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Reports: ND453415-ND4: Mechanistic Studies of Transition-Metal Catalyzed, MAO-Assisted Olefin Polymerization

Rainer Ernst Glaser, Ph.D., MS, Dipl.-Chem., University of Missouri, Columbia

Even though methylaluminoxane (MAO) is a common co-catayst in olefin polymerization, its structures and mechanistic functions remain unclear and a variety of species have been discussed (Figure 1).  There is no doubt that hydrolysis of alkyl–Al bonds will result in the formation of HO–Al bonds, the release of alkanes, and the formation of acyclic and cyclic systems with Al–O–Al bridges by further inter- and intramolecular alkane elimination.  Experimental studies by Sinn and theoretical studies by Hall et al. and by Linnolahti et al. provide guidance for the discussion of possible reactive MAO species derived from dimethylaluminum hydroxide (DMAH).

Figure 1.  MAO compounds discussed by Sinn, Hall et al. and Linnolathi et al. 
 
We recently presented results of ab initio studies of the hydrolysis of trimethylaluminum (TMA, 1, Figure 2, see Glaser, R.; Sun, X. J. Am. Chem. Soc. 2011, 133, 13323-13336).  We studied the hydrolysis of trimethylaluminum (TMA, 1) to dimethylaluminumhydroxide (DMAH, 2) and of the intramolecular 1,2-elimination of CH4 from 2 itself to form methylaluminumoxide 3, from its dimeric aggregate 4 to form hydroxytrimethyldialuminoxane 5 and dimethylcyclodialuminoxane 6, and from its TMA-aggregate 7 to form 8 and/or 9, the cyclic and open isomers of tetramethyldialuminoxane, respectively.  Each methane elimination creates one new Lewis acid site and dimethylether is used as a model oxygen-donor molecule to assess the most important effects of product stabilization by Lewis donor coordination.  It was found that the irreversible formation of aggregate 4 is about three times more exergonic than the reversible formation of aggregate 7, that the reaction free enthalpies for the formations of 5 and 6 both are predicted to be quite clearly exergonic, and that there is a significant thermodynamic preference for the formation of 6 over ring-opening of 5 to hydroxytrimethyldialuminoxane 10.  The mechanism for oligomerization was discussed based on the bonding properties of dimeric aggregates and involves the homologation of HO-free aluminoxane with DMAH (i.e., 9 to 13) and any initially formed hydroxydialuminoxane 10 is easily capped to trialuminoxane 13.  Our studies are consistent with and provide support for Sinn’s proposal for the formation of oligoaluminoxanes and, in addition, the results point to the crucial role played by the kinetic stability of 5 and the possibility to form cyclodialuminoxane 6.  PRF funding has enabled us explore these two lines of research. 

Figure 2.  Formation of dimethylcyclodialuminoxane 6 by two-fold 1,2-elimination of CH4 from the cyclic dimer 4 of dimethylaluminum hydroxide 2.  Methyl-bridged cycloadduct 7 results by aggregation of TMA with 2 and 1,2-elimination of CH4 from 7 leads to tetramethyldialuminoxane 9, the Sinn monomer.  Cycloadduct 5 is in equilibrium with hydroxytrimethyldialuminoxane 10. 
 
Aggregation of Acyclic Aluminoxanes.  We have been able to show how the structure of the Al16O12Me24 cluster can be understood as a tetrameric aggregate of the Sinn trimer.  We determined the structures of all of the intermediates of the aggregation sequence at higher levels of ab initio theory and the thermochemistry of all aggregation steps have been determined. 

Figure 3.  Molecular model of the MP2/6-31G* optimized structure of the Al16O12Me24 cluster. 

  Iron-Complexation by Cyclodialuminoxanes.  Dialuminoxanes 9 and 10 are reversed-polarity heterocumulenes and intramolecular O-to-Al dative bonding competes successfully with Al-complexation by Lewis donors, whereas intramolecular O-to-Al dative bonding is impeded in cyclodialuminoxane 6 and the dicoordinate oxygen in 6 therefore is a strong Lewis donor.  Ethylene polymerization catalysts contain highly oxophilic transition metals and our studies suggest that these transition metal catalysts should discriminate strongly in favor of cycloaluminoxane-O donors even if these are present only in small concentrations in the MAO cocatalyst.  Studies of the coordination of Sun-type iron catalysts with open and cyclic aluminoxanes (Figure 4) provide evidence in support of this hypothesis. 

Figure 4.  The complexes of iron catalysts formed with cyclic aluminoxanes are greatly preferred over the respective complexes formed with open aluminoxanes. 

  Collaboration and International Education.  PRF funding enabled us to continue and to strengthen on-going collaborations with Drs. W.-H. Sun and W. Yang of the Institute of Chemistry, Chinese Academy of Sciences, Beijing (ICCAS), Dr. C.-Y. Guo of the University of the Chinese Academy of Sciences, Beijing (UCAS), and Dr. B. Wu and Y. He of the Department of Chemistry, Northwest University, Xi’an (NWU).  The collaborations included mutual visits by faculty and students:  Dr. W. Yang visited MU for three months in the spring of 2014, NWU student K. Yang visited MU for a one-month internship in the spring semester of 2014, and the PI and MU chemistry students C. Camasta and E. Zars travelled to China for several months in the summer of 2014.  The PI spent close to three months in China during the summer of 2014 with additional support by a “Visiting Professorship for Senior International Scientists of the Chinese Academy of Sciences” and a “Guest Professorship” at NWU.  The PI was invited to present on Structural Chemistry And Thermochemistry Of Mao Formation. Studies Of Cycloaluminoxane Ligands And Of The Aggregation Of Acyclic Aluminoxanes at the 8th International Symposium On High-Tech Polymer Materials (HTPM-VIII, July 2, Fragrant Hill Hotel, Beijing) and he presented invited lectures at Northwest University, Xi’an (June 13), at UCAS, Beijing (June 24), at Xiamen University (July 14), at Ningbo University (July 16), and at Fudan University, Shanghai (July 18).  The impact of these visits went well beyond the contacts with the research collaborators, conference participants and colloquium audiences.  As in previous years, the PI taught courses on “Scientific Writing” at MU (spring 2014, >30 students) and at UCAS and NWU (summer, >300 students).  The PRF funding made it possible, for the first time, to teach all of these courses with the assistance of American students in China and with the assistance of visiting Chinese students in the US, respectively.  This pedagogical innovation improved classroom communication, contributed to the internationalization of the curriculum, and provided opportunities for internationalization-at-home efforts.