C–H Bond Activation of Silyl-Substituted Pyridines with Bis(Phenolate)Yttrium Catalysts as a Facile Tool towards Hydroxyl-Terminated Michael-Type Polymers

Abstract

Herein, silicon-protected, ortho-methylated hydroxy-pyridines were reported as initiators in 2-aminoalkoxy-bis(phenolate)yttrium complexes for rare earth metal-mediated group-transfer polymerization (REM-GTP) of Michael-type monomers. To introduce these initiators, C−H bond activation was performed by reacting [(ONOO)tBuY(X)(thf)] (X = CH2TMS, thf = tetrahydrofuran) with tert-butyl-dimethyl-silyl-functionalized α-methylpyridine to obtain the complex [(ONOOtBuY(X)(thf)] (X = 4-(4′-(((tert-butyldimethylsilyl)oxy)methyl)phenyl)-2,6-di-methylpyridine). These initiators served as functional end-groups in polymers produced via REM-GTP. In this contribution, homopolymers of 2-vinylpyridine (2VP) and diethyl vinyl phosphonate (DEVP) were produced. Activity studies and end-group analysis via mass spectrometry, size-exclusion chromatography (SEC) and NMR spectroscopy were performed to reveal the initiator efficiency, the catalyst activity towards both monomers as well as the initiation mechanism of this initiator in contrast to commonly used alkyl initiators. In addition, 2D NMR studies were used to further confirm the end-group integrity of the polymers. For all polymers, different deprotection routes were evaluated to obtain hydroxyl-terminated poly(2-vinylpyridine) (P2VP) and poly(diethyl vinyl phosphonate) (PDEVP). Such hydroxyl groups bear the potential to act as anchoring points for small bioactive molecules, for post-polymerization functionalization or as macroinitiators for further polymerizations.

Uniting Group-Transfer and Ring-Opening Polymerization─Block Copolymers from Functional Michael-Type Monomers and Lactones

“…For thf, during homopolymerization of 2VP, a competitive coordination of thf and 2VP took place decreasing the turnover frequency (TOF) and increasing the polydispersity hindering successful block copolymerization. 27 A similar behavior is expected for pyridine, and a complete inhibition of 2VP polymerization was already observed in the literature, 28 while polar protic solvents are not applicable due to incompatibility with the yttrium complex. Using toluene, P2VP slightly precipitates over the course of the reaction; however, upon the addition of CL, the polymer is fully dissolved again, facilitating controlled copolymerization with narrow polydispersity of the observed polymer (AB 9 ).…”

“…To investigate the influence of the reaction solvent, AB 8 was prepared using tetrahydrofuran and AB 9 using toluene instead of dichloromethane. For thf, during homopolymerization of 2VP, a competitive coordination of thf and 2VP took place decreasing the turnover frequency (TOF) and increasing the polydispersity hindering successful block copolymerization . A similar behavior is expected for pyridine, and a complete inhibition of 2VP polymerization was already observed in the literature, while polar protic solvents are not applicable due to incompatibility with the yttrium complex.…”

“…While the micellization of pH-responsive P2VP-based block copolymers ,,, and PCL-based block copolymers − as well as the combination of P2VP and PCL − are known in the literature, those block copolymers are usually prepared by (living) anionic polymerization instead of applying catalytic techniques. Especially, the susceptibility of such copolymers toward ester bond cleavage by hydrolysis or enzymatic degradation makes these systems interesting tools for drug-delivery applications. − The presented novel preparation of such copolymers via yttrium catalysis is a facile tool not only for precisely tuning the micelle properties but also to offer the advantage of end-group functionalization for drug conjugation or imaging. ,,, …”

Azide‐Modified Poly(diethyl vinylphosphonate) for Straightforward Graft‐to Carbon Nanotube Functionalization

“…Verification of whether the end-group is attached to the polymer is usually done by 1 H-NMR or diffusion ordered (DOSY)-NMR spectroscopy. [17,18] However for the herein prepared polymers this approach is not feasible due to the high molecular weight of the polymers and the overall low number of detectable protons of the PyN 3 end-group. Additionally, the intactness of the azide moiety cannot be ensured by means of NMR spectroscopy.…”

“…[6][7][8][9][10][11] Using these two methods, various different polymers like poly(N-isopropylacrylamide), [12] polycaprolactone, [13] poly(methyl methacrylate) (copolymers) [13,14] or polystyrene [13,15] have successfully been anchored to carbon nanotubes, for further functionalization examples a variety of comprehensive reviews exists in literature. [10,16] As a versatile functionalization technique for modifying group-transfer polymerization-based polymers, C-H bond activation of substituted 𝛼-methylpyridines has been utilized in literature, successfully introducing a variety of different moieties, including alcohols, [17,18] amines and thiols, [18] bipyridines, [19] di-and tripyridine initiators, [20,21] or double-bonds [22] as end-groups. In this approach, an azide-substituted 𝛼-methylpyridine is introduced to rareearth metal-based pre-catalysts like Cp 2 Ln(CH 2 TMS)(thf) or [(ONOO) tBu Ln(CH 2 TMS)(thf)] (Ln = Y, Lu) by means of C-H bond activation.…”