researchA

Jäkle Group
Research in Organometallic & Polymer Chemistry

Research

Supported by:

Project A. Well-Defined Side-Group Functionalized Polymeric Lewis Acids

The incorporation of Lewis acidic centers into polymers is of great interest for the development of supported Lewis acid catalysts and provides new opportunities for applications in the field of materials chemistry. We have recently reported a new route to well-defined polymeric Lewis acids.[1] Our methodology involves the controlled polymerization of a functional monomer (S-Si) followed by a highly selective polymer modification step that provides facile access to the desired boron-functionalized polymeric Lewis acids (PS-BR2).

Importantly, our approach can directly be applied to the synthesis of random copolymers, end-functionalized polymers, and block copolymers containing Lewis acid centers at well-defined positions of the polymer chain.[2,3] For instance, we have recently prepared the first boron-containing diblock copolymers,[2] which are expected to lead us into intriguing new areas including micellar catalysis, nano-structured materials, and in the case of B(OH)2-functionalized polymers drug delivery applications. One of the most appealing aspect of this unusual class of polymers is that the Lewis acidity can readily be fine-tuned through selective high-yield substitution reactions providing access to a family of new polymeric Lewis acids (PS-BR2 ; R = alkyl, aryl, alkoxy, amino groups).[1,4]

We are currently exploring these new polymeric Lewis acids in the following areas:

	*i) Polymeric Lewis Acid - Lewis Base Complexes and Supramolecular Polymer Chemistry with Organoboron Polymers.* We study the coordination of Lewis bases to our polymeric Lewis acids with special emphasis on the aggregation behavior with donor-functionalized polymers. We have prepared several well-defined polymeric Lewis acid – Lewis base complexes.[5,6] Fine-tuning of the Lewis acidity of the boron centers allows us to control the strength and reversibility of the donor acceptor bonding. For instance, triarylborane polymers PS-BR2 (R = thienyl, C6F5) [5] and borane polymers PS-BH2 [6] readily coordinate strong nucleophiles as exemplified in the formation of soluble, isolable polymeric donor acceptor complexes with pyridine ligands. A temperature-dependent equilibrium between coordinated and uncoordinated sites on the other hand is established with weaker donors.[5]
			*ii) Organoboron Polymers for Sensor Systems and Device Applications.* Molecular species such as aluminum tris(8-hydroxyquinolate) (Alq3) and the related diphenylboronquinolate (Ph2Bq) are known to possess excellent properties as electron-conductionand light emitting components in organic light emitting diodes (OLEDs) and related devices.The use of polymeric materials may be advantageous due to improved processability, which facilitates device fabrication. We found that polymer substitution reactions as outlined above provide a straightforward route to suitable organoboron polymers such as the novel quinolate polymers shown to the left.[7,8] These polymers are highly soluble and luminescent thin films can be cast from solution. Current work involves the tuning of the emission characteristics of these materials.[8]
	We use a similar strategy for the development of novel sensor materials that rely on the selective interaction of nucleophiles (e.g. fluoride and cyanide) with the Lewis acid centers in our polymers. Coordination of the substrate results in a response in the absorption/emission characteristics of suitably substituted luminescent organoboron polymers (e.g. bithiophene and mesityl substituents on boron).[9] This response is specific to the nature of the nucleophile under investigation and can readily be verified via spectroscopic screening.
	*iii) Organoboron Polymers for Applications in Catalysis.* We have also succeeded in the synthesis of a highly Lewis acidic polymer containing pentafluorophenyl substituents on boron (PS-BArF). This unusual polymer represents a supported analog of the industrially important class of fluorinated organoborane Lewis acid catalysts.[1] The key step in the synthesis of PS-BArF relies on the selective exchange of bromine substituents in PS-BX2 with pentafluorophenylcopper, a novel aryl transfer reagent developed in our laboratory.[10,11] We are in the process of evaluating the performance of PS-BArF and related supported Lewis acids as catalysts in organic synthesis and as activators for olefin polymerization.

References :

(1) Y. Qin, G. Cheng, A. Sundararaman, F. Jäkle, J. Am. Chem. Soc. 2002, 124, 12672-12673; "Well-Defined Boron-Containing Polymeric Lewis Acids". Y. Qin, G. Cheng, K. Parab, O. Achara, F. Jäkle, Macromolecules 2004, 37, 7123-7131; "Highly Selective Polymer Modification Reactions as a New Route to Well-Defined Organoboron Polymers".

(2) Y. Qin, V. Sukul, D. Pagakos, C. Cui, F. Jäkle, Macromolecules 2005, 38, 8987-8990; "Preparation of Organoboron Block Copolymers via ATRP of Silicon and Boron-Functionalized Monomers".

(3) Y. Qin, C. Cui, F. Jäkle, Macromolecules 2007, 40, 1413-1420; "Silylated Initiators for the Efficient Preparation of Borane-End-Functionalized Polymers via ATRP".

(4) Y. Qin, G. Cheng, K. Parab, A. Sundararaman, F. Jäkle, Macromol. Symp. 2003, 196, 337-345; "Lewis Acidic Organoboron Polymers".

(5) Y. Qin, F. Jäkle, J. Inorg. Organomet. Polym. Mater. 2007, 17, 149-157; "Formation of Polymeric Lewis Acid – Lewis Base Complexes with Well-Defined Organoboron Polymers"

(6) A. Doshi, F. Jäkle, Main Group Chem. 2006, 5, 309-318; "Polystyrene-Supported Borane Complexes PS-BH2 D"

(7) Y. Qin, C. Pagba, P. Piotrowiak, F. Jäkle, J. Am. Chem. Soc. 2004, 126, 7015-7018; "Luminescent Organoboron Quinolate Polymers".

(8) (a) Y. Qin, I. Kiburu, K. Venkatasubbaiah, S. Shah, F. Jäkle, Org. Lett. 2006, 8, 5227-5230; "Luminescence Tuning of Organoboron Quinolates through Substituent Variation at the 5-Position of the Quinolato Moiety". (b) Y. Qin, I. Kiburu, S. Shah, F. Jäkle, Macromolecules 2007, 17, 149-157; "Synthesis and Characterization of Organoboron Quinolate Polymers with Tunable Luminescence Properties".

(9) K. Parab, K. Venkatasubbaiah, F. Jäkle, J. Am. Chem. Soc.2006, 128, 12879-12885. "Luminescent Triarylborane-Functionalized Polystyrene: Synthesis, Photophysical Characterization, and Anion Binding Studies". Highlighted in C&E News (http://pubs.acs.org/cen/news/84/i38/8438thiophene.html; September 25, 2006, p. 94)

(10) A. Sundararaman and F. Jäkle, J. Organomet. Chem. 2003, 681, 134-142; "A Comparative Study of Base-Free Arylcopper Reagents for the Transfer of Aryl Groups to Boron Halides".

(11) F. Jäkle, Dalton Trans. 2007, 2851-2858; "Pentafluorophenyl copper: aggregation and complexation phenomena, photoluminescence properties, and applications as reagent in organometallic synthesis".