Radical/anionic SRN1-Type Polymerization for Preparation of Polyarenes

• Applicant: Professor Dr. Armido Studer
• Subject Area: Organic Molecular Chemistry - Synthesis and Characterisation ; Polymer Materials
• Term: from 2013 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 238544132

• Final Report Year: 2017

Final Report Abstract

SRN1 chemistry has been successfully used within the frame of this project for the preparation of polyphenylenes, sulfur containing polymers (PPS and PMPS) and for estimating the pi-conjugation in various (poly)aromatic structures. The starting point for this project was the development of a transition-metal-free SRN1-type polymerization if diiodoarenes to give polyphenylenes. Diiodoarenes could be polymerized by monomagnesiation followed by oxidative initiation using TEMPO or O2. The initiation step of the SRN1-type polymerization of diiodoarenes was studied in more detail. It could be shown that the TEMPO-initiation most likely proceeds through a diarylmagnesium species which is SET oxidized forming the corresponding biaryl radical anion as a key intermediate. DFT calculations support this claim further. Based on these results, different SET oxidants and reagent combinations were tested with the goal to achieve initiation at low temperature. However, it was found that efficient initiation can be achieved only at higher temperature. A main challenge of the SRN1-type polymerization of diiodoarenes was found to be the selective monometallation of the starting diiodoarenes. To overcome this problem, a new approach was developed using monoiodoarenes containing a sulfonate group as an anionic leaving group. These substrates can be readily monometallated. Using this approach, the scope of the polymerization with respect to the monomer moiety could be extended to core structures like fluorenes and carbazoles, whose polymers have interesting spectroscopic properties. Furthermore, sulfur containing polymers were successfully prepared using the SRN1 reaction by employing iodinated phenylthiolates as monomers. Thiolates are prime nucleophiles in SRN1 chemistry and are known to readily react with iodoarenes. The resulting poly(paraphenylene sulfide) (PPS) and poly(metaphenylene sulfide) (PMPS) are industrially relevant materials. In a different approach, the SRN1 reaction was used to qualitatively estimate the pi-conjugation length in (extended) aromatic systems. This was achieved by using diiodinated aromatic structures which were subjected to SRN1 conditions using thiolate nucleophiles. Analysis of the reaction outcome at low conversion allowed for estimating pi-conjugation efficiency in various bisiodinated pi-systems.

Publications

• TEMPO-Mediated Homocoupling of Aryl Grignard Reagents: Mechanistic Studies, Org. Biomol. Chem. 2015, 13, 2762-2767
  S. Murarka, J. Möbus, G. Erker, C. Mück-Lichtenfeld, A. Studer
  
• Poly(paraphenylene sulfide) and Poly(metaphenylene sulfide) via Light-initiated SRN1-type Polymerization of Halogenated Thiophenols, Rapid Commun. 2016, 37, 1494-1498
  N. B. Heine, A. Studer
  
• Intra- versus Intermolecular Electron Transfer in Radical Nucleophilic Aromatic Substitution of Dihalo(hetero)arenes – a Tool for Estimating π-Conjugation in Aromatic Systems, Chem. Sci. 2017, 8, 3547-3553
  B. Janhsen, C. G. Daniliuc, A. Studer