Copolymerization kinetics of hydrogels based on oligo(ethylene glycol) methacrylates and acrylic acid using isoconversional methods

Monomers consisting of a methacrylate moiety attached to a short poly(ethylene glycol) (PEG) chain can be polymerized to form hydrogels with several applications such as the removal of dyes and heavy metals from wastewater. In this study, the radical copolymerization kinetics of oligo(ethylene glycol) methyl methacrylate (OEGMMA), and oligo(ethylene glycol) hydroxyethyl methacrylate (OEGHEMA), with acrylic acid (AAc) was investigated. In both cases, hydrogels were formed with cross-linked structure. The rate of polymerization and degree of conversion were measured using differential scanning calorimetry (DSC) operating under non-isothermal conditions, at several constant heating rates, or under isothermal conditions, at different constant reaction temperatures. Isoconversional kinetics were employed to estimate the effective activation energy of the polymerization. It was found that, in the homopolymer (POEGHEMA) bearing hydroxyl groups, polymerization under both isothermal and non-isothermal conditions proceeds faster compared to the polymer with methoxy groups (POEGMMA). This behavior is attributed to the interaction between the hydrogen in the hydroxyl group with the carbonyl oxygen of the methacrylic ester, which results in a reduction of the electron density at the double bond and therefore increases its reactivity. Monomer–monomer association through hydroxyl groups results in initially lower activation energy of POEGHEMA. As polymerization proceeds, the existence of aggregated hydroxyl structures in the POEGHEMA macromolecular chains result in higher activation energies and a more abrupt increase in the conversion time curve. The addition of acrylic acid results in higher copolymerization rates for both methacrylates, with P(OEGMMA-AAc) affected more, since in the case of OEGHEMA specific functional (i.e. hydroxyl) groups already exist in the macromolecules. The association of monomer-AAc through hydroxyl‑carbonyl groups, results in lower activation energies for both copolymers compared to the corresponding homopolymers. The significant contribution of isoconversional methods in the study of polymerization kinetics of hydrogels it was thus verified.