Protonated chitosan/poly(ε-caprolactone) graft copolymer coatings for pH-modulated release of phosphorus fertilizers

This study reports the synthesis of protonated chitosan-g-poly(ε-caprolactone) copolymers and their use as pH-modulated coatings for the controlled release of phosphorus from granular monoammonium phosphate (MAP) and diammonium phosphate (DAP) fertilizers. Copolymers were synthesized in two steps: first, chitosan amino groups were protonated using dilute hydrochloric acid (HCl) to limit their participation in the polymerization reaction; then, ring-opening polymerization (ROP) of ε-caprolactone was performed from the hydroxyl groups using tin(II) 2-ethylhexanoate (Sn(Oct)₂) at 120 °C. Five copolymers were synthesized with increasing proportions of chitosan-HCl (2%, 3%, 5%, 7% and 10%). The materials were applied as polymeric coatings on MAP and DAP fertilizers using a rotary pan technique. Phosphorus release kinetics were studied in sandy and aqueous media under varying pH conditions. The results demonstrate a significant delay in phosphorus release from coated MAP and DAP compared with uncoated systems or those coated with neat poly(ε-caprolactone), with release behaviour influenced by the surrounding pH.