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.
Highlights
Since the initial discovery of group-transfer polymerization (GTP) of methyl methacrylate (MMA)using silyl ketene acetals by Webster et al in 1983 [1], this polymerization type has been continuously refined
Since the first polymerization attempts, various organocatalysts, andofnon-metallocenes were established for the polymers synthesis [3,5,7,8,9,10,11,12,13,14]
4-chloro-2,6-dimethylpyridine with 4-hydroxymethyl benzyl boronic acid resulting in the coupling protection group chemistry
Summary
Since the initial discovery of group-transfer polymerization (GTP) of methyl methacrylate (MMA)using silyl ketene acetals by Webster et al in 1983 [1], this polymerization type has been continuously refined. The application of a neutral samarocene-based complex for the polymerization of MMA by Yasuda et al in 1992 [2] introduced the field of rare earth metal-based group-transfer polymerization (REM-GTP). Mechanism elucidation on this reaction revealed a repeated 1,4-conjugate addition (Michael-addition) during propagation with a keto-enolate 8-membered cyclic intermediate [1,2,3,4,5,6,7,8]. Since the first polymerization attempts, various organocatalysts, andofnon-metallocenes were established for the polymers synthesis [3,5,7,8,9,10,11,12,13,14]. Of highly were established metallocenes for the synthesis highly precise, tailor-made and functional precise, tailor-made and functional polymers [3,5,7,8,9,10,11,12,13,14].
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