Polymerization of ε-caprolactone on calcium alkoxide-functionalized silica substrates†

Biodegradable composite materials free from toxic initiator residues have been prepared using calcium alkoxides bound to silica. Polycaprolactone with low polydispersity was synthesized by ring-opening polymerization of e-caprolactone at 100 °C on a calcium alkoxide-functionalized silica substrate prepared by the grafting of a hydrolysed epoxy silane onto silica gel, followed by reaction with calcium hydride. The polymerization was also initiated by the product of the direct reaction of calcium hydride with unfunctionalized silanol groups. Characterization of the reaction products and the real-time reaction kinetics using Raman spectroscopy suggests a living coordination–insertion mechanism. There is a difference in reactivity of a factor of ca 7 between the calcium alkoxide formed on the silanized and unsilanized silica surface, resulting in a monomer conversion of 90% in 100 h for the silanized silica compared to only 30% for the unsilanized silica. The degree of grafting ranges from a maximum of 30% for the silanized surface to 10% for the unsilanized silica alone. There is evidence for cyclic transesterification products, and release of the polymer from the silica occurs in this way and also by adventitious hydrolysis. Copyright © 2010 Society of Chemical Industry