtained are in order of k_p,bulk∼k_p,BP > k_p,DMF, suggesting that the intermolecular hydrogen bonding at carbonyl moieties critically affects the geometry of transition state quasi-equilibrium in propagation. We underpinned this observation by analyzing the carbonyl and alkene moieties of HEMA in different solvents using infrared (IR) spectroscopy. The mechanistic analyses and insights relating to hydrogen bonding and functional moieties can be relevant for future studies involving non-aqueous solvents promoting hydrogen bonds.