Synthesis of phosphate hydrogels with excellent swelling behavior prepared by ascorbic acid -mediated ARGET ATRP

Abstract

This manuscript explores the synthesis of hydrogels through reversible deactivation radical polymerization (RDRP) methods, specifically, activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) and Cu0-mediated ATRP. These techniques are known to offer improved control over polymer network structures due to their slow polymerization kinetics. However, these methods have historically been challenging to apply to stimuli-responsive polymers, such as those with acidic or basic moieties. In this study, the direct synthesis of poly(2-(methacryloyloxy) ethyl phosphate) (PMOEP), a phosphate-pendant hydrogel, was investigated in aqueous media by both ascorbic acid (AsA)-mediated ARGET ATRP and Cu0-mediated ATRP. 2-(methacryloyloxy) ethyl phosphate (MOEP) was utilized as the active monomer with tris(2-pyridylmethyl) amine (TPMA) as the ligand. A unique feature of this work is that the diene moiety inherently present in the commercial monomer, bis[2-(methacryloyloxy) ethyl] phosphate (BMOEP), served as the crosslinking agent to obtain the novel PMOEP hydrogel. Formulations were tuned to optimize the swelling characteristics of the hydrogels, with the restriction that the hydrogels should swell homogeneously without fracture. Importantly, using AsA as the reducing agent in the ARGET ATRP reaction resulted in improved homogeneity and enhanced swelling properties of the hydrogel. Furthermore, tuning the target degree of polymerization (Dp) ([monomer]:[initiator]) had a significant impact on swelling characteristics. Specifically, PMOEP hydrogels synthesized via ARGET ATRP with a target Dp of 300 and 1 mM CuII catalyst demonstrated outstanding water absorption capacity of more than 2500% (relative to the dry mass) in phosphate buffer solution (PBS) at pH 7.4 and over 6000% in pure water. The morphology of these hydrogels was imaged using field emission environmental scanning electron microscopy (FE-ESEM) and the chemical composition was examined using Fourier transform infrared (FTIR) spectroscopy. Furthermore, nuclear magnetic resonance (NMR) analysis was performed to investigate the conversion rate of monomer during Cu0-mediated ATRP. To the authors’ knowledge, the synthesis of a 100% phosphate-pendant hydrogel using AsA-mediated ARGET ATRP and an internal phosphodiester crosslinking moiety without dilution or comonomer, has not been reported previously. This facile method for synthesizing and controlling phosphate-based hydrogel properties also provides an opportunity to include additional functional groups and holds significant promise for applications in biomedical engineering, agriculture, and membrane separations.