Abstract The synthesis of tris(π‐allyl)uranium halides ( 2a–c ) and their behaviour in the stereospecific polymerization of butadiene are reported. Independently from the nature of the halogen bound to uranium, very high 1,4‐ cis ‐polymer with a fairly good yield is obtained. The catalytic activity of allyluranium derivatives is very much improved when a Lewis acid is added as a co‐catalyst. It is characteristic of the polymer to show a cis ‐content up to 99% and an exceptionally high chain regularity at least for long chain sequencies.
Abstract Nur mit der dreifachen Menge primärer Amine (I) oder Nitrile (IV) bilden die Alu= ′ minumhydride (II) definierte Verbindungen, denen die polymere Struktur (III) mit wechselnder Anzahl assoziierter THF‐Moleküle zugeordnet wird.
Abstract Through the use of a Ti(OR′)4-AlRCl2 catalyst system, high 1,4-cis isoprene polymers and crystalline 1,4-trans polybutadiene are obtained. Neither monomer is polymerized at a Al/Ti mole ratio of less than 4. The maximum activity and stereospecificity for isoprene is observed at Al/Ti = 4. For 1,4-trans butadiene polymers the activity increases progressively with increasing Al/Ti ratio. The investigations carried out on this catalyst system show that at a AI/Ti mole ratio of 4 the formation of crystalline β-TiCl3 takes place, while at lower ratios insoluble chloro-alkoxide derivatives of TiIII with different compositions separate. Soluble complexes containing aluminium and titanium are initially formed before precipitation occurs. Chemical data and investigations by IR and NMR spectroscopy indicate exchange reactions between Al-Cl, Al-R, and Ti-OR groups, together with reduction of the transition metal. A reaction mechanism and a hypothesis on the nature of the active catalyst are given.
New complexes of general formula [U(η-C3H5)2(OR)2][R = Et(1), Pri(2), or But(3)] have been prepared by reaction of [U(η-C3H5)4] with ROH. The crystal structure of (2) has been determined from single-crystal X-ray diffraction data (Mo-Kα radiation). The complex crystallizes in the space group P21/c with unit-cell dimensions a= 12.324(7), b= 8.535(1), c= 17.897(9)Å, β= 126.71(3)°, Z= 4, and R 0.030 for 1 408 independent observations. In the dimeric molecular structure, with crystallographic site symmetry, two uranium atoms are joined by two alkoxide bridges. The co-ordination polyhedron of the uranium can be regarded as a distorted pentagonal bipyramid, whose equatorial plane is defined by the terminal carbons of the η-allyl ligands and by a bridging oxygen. The axial positions of the bipyramid are occupied by the remaining bridging oxygen and by a terminal oxygen. The latter shows a linear co-ordination [U–O–C 178.0(10)°]. Other relevant bond distances are: U–O(bridging) 2.271(10) and 2.413(10); U–O(terminal) 2.056(13); and U–C(mean) 2.679(14)Å. Infrared data for (1)–(3) suggest that the uranium–allyl π bond is predominantly ionic; this is consistent with some structural peculiarities of the allyl ligands in (2). Hydrogen-1 n.m.r. results indicate monomeric structures in tetra-hydrofuran [(1)–(3)] and dimeric ones in toluene solution for (1) and (2).
