Analysis of Polymeric Methylaluminoxane (MAO) via Small Angle Neutron Scattering
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- 作者：Jö ; rg Stellbrink ; Aizhen Niu ; Jü ; rgen Allgaier ; Dieter Richter ; Bernd W. Koenig ; Rudolf Hartmann ; Geoffrey W. Coates ; Lewis J. Fetters
- 刊名：Macromolecules
- 出版年：2007
- 出版时间：July 10, 2007
- 年：2007
- 卷：40
- 期：14
- 页码：4972 - 4981
- 全文大小：183K
- 年卷期：v.40,no.14(July 10, 2007)
- ISSN：1520-5835
文摘
Since its discovery as a crucial cocatalyst in metallocene and post-metallocene olefin polymerizationsmethylaluminoxane (MAO) has retained commercial and academic status. In spite of continued interest the MAOstructure remains ambiguously defined. Because of limited alkane solubility toluene emerged as the MAO solventof necessity. With time these toluene solutions can develop a gel fraction. The MAO structures proposed includelinear, ring, ladder, and cyclic with the latter involving fused four and six membered rings along with cage anddrumlike architectures. The linear and ring structures have aluminum and oxygen valences of three and tworespectively while the other structures require Al/O co-ordination numbers of four and three. MAO structuralinformation has been gathered from colligative property measurements, various NMR formats and quantumchemical calculations. We have used small-angle neutron scattering (SANS)-fortified with static and dynamiclight scattering (SLS/DLS)-as the primary analysis tool for dilute MAO solutions (
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0.6). The main structureassayed is a linear polymer chain consisting of [-Al(CH3)-O-] monomer units with an Mw of 20 kg/mol anda negative second virial coefficient. The latter fortifies the recognized state of play that toluene is a poor MAOsolvent. About 0.8 wt % of MAO was captured as large-scale three-dimensional aggregates. Elemental analysisresults show that in these large-scale aggregates the ratio of O/Al is significantly larger than for the smallerpolymer chains. Additionally the large-scale aggregates contain only a small fraction of CH3 units. Those structuresmay be the precursor aggregates to the previously observed macroscopic gel fraction reported to form over timeat room temperature. The solutions studied in this work did not show gel formation over an 8-month period at~0 
C.
/* 0.6). The main structureassayed is a linear polymer chain consisting of [-Al(CH3)-O-] monomer units with an Mw of 20 kg/mol anda negative second virial coefficient. The latter fortifies the recognized state of play that toluene is a poor MAOsolvent. About 0.8 wt % of MAO was captured as large-scale three-dimensional aggregates. Elemental analysisresults show that in these large-scale aggregates the ratio of O/Al is significantly larger than for the smallerpolymer chains. Additionally the large-scale aggregates contain only a small fraction of CH3 units. Those structuresmay be the precursor aggregates to the previously observed macroscopic gel fraction reported to form over timeat room temperature. The solutions studied in this work did not show gel formation over an 8-month period at~0 C.