C−H Bond Activation by σ‑Bond Metathesis as a Versatile Route toward Highly Efficient Initiators for the Catalytic Precision
Polymerization of Polar Monomers
Benedikt S. Soller, Stephan Salzinger, Christian Jandl, Alexander Pöthig, and Bernhard Rieger*
WACKER-Lehrstuhl für Makromolekulare Chemie, Technische Universitaẗ München, Lichtenbergstraße 4, 85748 Garching bei
München, Germany *S Supporting Information
ABSTRACT: Rare earth metals show high activities toward C−
H bond activation of heteroaromatic substrates and even methane. In this work, we demonstrate the suitability of this synthetic approach to rare earth metallocenes and show the applicability of the resulting complexes as highly efficient initiators for rare earth metal-mediated group transfer polymerization. Bis(cyclopentadienyl)(4,6-dimethylpyridin-2-yl)methyl lanthanide complexes exhibit unprecedented initiation rates for rare earth metal-mediated dialkyl vinylphosphonate polymerization and facilitate an efficient initiation for a broad scope of
Michael acceptor-type monomers.
Since the first reports on living polymerizations of acrylicmonomers using early transition metal initiators by Collins,
Ward,1 and Yasuda et al.2 in 1992, researchers have devoted their efforts in the optimization of reaction conditions and initiator efficiency and the extension of this method to a variety of (meth)acrylates and (meth)acrylamides.3 With respect to the propagation mechanism, this type of polymerization is recognized as coordinative-anionic or coordination−addition polymerization, and due to its similarity to silyl ketene acetalinitiated group transfer polymerization, it is also referred to as transition metal-mediated GTP.3d,4 Rare earth metal-mediated group transfer polymerization (REM-GTP) is of particular interest, as recent publications have shown that its applicability is not limited to common acrylic monomers, but also facilitates the polymerization of several other monomer classes, i.e., dialkyl vinylphosphonates (DAVP), 2-isopropylene-2-oxazoline (IPOx), and 2-vinylpyridine (2VP).4b,5 Moreover, our group reported on the development of a surface-initiated group transfer polymerization (SI-GTP) mediated by rare earth metal catalysts allowing the perfect decoration of substrates with polymer brushes of specific functionality.6 Recently, the modification of silicon nanoparticles to form thermoresponsive and photoluminescent hybrid materials using SI-GTP was published.7
The applicability of REM-GTP to new monomers enables the precise synthesis of tailor-made functional materials, as this polymerization method combines the advantages of both living ionic and coordinative polymerizations. According to its highly living character, REM-GTP leads to strictly linear polymers with very narrow molecular weight distribution (PDI < 1.1), exhibits a linear increase of the average molar mass upon monomer conversion, and allows the synthesis of block copolymers as well as the introduction of chain end functionalities.3 The coordination of the growing chain end at the catalyst suppresses side reactions and allows stereospecific polymerization as well as activity optimization by variation of both the metal center and the catalyst ligand sphere.3,4b
REM-GTP initiation usually proceeds via nucleophilic transfer of a strongly basic ligand, e.g., hydride, methyl, or
CH2TMS, to a coordinated monomer (Scheme 1a; this is not the case for divalent rare earth metal centers, for which redox initiation occurs).3d,8
Accordingly, for zirconocene systems, a variety of strategies for the synthesis of enolate initiators, which follow a faster initiation mechanism over an eight-electron process (Scheme 1b), has been presented.3d,9 Surprisingly, only little effort was devoted to the development of new initiating species for rare
Special Issue: Mike Lappert Memorial Issue
Received: November 20, 2014
Scheme 1. Possible Initiation Reactions for REM-GTP of
DAVP: Nucleophilic Transfer via a (a) 6e− or (b) 8e−
Process and (c) Deprotonation of the Acidic α-CH
Note pubs.acs.org/Organometallics © XXXX American Chemical Society A DOI: 10.1021/om501173r
Organometallics XXXX, XXX, XXX−XXX earth metal-based catalysts, which would in turn allow the introduction of novel chain end functionalities.
In previous work, we have shown that late lanthanide metallocenes are highly active catalysts for DAVP polymerization.5c However, in detailed mechanistic studies we found that the traditionally used strongly basic methyl and CH2TMS initiators lead to an inefficient, slow initiation by deprotonation of the acidic α-CH (Scheme 1c).10 Other initiating groups, such as sterically crowded Cp3Ln complexes and thiolato complexes [Cp2Ln(StBu)]2, were found to efficiently initiate DAVP polymerization; however, further development of Cp3Ln complexes is limited and thiolate end groups were found to be prone to elimination.10 Moreover, these complexes are not suitable initiators for sterically less demanding or weaker coordinating monomers such as IPOx or 2VP.5f,10
Accordingly, the development of new initiators for REMGTP, which facilitate an efficient initiation for a broad scope of monomers and which lead to a stable end group functionalization via a C−C bond, is still of current interest. Inspired by the use of enolate-type initiators in zirconium-mediated GTP, our group focused on the development of enolate or enamide initiators (Scheme 2) in order to facilitate an initiation over an eight-electron process. Such initiators simulate the active propagating species and bypass the ineffective initiation step starting from the alkyl initiator.
As synthetic routes via salt metathesis from lithium enolates and rare earth metal chlorides and via thermolysis of alkyl complexes in the presence of tetrahydrofuran are restricted to selected systems only,11 we decided to investigate the accessibility of rare earth enolates via α-CH-deprotonation of the respective carbonyls (or oxazoline/phosphonate) by amide and alkyl precursors Cp2Ln(bdsa)(thf) and Cp2Ln(CH2TMS)(thf) (bdsa = bis(dimethylsilylamide, N(SiMe2H)2). We first evaluated the reaction between Cp2Ln(bdsa)(thf) and acetone resulting in the quantitative formation of Cp2Ln(N(SiMe2OiPr)(SiMe2H)) by hydrosilylation of the carbonyl moiety.