Synthesis of mixed poly(ε-caprolactone)/polystyrene brushes from Y-initiator-functionalized silica particles by surface-initiated ring-opening polymerization and nitroxide-mediated radical polymerization

Abstract

This article reports on the synthesis of mixed homopolymer brushes grafted on silica particles composed of poly(ε-caprolactone) (PCL), a chemically and biologically degradable polymer, and polystyrene (PS), a vinyl polymer, by sequential surface-initiated ring-opening polymerization (ROP) of ε-caprolactone (εCL) at 75 °C and nitroxide-mediated radical polymerization (NMRP) of styrene at 120 °C. The silica particles were functionalized with an asymmetric difunctional initiator (Y-initiator) bearing a hydroxyl group, an initiator for ROP, and an alkoxyamine moiety, an initiator for NMRP. We found that for ROP, PCL with M_n,SEC values of ∼25 kDa and narrow polydispersity indexes (≤1.20) relative to PS standards can be reproducibly synthesized at 75 °C using tin(II) 2-ethylhexanoate as catalyst after the Y-initiator-functionalized silica particles and the catalyst were thoroughly dried. Both surface-initiated ROP and NMRP polymerizations were carried out in the presence of a free initiator. The polymer brushes were cleaved from silica particles using hydrofluoric acid, and the SEC analysis showed that the molecular weights of the grafted polymers were close to those of the free polymers formed from the free initiators. The overall grafting density of mixed PCL/PS brushes can be tuned from 1.24 to 0.40 chains per nm² by varying the mass ratio of Y-initiator-terminated triethoxysilane to bare silica particles in the initiator immobilization step, though the grafting density of PS was always higher than that of PCL.