Crosslinked Nanoparticles from Stars by Intramolecular Crosslinking: A Bottom Up Approach to Polymeric Nanoparticles

INTRODUCTION Polymer structure and architecture are critical features in nanoscience and nanotechnology given the potential of nanometersized features for a variety of applications. For this reason, there has been an explosion of interest in nanoparticles in Chemistry, Material Science and Biology. The formation of nanoparticles is usually characterized by two distinct processes: (1) assembly of molecules into nanostructures and (2) stabilization of these nanostructures. Microand miniemulsion polymerization is a useful way of producing polymer particles in varying sizes. Likewise self assembly of monomers and polymers followed by stabilization of the assembled structures is receiving increased attention. In this regard, amphiphilic block copolymers self assembly for form micelles which can be stabilized by crosslinking in either the core or the shell of the structure. We have been interested in the production of crosslinked nanoparticles from single polymer molecules by chain collapse initiated by intramolecular carbon-carbon bond forming reactions and have demonstrated the templating effect of such thermally labile nanoparticles in the formation of nanoporosity in thin films. Here we demonstrate the formation of small, functionalized, crosslinked, polymeric nanoparticles from reactively functionalized multiarm stars prepared by anionic polymerization. The size of the crosslinked nanoparticle is dictated by that of the polymeric precursor. In an arm-first synthetic approach, multifunctional stars can be generated by the addition of living, linear polystyrene to divinylbenzene (DVB). The reaction proceeds in two main stages; first the addition of the macroinitiator to DVB produces a crosslinked polyanionic core decorated with linear polystyrene arms. In the presence of a polymerizable monomer, the core provides polymer growth sites which can be decorated with additional polymer chains. In the absence of any termination, the number of polymer arms added in each stage will be the same. This procedure can be used to generate multiarm star homopolymers or miktoarm block copolymers. The morphology of these materials is best described as a star with pendant arms eminating from a small crosslinked core. The synthetic procedure provides considerable versatility. For example, the use of a functionalized initiator results in the positioning of the functionality at the ends of the initial pendant arms; alternatively quenching the polymeric arms which grow from the polyanionic core locates the functionality at the ends of the chains added in the second step. In principle, the incorporation of latent crosslinking substituents into the growing arms provides a route to highly crosslinked nanoparticles with dimensions commensurate with the stars. The latent crosslinking functionality can be incorporated into either the initial, final or even both arms. A suitable latent crosslinking group must be inert to the anionic polymerization conditions and be activatible on demand (e.g. light, heat, chemical transformation etc.). Ideally the latent functionality should be selectively transformed into a highly reactive intermediate capable of reacting with other such intermediates on other arms in the vicinity in an irreversible fashion to produce a satble linkage. Alternatively, crosslinking could be achieved by reaction of the activated intermediate with appropriate functionality on other arms or by intermolecular reaction with added polyfunctional reagents. In the case of linear polymers, intramolecular reaction leads to chain collapse accompanied by large changes in the molecular dimensions. For star polymers, smaller volume changes would be expected since the connectivity of the star largely defines the shape of the molecule. Benzocyclobutene substituents constitute one class of latent reactants which are generally stable to the anionic polymerization conditions. This substituent is thermally converted by ring opening to a highly reactive o-xylylene which can stabilize by bimolecular reaction or polymerization. Either route would be effective for crosslinking the polymer arms.