Toolbox of Nonmetallocene Lanthanides: Multifunctional Catalysts in Group-Transfer Polymerization

Abstract

Herein, we present a fundamental study of isostructural 2-methoxyethylamino-bis(phenolate)-lanthanide complexes [(ONOO)M(X)(THF)] (M = Lu, Y; R = Bu, CMePh, X = CHTMS, collidine; THF = tetrahydrofuran; TMS = trimethylsilyl) for rare-earth metal-mediated group-transfer polymerization (GTP). This analysis includes the differentiation of electron-donating and nondonating vinyl monomers and two metal centers with regard to the ionic radius (yttrium and lutetium). In addition, highly nucleophilic alkyl initiators are compared with electron-donating heteroaromatic initiators. Our examinations include the impact of these parameters on the activity, initiator efficiency, and tacticity of the obtained polymers. Density functional theory calculations and proposed catalyst structure determinations via X-ray analysis support these investigations. This facilitates the selection of the best metal and initiator combination to address efficient and stereospecific polymerization of a broad range of Michael monomers. [(ONOO)Lu(X)(THF)] shows the highest activity of 2220 h (normalized turnover frequency) for the polymerization of 2-vinylpyridine due to the higher Lewis-acidity of lutetium. Through C(sp)-H bond activation, catalysts with higher initiator efficiency in N,N'-dimethylacrylamide (DMAA) and diethylvinylphosphonate polymerization were synthesized. Remarkably, [(ONOO)Y(collidine)(THF)] was capable of stereospecifically polymerizing DMAA to highly isotactic poly(DMAA) (P = 0.94). Overall, the kinetics studies reveal a living-type GTP mechanism for all of the tested catalysts, enabling precise molecular-weight predeterminations with narrow molecular weight distributions (Đ ≤ 1.06).

Metal‐Catalyzed Group‐Transfer Polymerization: A Versatile Tool for Tailor‐Made Functional (Co)Polymers

“…This type of C−H bond activation of trivalent lanthanides and d 0 ‐transition metals is a common way for C−H‐bond cleavage, as these metals do not possess any electrons for oxidative addition‐ and reductive elimination‐reactions. C−H bond activation of a variety of alkynes and heteroaromatic compounds such as 2,4,6‐trimethylpyridine ( sym ‐collidine), 1‐trimethylsilyl‐1‐propyne, or 2,3,5,6‐trimethylpyrazine was previously reported by the groups of Rieger and Mashima (Figure ) …”

“…Isotacticities up to 92 % were reached with a bis(phenolate) catalyst bearing a tris(3,5‐dimethylphenyl)methyl‐moiety . With regard to the stereospecific polymerization of DMAA, the 2‐aminoalkoxy‐bis(phenolate)yttrium complexes and the before mentioned a nsa ‐zirconocenium complexes are the only catalysts that produce highly isotactic PDMAA with isotacticities up to 99 % in a living polymerization …”

“…Rieger et al. utilized similar 2‐aminoalkoxy‐bis(phenolate)catalysts with the general form (ONOO) R Ln(CH 2 TMS)(THF) (Ln=Y, Lu) for the polymerization of 2VP, DMAA, DEVP and IPOx under mild conditions with moderate to exceptional activities . In situ ATR‐IR measurements assumed a monometallic Yasuda‐type mechanism analogue to that of lanthanocenes.…”

“…Contrary to observations in DEVP and DMAA polymerization, 2‐aminoalkoxy‐bis(phenolate)yttrium complexes with heteroaromatic initiators showed a decreased efficiency in polymerization with electron‐donating monomers (2VP and IPOx) as an overload of electron density is provoked at the yttrium center. Computational studies underlined that the coordination of nitrogen‐donating monomers was hindered with these yttrium catalysts, if an electron‐donating initiator was used …”

“…CÀHb ond activation of av ariety of alkynes and heteroaromaticc ompounds such as 2,4,6-trimethylpyridine( symcollidine), 1-trimethylsilyl-1-propyne, or 2,3,5,6-trimethylpyrazine was previously reportedb yt he groups of Rieger and Mashima ( Figure 3). [26,27,29] As yttrium complexes with alkylinitiators, in general, show relativelyl ow initiator efficiencies in the polymerization of DEVP,h eteroaromatic initiatorsc an be introduced through sbond metathesis to improve the initiator efficiency.I nefficient initiation by deprotonation of the acidic a-CH is avoided and initiation by nucleophilic attack of the initiator to the first monomer takes place. [16a, 30] Contraryt oo bservations in DEVP and DMAA polymerization, 2-aminoalkoxy-bis(phenolate)yttrium complexes with heteroaromatic initiators showedadecreased efficiency in polymerization with electron-donating monomers (2VP and IPOx) as an overload of electrond ensity is provoked at the yttrium center.C omputational studies underlined that the coordination of nitrogen-donating monomers was hindered with these yttrium catalysts, if an electron-donatingi nitiator was used.…”