Journal Article

PreJuSER-58101

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Analysis of Polymeric Methylaluminoxane (MAO) via Small Angle Neutron Scattering

Stellbrink, J.FZJ* ; Niu, A.FZJ* ; Allgaier, J.FZJ* ; Richter, D.FZJ* ; Koenig, B. W.FZJ* ; Hartmann, R.FZJ* ; Coates, G. W. ; Fetters, L. J.

2007
Soc. Washington, DC

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Please use a persistent id in citations: doi:10.1021/ma062940v

Abstract: Since its discovery as a crucial cocatalyst in metallocene and post-metallocene olefin polymerizations methylaluminoxane (MAO) has retained commercial and academic status. In spite of continued interest the MAO structure remains ambiguously defined. Because of limited alkane solubility toluene emerged as the MAO solvent of necessity. With time these toluene solutions can develop a gel fraction. The MAO structures proposed include linear, ring, ladder, and cyclic with the latter involving fused four and six membered rings along with cage and drumlike architectures. The linear and ring structures have aluminum and oxygen valences of three and two respectively while the other structures require Al/O co-ordination numbers of four and three. MAO structural information has been gathered from colligative property measurements, various NMR formats and quantum chemical calculations. We have used small-angle neutron scattering (SANS)fortified with static and dynamic light scattering (SLS/DLS)as the primary analysis tool for dilute MAO solutions (phi/phi* <= 0.6). The main structure assayed is a linear polymer chain consisting of [-Al(CH3)-O-] monomer units with an M-w of 20 kg/mol and a negative second virial coefficient. The latter fortifies the recognized state of play that toluene is a poor MAO solvent. About 0.8 wt % of MAO was captured as large-scale three-dimensional aggregates. Elemental analysis results show that in these large-scale aggregates the ratio of O/Al is significantly larger than for the smaller polymer chains. Additionally the large-scale aggregates contain only a small fraction of CH3 units. Those structures may be the precursor aggregates to the previously observed macroscopic gel fraction reported to form over time at room temperature. The solutions studied in this work did not show gel formation over an 8-month period at similar to 0 degrees C.

Keyword(s): J

Note: Record converted from VDB: 12.11.2012

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Contributing Institute(s):

1. Neutronenstreuung (IFF-5)
2. Molekulare Biophysik (INB-2)

Research Program(s):

1. Kondensierte Materie (P54)
2. Großgeräte für die Forschung mit Photonen, Neutronen und Ionen (PNI) (P55)

Appears in the scientific report 2007

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