Synthesis and characterization of arborescent graft copolymers

Abstract

Synthetic techniques are presented for the preparation of novel arborescent copolymers containing either polyisoprene, poly(2-vinylpyridine), poly(tert-butyl methacrylate), and poly(methyl methacrylate) side chains grafted onto polystyrene substrates of different architectures. Polyisoprene and poly(2-vinylpyridine) macroanions can be reacted directly with chloromethylated polystyrene substrates for this purpose. A convenient method for converting chloromethylated polystyrenes to their bromomethylated analogues is described. The more reactive bromomethyl sites are necessary for the synthesis of arborescent copolymers with either poly(tert-butyl methacrylate) or poly(methyl methacrylate) side chains. In each example provided, the graft-on-graft approach used provides control over the side chain molecular weight and the branching density, while a low apparent polydispersity is maintained for all copolymers (Mw/Mn < 1.2).

The arborescent copolymers are characterized by a compact, highly branched structure leading to interesting physical properties that are strongly influenced by the chemical composition of the side chains. For example, scanning force microscopy measurements in the phase contrast mode show that the isoprene copolymers display phase-separated morphologies (glassy core surrounded by a rubbery shell) to different extents, depending on their structure. Arborescent poly(2-vinylpyridine) molecules expand considerably more than their linear homologues when protonated with HCl. This enhanced polyelectrolyte effect is attributed to the higher charge density attained for the branched copolymers. This effect is even more noticeable for the arborescent poly(tert-butyl methacrylate) copolymers after hydrolysis and neutralization with NaOH.