Polybenzoxazines are new class of materials, considered as a promising substitute of commercial thermosets due to their outstanding properties. However, polybenzoxazines have some shortcomings constraining their industrial application such as difficult processability. One strategy to overcome this drawback is the incorporation of benzoxazine groups onto a processable polymer chain. This approach would allow the processing of the materials before completing their shaping by thermal curing. In this work has been investigated the functionalization of polymers bearing hydroxyl groups, like polyethylene glycol, via esterification with phenols bearing carboxylic acid groups. In particular, phloretic acid, a naturally occurring phenylpropanoid phenol, has been used as a chemical platform to provide phenol-terminated molecules, further followed by the assembling of benzoxazine monomer. The two-step synthesis via solventless method has been successfully performed and validated by molecular characterizations (NMR, FTIR and MALDI-TOF/MS), leading to fully bio-based molecules with benzoxazine groups anchored at the end of the polymers. Interestingly, it has been evidenced that the ester linkages are thermally stable, allowing the polymerization of the benzoxazine moieties without triggering early stage degradation. The formation of a thermoset has finally been reached, as the material is cross-linking in 9 minutes at 180 °C, highlighting the success of this strategy. This totally novel approach paves the way toward the benzoxazine functionalization of hydroxyl bearing compounds like (bio)polymers, nanoparticles and even metallic surfaces like steel.