Rare Earth Metal-mediated Group Transfer Polymerization Research Articles

Cytocompatible Hydrogels with Tunable Mechanical Strength and Adjustable Swelling Properties through Photo-Cross-Linking of Poly(vinylphosphonates).

Herein, the synthesis, characterization, and application of a novel synthetic hydrogel based on the photoinitiated cross-linking of poly(vinylphosphonates) is presented. First, statistical copolymers with adjustable ratios of the monomers diallyl vinylphosphonate (DAlVP) and diethyl vinylphosphonate (DEVP), as well as different molecular weights, were obtained via rare earth metal-mediated group-transfer polymerization (REM-GTP) while maintaining narrow polydispersities. The copolymers were cross-linked by applying photoinitiated thiol-ene click chemistry (λ = 365 nm). The network formation was monitored via oscillatory rheology coupled with UV-irradiation, revealing the high spatiotemporal control of the reaction. Moreover, the equilibrium storage moduli of poly(vinylphosphonate)-based hydrogels increased with a growing number of DAlVP units and upon application of a different cross-linker, which was additionally confirmed by nanoindentation experiments. In contrast, the water uptake of hydrogels decreased with higher DAlVP amounts in the corresponding hydrogels due to lower chain mobility and an overall increase in the hydrophobicity of the samples. Upon successful functionalization of P(DEVP-stat-DAlVP) copolymers with sodium 3-mercaptopropane-1-sulfonate, as indicated via 1H DOSY NMR, the respective cross-linked materials displayed a remarkable increase in the water uptake; thus, presenting highly hydrophilic gels with an apparent interplay between water uptake, cross-linking density, and functionalization degree. Finally, the purified hydrogels showed cytocompatibility and enabled cell adhesion of human umbilical artery smooth muscle cells (HUASMCs) after direct seeding. The materials further allowed the adhesion and growth of an endothelial layer, triggered no pro-inflammatory response as evidenced by cytokine release of M0 macrophages, and exhibited antibacterial properties toward S. aureus and E. coli.

Expanding the Scope of Organic Radical Polymers to Polyvinylphosphonates Synthesized via Rare-Earth Metal-Mediated Group-Transfer Polymerization

The synthesis of organic radical polymers (ORPs) is expanded to the field of rare-earth metal-mediated group-transfer polymerization (REM-GTP). The compatibility of this polymerization technique wi...

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

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.

Heteronuclear, Monomer-Selective Zn/Y Catalyst Combines Copolymerization of Epoxides and CO2 with Group-Transfer Polymerization of Michael-Type Monomers.

Terpolymerizations of cyclohexene oxide (CHO), CO2, and the Michael-type monomer 2-vinylpyridine (2VP) are presented. The combination of two distinct polymerization mechanisms was enabled by the synthesis of a heterobifunctional complex (3). Its β-diiminate zinc moiety allows the ring-opening copolymerization of CHO and CO2, whereas the yttrium metallocene catalyzed the rare earth metal-mediated group-transfer polymerization of the polar vinyl monomer. Both units were connected via the CH-bond activation of a pyridyl-alkoxide linker. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) revealed the successful transfer of the linker to the end-group of the respective homopolymers poly(cyclohexene carbonate) (PCHC) and poly(2VP) (P2VP) being the prerequisite for copolymer formation. Aliquot gel-permeation chromatography (GPC) analysis and solubility behavior tests confirmed the P2VP-block(b)-PCHC terpolymer formation via two pathways, a sequential and a one-pot procedure. Furthermore, the versatility of the method was demonstrated by introducing 2-isopropenyl-2-oxazoline (IPOx) as the second Michael-type monomer that yielded the terpolymer poly(IPOx)-b-PCHC.

From lanthanide-mediated, high-precision group transfer polymerization of Michael-type monomers, to intelligent, functional materials

A wide range of catalytic systems is already established for the precise polymerization of functional α,β unsaturated monomers. The rare earth metal-mediated group transfer polymerization (REM-GTP) represents a well-known method which not only enables the fast and controlled conversion of (meth)acrylates, but also of other Michael-type monomers, like the stimuli-responsive 2–vinylpyridine (2VP) and dialkyl vinylphosphonates (DAVP). The next step is to connect and combine the defined macromolecules with smart functions. This feature article is focused on the recent application-oriented advances in the fields of 2VP/DAVP (co)polymers synthesized with REM-GTP catalysts. On the one hand, these copolymers function as dual-responsive drug-delivery vehicles, while exhibiting high biocompatibility during cytotoxicity screenings and in vitro studies. On the other hand, post-polymerization functionalization of PDAVPs allows access to polymer-biomolecule conjugates and polymeric fluorescent markers. Combing these two features, p(DEVP-co-2VP), polymerized with a tailor-made initiator, facilitated the complexation of a rhenium compound, resulting in a photocatalytically-active copolymer.

