Abstract

Abstract

Abstract

Abstract

Abstract

A kinetically inert, reversible coordination polymer (3) was obtained through complexation of dicyclohexylphosphine telechelic poly(tetrahydrofuran) with palladium(II) dichloride. This coordination polymer is unreactive towards palladium(II) dichloride bis(1-diphenylphosphino)dodecane (4), because ligand dissociation in the coordination polymer is slow. However, upon ultrasonication of solutions of 3 in toluene in the presence of 4, formation of palladium(II) heterocomplexes was observed with ³¹P NMR spectroscopy. Heterocomplex formation, the consumption of 4, and changes in molecular weight were used to quantify the scission process. In the presence of 60 equivalents of the alkyldiphenylphosphine stopper complex, the reduction in molecular weight was strongly enhanced; over a period of eight hours the weight-averaged molecular weight was reduced from 1.1×10⁵ to 2.3×10⁴ g mol⁻¹ while 47 % of the palladium(II) complexes in the coordination polymer had been converted into heterocomplexes. These results show that the system of 3 in combination with scavenger 4 is a suitable system to study the efficiency of ultrasound-induced chain scission of coordination polymers.

Aus zwei wird eins: Ein AB-Monomer mit jeweils vier H-Brücken bildenden 2-Ureido-4[1H]-pyrimidinon- und 2,7-Diamido-1,8-naphthyridin-Einheiten entsteht durch selektive Olefin-Kreuzmetathese. Mit ihm lässt sich das stöchiometrische Ungleichgewicht bei der Selbstorganisation von supramolekularen AA+BB-Polymeren umgehen. Da der Linker zwischen den beiden Motiven flexibel ist, liegt das Monomer bei niedriger Konzentration als Cyclus vor.

The history of ultrasound in polymer chemistry goes back a long way. Initially, its uses were limited to being an alternative method of initiating radical polymerizations through the decomposition of solvents to form radicals or through the breakage of polymers leading to macroradicals. Recently, the raw power of ultrasound has been focused through the use of weak linkages in polymer chains, which enables the production of well-defined macroradicals and coordinatively unsaturated metal complexes. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5445–5453, 2006

Mit Schall: Das rheologische Verhalten von Fluiden kann durch externe Stimuli beeinflusst werden, und elektrochemisch und photochemisch induzierte Viskositätsänderungen und Sol-Gel-Übergänge sind schon seit einiger Zeit bekannt. Neuere Arbeiten beschreiben die Anwendung von Ultraschall als Rheologieschalter für supramolekulare Polymere und Netzwerke.

Ein chiraler supramolekularer „Aufpasser“ als strukturdirigierendes Agens beeinflusst die Rückgratkonfiguration während der Polymerisation der achiralen monomeren „Untergebenen“ (siehe Bild). Das erhaltene Polymer ist trotz seiner gemischten Mikrostruktur bevorzugt helical gefaltet.

A chiral supramolecular “sergeant” as the structure-directing agent affects the backbone stereochemistry during polymerization of the achiral “soldier” monomers. The polymer obtained shows preferred helical folding despite its mixed microstructure.

Blocked isocytosine isocyanates are conveniently obtained by the reaction of 1,1′-carbonyldiimidazole (CDI) with isocytosines. The resulting precursors for quadruple hydrogen-bonded structures can be isolated and stored for further use. Reaction with either aliphatic or aromatic amines gives the corresponding mono-, bi-, or trifunctional ureidopyrimidinone derivatives in good to excellent isolated yields. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Auseinander und wieder zusammen: Die Spaltung hochmolekularer linearer Koordinationspolymere aus Phosphan-telechelem Polytetrahydrofuran und Palladiumdichlorid in verdünnter Lösung ist reversibel (siehe Bild). 1 h Beschallung führte zu einer Abnahme des gewichtsgemittelten Molekulargewichts des Polymers von 1.7×10⁵ auf 1.0×10⁵ g mol⁻¹. Nach 24 h Stehen war wieder das ursprüngliche Molekulargewicht erreicht.

Breaking up and making up: The ultrasonic cleavage of high-molecular-weight linear coordination polymers of phosphane telechelic polytetrahydrofuran and palladium dichloride in dilute solution is a reversible process (see picture). Sonication for 1 h led to a decrease in the weight-averaged molecular weight of the polymer from 1.7×10⁵ to 1.0×10⁵ g mol⁻¹. Upon leaving the sample to stand for 24 h, the original molecular weight was completely restored.

A series of mono- and bifunctional compounds 2–7, based on the ureido pyrimidinone quadruple hydrogen bonding unit, was prepared to study the mode of aggregation of these compounds in the bulk and in solution. Compounds 2–7 exhibit thermotropic liquid crystalline properties, as evidenced by differential scanning calorimetry and optical polarization microscopy. The presence of an ordered hexagonal discotic (D_ho) phase of 2 a was confirmed by X-ray diffraction on an aligned sample. In chloroform, the bifunctional compounds form cyclic dimers at millimolar concentrations, and these dimers exist in equilibrium with linear species above a critical concentration, which may be from 6 mM to greater than 260 mM, depending on the structure of the spacer. Circular dichroism measurements in chloroform did not show a Cotton effect. Dodecane solutions of compounds 3, 4 b, and 7 b display a Cotton effect at the absorption band of the phenyl-pyrimidinone unit. Amplification of chirality was observed in mixtures of 7 a and 7 b, but not in mixtures of 4 a and 4 b, indicating that 7 a and 7 b form mixed polymeric aggregates with a helical architecture in dodecane solution, whereas 4 a and 4 b do not. The Cotton effect is lost upon increasing the temperature. Half of the helicity is lost at 25 °C for 3 and at 60 °C for 4 b, suggesting that 3, bearing the shorter spacer, forms less stable columns than 4 b. Compound 7 b loses half of its helicity at 45 °C. Compounds 2 b, 5, and 6 do not exhibit helical organization, as evidenced by the absence of Cotton effects.

The article discusses the development and properties of supramolecular polymers based on quadruple hydrogen bonds between self-complementary ureidotriazine (UTr) and ureidopyrimidinone (UPy) functional groups. The high association constant with which these groups dimerize leads to polymers with a high degree of polymerization in isotropic solution. Application of these units for the functionalization of telechelic polymers results in new materials with mechanical properties approaching those of covalent polymers, but with a much stronger temperature-dependent behavior. Solvophobic interactions between the hydrogen bonding moieties may be used to obtain supramolecular polymers with a well defined helical columnar architecture. Another consequence of the high dimerization constant of the UPy group is the phenomenon of a critical concentration in solutions of many bifunctional monomers. Below this concentration, only cycles are present, while above the critical concentration, the amount of cycles remains constant, and a polymer is formed. Conformational properties of the linker units are used to control the equilibrium between polymers and cycles, and are proposed to form a promising strategy toward tunable materials.