Isotactic Poly(2-vinylpyridine), Coordination Polymers, and Magnetic Nanocomposites from Chromium Atoms

摘要

Alternatives to homogeneous metallocene isospecific polymerization catalysts are few. Such catalysts are important for "engineering" macromolecules that offer enhanced performance based on control of tacticity, molecular weight, polydispersity, chirality, and compositional heterogeneity. In the course of our search for new high-energy atom routes to non-metallocene polymerization initiators and catalysts, we discovered an unusual method of activating vinyl monomers for isospecific polymerization. In this communication we describe our approach to isotactic polymerization and an adaptation to synthesize magnetic polymer nanocomposites in a single reaction vessel. In the first step, we deliver a "free" chromium atom to a monomer molecule of 2-vinylpyridine (2-VP), converting it to an orga-nometallic species active for isospecific polymerization. The 2-VP monomer is thus co-opted into behaving as the latent agent of its own activation for polymerization by assigning it a crucial role as a stereodirecting active center. This approach to isospecific polymerization is unprecedented in the literature. Second, polymerization occurs with the conversion of poly(2-VP) into a coordination polymer in which chromium is dispersed as a bound complex. By starting only with atoms and organic monomer, this provides a new synthetic entry to metal-containing polymers. Third, we adapt the above chemistry to prepare a poly(2-VP)anoparticle magnetic composite. Geometric (size) constraints are intentionally imposed to alter magnetic properties by forcing an allotropic hcp → fcc phase transition in entrained 20 A cobalt particles.