Poly(ε-caprolactone)-grafted-chitosan copolymers: Synthesis and use as tunable and biodegradable coating for water soluble fertilizers

The urgent need for efficient agriculture to increase crop and food has fueled the development of coated fertilizers with a slow-release kinetics, water economy and full degradability to avoid negative environmental impact. In this context, we herein report the synthesis of poly(ε-caprolactone) (PCL) grafted from chitosan (CS) as a new class of biodegradable coating materials. The CS-g-PCL were prepared following a one-pot, two-step strategy, in which the primary amines of CS first react with ε-caprolactone (CL) to form CL-oligomers linked to CS via amide bonds terminated with hydroxyls, which together with the native hydroxyls of CS initiate the ring-opening polymerization of CL after addition of Sn(C ≡ CPh)4 as catalyst. Owing to the great stability of the used catalyst, the high molar mass of PCL grafted onto CS was successfully obtained under trivial open-air conditions. Different copolymers (CS-g-PCL) were prepared by varying the CS content. The structural characterization of CS-g-PCL was carried out using FTIR, 1H NMR, X-ray diffraction, contact angle and size exclusion chromatography (SEC) while the thermal and mechanical characterization of the prepared CS-g-PCL were compared to the neat PCL. Next, using a laboratory rotary drum, CS-g-PCL was implemented to uniformly coat granular diammonium phosphate fertilizer (DAP). Consequently, the kinetic releases of nitrogen and phosphorus were significantly delayed compared to that observed from uncoated DAP (conventional fertilizers). Finally, the degradation of CS-g-PCL was examined under aerobic conditions and showed a significant increase in their biodegradability with respect to neat PCL.