Bottle-Brush Brushes: Surface-Initiated Rare Earth Metal Mediated Group Transfer Polymerization from a Poly(3-((2,6-dimethylpyridin-4-yl)oxy)propyl methacrylate) Backbone

A general and efficient method for the preparation of bottle-brush brushes is presented via the combination of self-initiated photografting and photopolymerization (SIPGP) and rare earth metal mediated group transfer polymerization (REM-GTP). This strategy involves the creation of poly(3-((2,6-dimethylpyridin-4-yl)oxy)propyl methacrylate) (PDMPPMA) brushes and the successive conversion to surface-grafted macroinitiator brushes through the C–H bond activation by Cp2YCH2TMS(THF) complex. The surface-bound macroinitiator exhibits high activity and no initiation period in the polymerization of monomers following a group-transfer polymerization mechanism, resulting in the formation of side chains along the PDMPPMA backbone. The versatile approach is not only applicable to the polymerization of conventional MMA, but also active for the polymerization of heteroatom containing monomers, e.g., vinylphosphonate, 2-vinylpyridine and 2-isopropenyl-2-oxazoline, thus affording bottle-brush brushes which cannot be effic...

Core-First Synthesis of Three-Armed Star-Shaped Polymers by Rare Earth Metal-Mediated Group Transfer Polymerization

Addressing polymer topologies is one of the key methods for tailoring polymer properties. Herein, we report for the first time on the core-first synthesis of three-armed star-shaped polymers with adjustable molecular weights via rare earth metal-mediated group transfer polymerization (REM-GTP). Based on the versatility of REM-GTP, enabling polymerization of a broad variety of functional monomers not accessible via conventional techniques, a novel and fast method toward directed polymeric structures was established. Therefore, the trinuclear catalyst was synthesized by 3-fold C–H bond activation of 1,3,5-tris(3,5-dimethyl-4-pyridinyl)benzene using Cp2YCH2TMS(THF) as precursor complex. Kinetic investigations in comparison to monometallic Cp2Y(sym-collidinyl) on the polymerization of diethyl vinylphosphonate (DEVP) and 2-isopropenyl-2-oxazoline (IPOx) evidenced activity of all three metal centers. However, in REM-GTP generally occurring incomplete initiation provoked by the interaction of initiators and mono...

2-Methoxyethylamino-bis(phenolate)yttrium Catalysts for the Synthesis of Highly Isotactic Poly(2-vinylpyridine) by Rare-Earth Metal-Mediated Group Transfer Polymerization

Highly isotactic poly(2-vinylpyridine) (P2VP) was synthesized by the group transfer polymerization of the prochiral 2-vinylpyridine (2VP) with 2-methoxyethylaminobis(phenolate)yttrium complexes. Isotacticities of up to Pm = 0.92, narrow molecular weight distributions, and high molecular weights were achieved by steric modifications of the variable bisphenolate ligand structure. The resulting polymer samples were characterized by thermoanalysis (DSC, TGA), GPC, and 13C NMR. The origin of the isotactic microstructure was attributed to an enantiomorphic site control mechanism based on 13C NMR mechanistic studies and allowed new insights into 13C pentad assignments.

Next Generation Multiresponsive Nanocarriers for Targeted Drug Delivery to Cancer Cells.

C-H bond activation of 2-methoxyethylamino-bis(phenolate)-yttrium catalysts allowed the synthesis of BAB block copolymers comprised of 2-vinylpyridine (2VP; monomer A) and diethylvinylphosphonate (DEVP; monomer B) as the A and B blocks, respectively, by rare-earth-metal-mediated group-transfer polymerization (REM-GTP). The inherent multi-stimuli-responsive character and drug-loading and -release capabilities were observed to be dependent on the chain length and monomer ratios. Cytotoxicity assays revealed the biocompatibility and nontoxic nature of the obtained micelles toward ovarian cancer (HeLa) cells. The BAB block copolymers effectively encapsulated, transported, and released doxorubicin (DOX) within HeLa cells. REM-GTP enables access to previously unattainable vinylphosphonate copolymer structures, and thereby unlocks their full potential as nanocarriers for stimuli-responsive drug delivery in HeLa cells. The self-evident consequence is the application of these new micelles as potent drug-delivery vehicles with reduced side effects in future cancer therapies.

