Chitosan-Transition Metal Coordination Biopolymer: A Promising Heterogeneous Catalyst for Radical Ion Polymerization of Vinyl Acetate at Ambient Temperature.

The present study utilized chitosan obtained from crab shell and transition metal salts as precursors to synthesize chitosan-metal coordination biopolymers of Mn(II), Fe(III), Co(II) and Ni(II) [i.e Chit-Mn(II), Chit-Fe(III), Chit-Co(II) and Chit-NI(II) respectively]. The synthesized coordination biopolymers have been characterized using different instrumental techniques such as spectroscopic (UV-visible, FT-IR, XRD, EDS, and ICP-OES), thermal analysis (TGA and DTA), surface analysis (SEM), and hydrogen-temperature programmed reduction (H₂-TPR) analysis. Spectroscopic studies confirmed the successful incorporation of the metals into the biopolymer matrix. Thermal analysis and H₂-TPR revealed the reducibility of the Chit-Fe(III) at 120 ℃. While Chit-Fe(III) and Chit-Ni(II) were inactive, Chit-Co(II) and Chit-Mn(II) were found to be active towards vinyl acetate polymerization in the presence of aqueous Na₂SO₃. Furthermore, the polyvinyl acetate (PVAc) produced from Chit-Co(II) compared perfectly with a commercial PVAc and was in higher yield than PVAc produced from Chit-Mn(II). The polymerization has been shown to proceed via surface-initiated atom transfer radical polymerization (SI-ATRP), and the viscosity average molecular weight of PVAc produced has been measured as 25, 078. The density functional theory approach has been used to ascertain the coordination orientation of the Chit-Co(II) and explain its high efficiency towards vinyl acetate polymerization. The catalyst reusability test revealed an insignificant loss of activity for the Chit-Co(II) after seven cycles of polymerization. Kinetic studies show that the vinyl acetate polymerization suits the second-order kinetic model at ambient temperature. Thermodynamic studies also revealed that chain initiation is an endothermic process while chain propagation is an exothermic process. The result of this work also suggests an investigation of chitosan-metal coordination biopolymer via low-ppm ATRP approach for possible biomedical application.