A catalytic alkane metathesis provides a mild and selective degradation of polyethylene wastes into valuable fuels and waxes.

Abstract

Abstract

Abstract

This mini review uses two examples to illustrate the importance of supramolecular chemistry in natural and biomimetic polymers for advanced mechanical properties. In the first example, dragline silk, one of the strongest and toughest natural fibers, uses intermolecular weak forces to self-assemble into nanocomposites composed of β-sheet nanocrystals imbedded in an amorphous matrix. In the second example, a load-bearing protein, the sarcomere muscle protein titin, uses intramolecular weak forces to fold into repetitive modules for combined strength, toughness, and elasticity. Both examples show vividly that many natural polymers can combine important mechanical properties through programming supramolecular weak interactions into covalently formed biopolymers. This mini review attempts to summarize the current understanding of the molecular mechanisms that contribute to the exceptional mechanical properties for each natural polymer. Following that, the efforts from others and the author's own laboratory on developing synthetic polymers to mimic these natural counterparts are discussed. Copyright © 2007 Society of Chemical Industry

Ganz natürlich: Eine neue Klasse synthetischer Biomaterialien (die abgebildeten Saccharid-Peptid-Hybridcopolymere) ist biologisch abbaubar, ungiftig und nicht-immunogen. Die kationischen Saccharid-Peptid-Hybridcopolymere erwiesen sich auch als wirksam beim Verdichten von Plasmid-DNA und ihrem Einschleusen in Zellen.

Vorteile durch Design: Makrocyclische Cyclophanliganden erhöhen die thermische Stabilität von neuen Katalysatoren für die Polymerisation von Ethen zu Polyethylen mit hohem Molekulargewicht. Entscheidend für die hohe Aktivität und die Stabilität ist die strategische Platzierung des Zentralmetalls im Kern der Cyclophanarchitektur (siehe Bild; Pd rot, Cl grün, N blau, C grau).

Better by design: Exploitation of the macrocyclic architecture of a cyclophane-based ligand provides new highly active catalysts with improved thermal stability for ethylene polymerization. The strategic positioning of the metal center at the core of the cyclophane-based ligand is the key to the observed high activity and thermal stability, and to the high molecular weight of the polyethylene (see picture; red: Pd, green: Cl, blue: N, gray: C).

An Erratum has been published for this article in J Polym Sci Part A: Polym Chem (2004) 42 213

In this article, recent examples are reviewed of late-transition-metal catalysis applied to polymer topology control. By the judicious selection or design of late-transition-metal catalysts, polymers with a broad range of topologies, including linear, short-chain-branched, hyperbranched, dendritic, and cyclic topologies, have been successfully synthesized. A distinctive advantage of the catalyst approach is that polymers with complex topologies can be prepared in one pot from simple commercial monomers. A fundamental difference of the catalyst approach with respect to other approaches is that the polymer topology is controlled by the catalysts instead of the monomer structure. In our own laboratory, we have successfully used two strategies to control the polymer topology with late-transition-metal catalysts. In the first strategy, hyperbranched polymers are prepared by the direct free-radical polymerization of divinyl monomers through control of the competition between propagation and chain transfer with a cobalt chain-transfer catalyst. In the second strategy, polyethylene topology is successfully controlled by the regulation of the competition between propagation and chain walking with the Brookhart Pd^II-α-bisimine catalyst. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3680–3692, 2003

Chain-walking catalysts are demonstrated to control the branching topology of polyethylene by tuning the competition between monomer insertion and chain isomerization (walking). The topology of the polyethylene was controlled from linear with moderate branching to hyperbranched to dendritic. Although overall branching number and distribution of short chain branching are relatively constant, the topology of the polyethylene changes from linear polyethylene with moderate branches at high ethylene pressures to a dendritic polyethylene at low pressures. This provides a straightforward one-pot process for making polymers with a full range of tunable topologies.