Bernhard Rieger

Angewandte Chemie International EditionVolume 55, Issue 44 p. 13642-13642 Author ProfileFree Access Bernhard Rieger First published: 06 June 2016 https://doi.org/10.1002/anie.201603863AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract “My favorite composer is Dmitri Shostakovich. My favorite time of day is late evening. ...” This and more about Bernhard Rieger can be found on page 13642. The author presented on this page has recently published his 10th article in Angewandte Chemie in the last 10 years: “Silicon Nanocrystals and Silicon-Polymer Hybrids: Synthesis, Surface Engineering, and Applications”: M. Dasog, J. Kehrle, B. Rieger, J. G. C. Veinot, Angew. Chem. Int. Ed. 2016, 55, 2322; Angew. Chem. 2016, 128, 2366. The work of B. Rieger has been featured on the cover of Angewandte Chemie: “Transformation of Carbon Dioxide with Homogeneous Transition-Metal Catalysts: A Molecular Solution to a Global Challenge?”: M. Cokoja, C. Bruckmeier, B. Rieger, W. A. Herrmann, F. E. Kühn, Angew. Chem. Int. Ed. 2011, 50, 8510; Angew. Chem. 2011, 123, 8662. Bernhard Rieger Date of birth: January 21, 1959 Position: WACKER Chair of Macromolecular Chemistry and Director of the Institute of Silicon Chemistry, Technische Universität München E-mail: rieger@tum.de Homepage: www.makro.ch.tum.de Education: 1986 Diploma in chemistry, Ludwig-Maximilians-Universität (LMU), Munich 1988 Doctorate supervised by Prof. W. Beck, LMU 1988–1989 Postdoctoral work with Prof. Dr. J. C. W. Chien, University of Massachusetts at Amherst 1995 Habilitation (mentor: Prof. E. Lindner), University of Tübingen Awards: 2008 Elected to the Finnish Academy of Science and Letters; 2011 elected to the European Academy of Science; 2013 Wöhler Prize for Sustainable Chemistry, Gesellschaft Deutscher Chemiker (GDCh; German Chemical Society); 2013 elected to the National Academy of Science and Engineering “acatech”, Germany Current research interests: Novel catalysts for precision polymerization reactions. CO2 utilization for polymers and energy storage; photocatalytic CO2 conversion. Functional polymer/inorganic (nano)hybrids for energy conversion and optoelectronic applications. Main-group element catalysis. Hobbies: Literature, music, languages My favorite composer is Dmitri Shostakovich. My favorite time of day is late evening. The biggest challenge facing scientists is reintegration of the carbon atoms in CO2 and CH4 into chemical cycles. Chemistry is fun because synthesis is just another word for creativity. If I could be anyone for a day, I would be Maxwell's Demon on the end of a growing polymer chain. The most important future applications of my research are novel catalysts for functional precision polymers. My first experiment was charcoal, when I was six; then my parents’ barn was gone. My biggest inspiration is discussions with engaged students. The secret of being a successful scientist is never losing the fascination of explorative science. The most important thing I learned from my students is dedication. The principal aspect of my personality is definitely not patience. My favorite book is Discours de la méthode (R. Descartes). My 5 top papers: References 1“Rare Earth Metal-Mediated Precision Polymerization of Vinylphosphonates and Conjugated Nitrogen-Containing Vinyl Monomers”: B. S. Soller, S. Salzinger, B. Rieger, Chem. Rev. 2016, 116, 1993. (A catalytic route to functional polymers of precise microstructure.) 2“Thermoresponsive and Photoluminescent Hybrid Silicon Nanoparticles by Surface-Initiated Group Transfer Polymerization of Diethyl Vinylphosphonate”: J. Kehrle, I. M. D. Höhlein, Z. Yang, A.-R. Jochem, T. Helbich, T. Kraus, J. G. C. Veinot, B. Rieger, Angew. Chem. Int. Ed. 2014, 53, 12494; Angew. Chem. 2014, 126, 12702. (Catalytic formation of “breathing” photoluminescent nanocomposites.) 3“Flexibly Tethered Dinuclear Zinc Complexes: A Solution to the Entropy Problem in CO2/Epoxide Copolymerization Catalysis?”: M. W. Lehenmeier, S. Kissling, P. T. Altenbuchner, C. Bruckmeier, P. Deglmann, A.-K. Brym, B. Rieger, Angew. Chem. Int. Ed. 2013, 52, 9821; Angew. Chem. 2013, 125, 10004. (Novel concept for high-activity dinuclear polymerization catalysts with an altered rate law.) 4“High-Molecular-Weight Poly(vinylphosphonate)s by Single-Component Living Polymerization by Rare-Earth Metal Complexes”: U. B. Seemann, J. E. Dengler, B. Rieger, Angew. Chem. Int. Ed. 2010, 49, 3489; Angew. Chem. 2010, 122, 3567. (Catalytic polymerization on heteroelement monomers.) 5“Rare Earth Metal-Mediated Group-Transfer Polymerization: From Defined Polymer Microstructures to High-Precision Nano-Scaled Objects”: N. Zhang, S. Salzinger, B. S. Soller, B. Rieger, J. Am. Chem. Soc. 2013, 135, 8810. (The start of our work on catalytic precision polymerization.) Volume55, Issue44October 24, 2016Pages 13642-13642 ReferencesRelatedInformation

