Surface modification of natural halloysite clay nanotubes with aminosilanes. Application as catalyst supports in the atom transfer radical polymerization of methyl methacrylate

Abstract

The grafting of natural halloysite nanotubes (HNT) with aminosilanes exhibiting two (DAS) and three (TAS) amino groups has been investigated and compared to the physisorption of both silanes on halloysite nanotubes. Halloysite nanotubes were used as solid support for the heterogeneous Atom Transfer Polymerization of methyl methacrylate (MMA) into poly(methylmethacrylate) (PMMA) using CuBr as catalyst. Silane grafted on the nanoclay acts both as a ligand that bonds to CuBr and as a catalyst for the heterogeneous MMA polymerization. Grafting of halloysite nanotubes with DAS produced a polymer with polydispersities similar to those produced by the physically adsorbed diaminosilane catalyst, but conversion percentages were lower and a poorer control over the polymerization reaction was achieved. Grafting of halloysite nanotubes with TAS had a detrimental effect on the control of the polymerization reaction and a loss of catalytic activity due to the immobilization of the copper catalyst. The best control over the polymerization of methyl methacrylate is achieved when the catalyst is not immobilized by covalent bond on the surface of halloysite nanotubes. Therefore, physysorption of the CuBr/aminosilane complex on the halloysite nanotubes provided better control on the polymerization reaction compared to the grafting of the aminosilanes on the halloysite nanotubes.