ε-Caprolactone polymerization using titanium complexes immobilized onto silica based materials functionalized with ionic liquids: insights into steric, electronic and support effects

Heterogeneous catalysts comprised of titanium immobilized on silica imidazolium-based systems have been synthesized for the ring opening polymerization of epsilon-caprolactone. Titanium isopropoxide as a metallic precursor was grafted onto ionic liquid-functionalized silica surfaces or alternatively first grafted onto silica followed by reaction with 1-methyl-3-[(triethoxysilyl) propyl] imidazolium chloride. These materials have been characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), N-2 adsorption-desorption, transmission electron microscopy (TEM) with microanalysis by energy-dispersive X-ray (EDX), Si-29, C-13 and Ti-49/47 solid-state NMR spectroscopy (MAS NMR), Fourier-transform infrared spectroscopy (FT-IR), UV-vis diffuse reflectance spectroscopy (DRUV-vis), thermogravimetric analysis (TG-DTG) and solid state electrochemical techniques. Different ionic liquid compounds have been tested with different silica supports (mesoporous silica nanoparticles, SBA-15 and high surface area silica). Thus, the effect of the ionic liquid in the chemical environment of the anchored titanium complex has been determined. In addition, the stability and reusability of the catalysts have been studied.