Organolanthanide compounds are not only unique model systems for studying the elementary processes of a-olefin polymerization but they can also act as competitive precatalyst systems, [1] however, to date their implementation in this area has been very limited. In contrast, the interaction of lowagglomerated rare-earth metal ("neodymium") complexes, such as alkoxide or carboxylate derivatives, with various organoaluminum reagents is successfully exploited to generate high-performance catalysts for industrial 1,3-diene polymerization.[2] Solubility in aliphatic solvents, low Al:lanthanide(Ln) ratios, cis-stereospecificity, and medium polydispersity are the criteria to be met by such ternary "Ziegler Mischkatalysatoren" (Mischkatalysatoren = mixed catalysts), the mechanisms of which are still not completely understood.[3] Although the homoleptic tetraalkylaluminate complexes Ln(AlR 4 ) 3 have a unique preorganized set of bridged, heterobimetallic moieties, their application in olefin transformations has not been reported so far.[4] Moreover, the compounds Ln(AlR 4 ) 3 are exceptional for they are obtained as alkyl-only ligated monomeric systems, without the formation of ate complexes for the entire lanthanide series. Herein we describe the use of a Ln(AlR 4 ) 3 /Et 2 AlCl binary precatalyst system in highly (cis)stereoregular isoprene polymerization. Additionally, the use of grafted variants as storable singlecomponent heterogeneous catalysts is investigated by employing periodic mesoporous silica MCM-48 as a structured support material.Homoleptic tetramethylaluminate complexes of the trivalent lanthanide metals [Ln{(m-Me)

Steric factors govern the formation of half-sandwich complexes (C5Me4R)Ln[N(SiHMe2)2]2 according to acid-base reactions utilising Ln[N(SiHMe2)2)3(thf)2 and substituted cyclopentadienes. Subsequent trimethylaluminium-promoted silylamide elimination produces the first half-sandwich bis(tetramethylaluminate) complexes (C5Me4R)Ln(AlMe4)2.

The homoleptic ytterbium(II) tetraalkylaluminate complexes {Yb[AlR 4 ] 2 } n have been obtained according to a silylamide elimination reaction from Yb[N(SiMe 3 ) 2 ] 2 (THF) 2 and excess AlR 3 (R ) Me, Et, iBu). While the tetramethylaluminate derivative is a pyrophoric powder which is insoluble in aliphatic and aromatic hydrocarbons, the ethyl and isobutyl congeners are readily soluble in n-hexane. Perethylated polymeric {Yb[AlEt 4 ] 2 } n is constituted formally of the two molecular fragments [Yb(AlEt 4 )] + and [Yb(AlEt 4 ) 3 ] -, forming an intricate threedimensional network in the solid state. Both fragments are linked by bridging R-carbon atoms and secondary Yb‚‚‚H-C agostic interactions combining µ,η 1 , µ,η 2 , and µ,η 3 coordination modes which result in remarkably short Yb‚‚‚Al (2.809(2) Å) and a large range of Yb‚‚‚C (2.649(5)-3.364(6) Å) distances. DFT calculations on the molecular fragments [Yb(AlEt 4 ) 3 ]and [Yb(AlEt 4 )] + reproduced the X-ray geometry remarkably well. Moreover, the theoretical investigations on model systems for the aluminate coordination support the highly fluxional nature of the aluminate coordination (∆E(η 2 fη 3 ) ) -8 kcal/mol), which is also indicated by solution NMR spectroscopy. A topological analysis of the total electron density of the µ,η 2bonded aluminate ligand in the benchmark systems Y(AlR 4 ) 3 (R ) Me, Et) revealed the presence of two bond critical points between the Y-C b and C b -Al bonds (C b ) bridging carbon atom) and thus suggests a hypervalent character of the bridging carbon atom.

C 2-symmetric tetraalkylaluminate complexes rac-[Me2Si(2-Me-C9H5)2]Y(μ-R)2AlR2 and terminal alkyl complexes rac-[Me2Si(2-Me-C9H5)2]YR(THF) (R = Me, Et, iBu) are quantitatively formed via a special silylamide elimination reaction. The “reversibility” of tetraalkylaluminate coordination gives access to the first mixed-alkyl lanthanidocene complexes, which can be discussed as models for polymer chain transfer in organoaluminum-dependent Ziegler−Natta catalysts.

In the current work we present results on the controlled/living radical copolymerization of acrylonitrile (AN) and 1,3-butadiene (BD) via reversible addition fragmentation chain transfer (RAFT) polymerization techniques. For the first time, a solution polymerization process for the synthesis of nitrile butadiene rubber (NBR) via the use of dithioacetate and trithiocarbonate RAFT agents is described. It is demonstrated that the number average molar mass, $\overline M _{\rm n} $, of the NBR can be varied between a few thousand and 60 000 g · mol(-1) with polydispersities between 1.2 and 2.0 (depending on the monomer to polymer conversion). Excellent agreement between the experimentally observed and the theoretically expected molar masses is found. Detailed information on the structure of the synthesized polymers is obtained by variable analytical techniques such as infrared spectroscopy (IR), nuclear magnetic resonance (NMR) spectroscopy, differential scanning calorimetry, and electrospray ionization-mass spectrometry (ESI-MS).