Catalytic living ring-opening metathesis polymerization

In living ring-opening metathesis polymerization (ROMP), a transition-metal–carbene complex polymerizes ring-strained olefins with very good control of the molecular weight of the resulting polymers. Because one molecule of the initiator is required for each polymer chain, however, this type of polymerization is expensive for widespread use. We have now designed a chain-transfer agent (CTA) capable of reducing the required amount of metal complex while still maintaining full control over the living polymerization process. This new method introduces a degenerative transfer process to ROMP. We demonstrate that substituted cyclohexene rings are good CTAs, and thereby preserve the ‘living’ character of the polymerization using catalytic quantities of the metal complex. The resulting polymers show characteristics of a living polymerization, namely narrow molecular-weight distribution, controlled molecular weights and block copolymer formation. This new technique provides access to well- defined polymers for industrial, biomedical and academic use at a fraction of the current costs and significantly reduced levels of residual ruthenium catalyst

Phosphonate-based resins for the selective enrichment of uranium(VI)

Phosphonate-derivatized polystyrene-divinylbenzene-based (PS-DVB) resins have been prepared from chloromethylpolystyrene-divinylbenzene (Cl-CH(2)-PS-DVB) by the Arbuzov-reaction using lithium or sodium alkyl chlorophosphites. Base-promoted hydrolysis of the diethyl phosphite-derivatized resin in refluxing 1,4-dioxane resulted in phosphonic acid-derivatized PS-DVB resins with an average capacity of 0.8-2.2 mmol phosphonate, depending on the alkyl chlorophosphate used. Both the free phosphonic acid as well as the diethyl, dibutyl- and bis(2-ethylhexyl)-phosphonate-derivatized sorbents were investigated in terms of their extraction characteristics for U(VI), Th(VI) as well as for other transition metal ions such as Fe(III), Zn(II), Co(II), Cd(II), Ni(II) and Cu(II) from acidic aqueous solutions. In contrast to phosphonate resins, the phosphonic acid-derivatized resins exhibited a high sorption capacity and selectivity for U(VI). Breakthrough experiments were conducted for both U(VI) and Th(IV) within a pH-range of 0-4. U(VI)-uptake was measured as a function of the concentration of nitric and hydrochloric acids, respectively. The sorbent was found to possess a high selectivity for U(VI) with an optimum extraction pH of 4. Recoveries for U(VI) determined prior to breakthrough were found to be quantitative (97-99%) on the free phosphonic acid resin within the investigated pH-range. Finally, masking and synergetic effects on the extraction efficiency of the phosphonic acid-derivatized resin for U(VI) have been investigated using 5-sulfosalicylic acid as a masking reagent and dipyridyl amine as an auxiliary ligand. (C)1999 Elsevier Science B.V. All rights reserved