Date of Award
Chemistry and Biochemistry
Pure sciences, Frustrated Lewis pairs, Olefin polymerization, Small-molecule activation
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Recent work in the Stephan Group has identified the concept of "frustrated" Lewis pairs, in which traditional Lewis acid-base adducts of sterically demanding phosphines and the borane, B(C6F5)3 are not formed and alternative reactivity can occur.
Compounds derived from "frustrated" Lewis pair chemistry have been investigated as novel co-catalysts for ethylene polymerization. The phosphonium borates of the form, [HPR3][B(C6F5)4], [R2PHC6F4BF(C6F5) 2] and [R2PHC4H8OB(C6F 5)3], have been shown to be effective protic activators for the generation of electrophilic cationic Ti metal centers of the form, [CpTiMe(NP tBu3)]. The derivatization of the perfluoroaryl-linked phosphonium borates to form the perfluoroaryl-link phosphino-boranes of the form, R2PC6F4B(C6F5) 2, provides a unique family of potential Lewis acidic co-catalysts. These compounds were found to be excellent co-catalysts, as the interaction of the Lewis basic phosphine with the cationic Ti center increases ion-pair separation, resulting in a more active catalytic system.
Investigations of Lewis basic phosphine additives to the polymerization of ethylene using CpTiMe(NPtBu3)/B(C 6F5)3 systems resulted in the observation that the addition of sterically bulky phosphines, such as PtBu 3 and PCy3, increased the observed polymerization activity. It has been proposed that this phenomenon is a result of the greater ion-pair separation, due to interaction of the phosphine with the Ti metal center. This provides a novel way to view the active catalyst system and the methods involved with enhancement and activity optimization.
The synthesis of the sterically bulky phosphine-functionalized monomers, tBu2P(CH2)3CHCH2, and the polymerization of these monomers was investigated. The phosphine functionalized monomer was co-polymerized with 1-hexene, albeit in low percent yield and low incorporation of the functionality. Investigations of the potential inhibition pathways indicated that the co-polymerizations and homo-polymerizations of the phosphine-functionalized monomers are inhibited by reactivity with the co-catalyst, intermolecular coordination of the phosphine functionality, and intramolecular coordination of the phosphine.
Sterically frustrated Lewis pairs of bulky phosphines and the borane, B(C6F5)3 exhibit unprecedented reactivity with olefins, affording both intermolecular additions as well as intramolecular cyclizations. The expansion of the reactivity of the olefin activation is hindered by the nucleophilic aromatic substitution reactions.
These studies demonstrate the application of the concept of "frustrated" Lewis Pairs to the polymerization and activation of olefins.
McCahill, Jenny Sara Jean, "“Frustrated” Lewis Pairs: Applications in olefin polymerization and small molecule activation" (2008). Electronic Theses and Dissertations. 7912.