On novel hydrogels based on poly(2-hydroxyethyl acrylate) and polycaprolactone with improved mechanical properties prepared by frontal polymerization

This work aimed to improve the mechanical properties of poly(2-hydroxyethyl acrylate) (PHEA)-based hydrogels
by performing their polymerization in the presence of an ad hoc synthesized star-shaped polymer, whose arms are
composed of a biopolymer, i.e., polycaprolactone (PCL) end-capped with acrylic units to act as a crosslinker for
PHEA. Indeed, this system was designed to have (i) biocompatible, biodegradable, and mechanical strong arms as
well as (ii) a star structure with acrylic functionalities to promote the crosslinking of the material. 1H NMR, IR
and TGA measurements confirmed the star PCL functionalization, which was carried out starting from a
hydroxyl-terminated polymer. Hydrogels were prepared by varying the concentration of the synthesized star
polymer, and the properties of the materials obtained by frontal (FP) and bulk polymerization (BP) were
compared. Moreover, to evaluate the specific effect of PCL-star-4-2 k-tetraacrylate (PCL-TA) on the crosslinking
of the systems, samples were also synthesized using a commercial acrylate crosslinker. For what concerns the
frontal polymerization process, fronts were found to be stable in the presence of PCL-TA. The thermal charac-
terization results showed a significant decrease in the PHEA glass transition temperature with increasing PCL-TA
content, which was particularly evident in the samples prepared by FP. This result can be attributed to the partial
miscibility of the two polymers, which become compatible during the polymerization process by forming a
copolymer system. This was confirmed by analyzing the thermal behavior of hydrogels polymerized in the
presence of a hydroxyl terminated PCL, i.e., a polymer without active functional end groups. The samples
synthesized by FP with the star polymer and the commercial crosslinker, as well as neat PHEA, were subjected to
mechanical tests. The mechanical behavior of PCL-based hydrogels was outstanding, exhibiting three time higher
modulus than that of crosslinked PHEA, and were structurally stable even under high compression loadings. This
remarkable property, combined with the fast and efficient polymerization method and the environmentally
friendly properties of PCL, make the developed hydrogels promising systems for practical applications in various
fields.