Rare Earth Metal-Mediated Precision Polymerization of Vinylphosphonates and Conjugated Nitrogen-Containing Vinyl Monomers.

This review focuses on introducing and explaining the rare earth metal-mediated group transfer polymerization (REM-GTP) of polar monomers and is composed of three main sections: poly(vinylphosphonate)s, surface-initiated group transfer polymerization (SI-GTP), and extension to N-coordinating Michael-type monomers (2-vinylpridine (2VP), 2-isopropenyl-2-oxazoline (IPOx)). The poly(vinylphosphonate)s section is divided into two parts: radical, anionic, and silyl ketene acetal group transfer polymerization (SKA-GTP) of vinylphosphonates in comparison to REM-GTP, and properties of poly(vinylphosphonate)s. The mechanism of vinylphosphonate REM-GTP is discussed in detail for initiation and propagation including activation enthalpies ΔH(‡) and entropies ΔS(‡) according to the Eyring equation. SI-GTP is presented as a method for surface functionalization, and recent trends for 2VP and IPOx polymerization are summarized. This review will serve as a good resource or guideline for researchers who are currently working in the field of rare earth metal mediated polymerization catalysis as well as for those who are interested in beginning to employ rare earth metal complexes for the synthesis of new materials from polar monomers.

Poly(vinylphosphonate)s as Macromolecular Flame Retardants for Polycarbonate

A series of poly(dialkyl vinylphosphonate)s (PDAVPs) have recently been reported to be available via rare earth metal-mediated group transfer polymerization (REM-GTP). We extend the existing portfolio of polyvinylphosphonates (PVPs) by presenting poly(ditolyl vinylphosphonate) (PDTVP) as the first example of a poly(diaryl vinylphosphonate) (PDArVP). Thermogravimetric analyses revealed that, for PDTVP, in contrast to the selected PDAVPs, side group cleavage does not occur. Instead, thermal decomposition takes place in a one-step mechanism at high temperatures above 350 °C. A series of PDAVPs and PDTVP were tested for their performance as flame-retardant additives (FRA) as well as flame-retardant coatings (FRC) for polycarbonate (PC). We thereby found that PDTVP is a promising FRA, because of its high thermal stability and its compatibility with polycarbonate. Poly(diisopropyl vinylphosphonate) (PDIVP) shows excellent performance as an FRC, because it forms a stable, blistered crust of poly(vinylphosphonic ...

Versatile 2-Methoxyethylaminobis(phenolate)yttrium Catalysts: Catalytic Precision Polymerization of Polar Monomers via Rare Earth Metal-Mediated Group Transfer Polymerization

The present study is one of the first examples for rare earth metal-mediated group transfer polymerization (REM-GTP) with non-metallocene catalyst systems. 2-Methoxyethylaminobis(phenolate)yttrium trimethylsilylmethyl complexes were synthesized and showed moderate to high activities in the rare earth metal-mediated group transfer polymerizations of 2-vinylpyridine, 2-isopropenyl-2-oxazoline, diethyl vinylphosphonate, diisopropyl vinylphosphonate, and N,N-dimethylacrylamide as well as in the ring-opening polymerization of β-butyrolactone. Reaction orders in catalyst and monomer were determined for the REM-GTP of 2-vinylpyridine. The mechanistic studies revealed that the catalyst systems follow a living monometallic group transfer polymerization mechanism allowing a precise molecular-weight control of the homopolymers and the block copolymers with very narrow molecular weight distributions. Temperature-dependent reaction kinetics were conducted and allowed conclusions about the influence of the bulky substituents around the metal center on the polymerization activity. Additional polymerization experiments concerning the combination of REM-GTP and ROP to obtain block copolymers were performed.

Catalytic Precision Polymerization: Rare Earth Metal‐Mediated Synthesis of Homopolymers, Block Copolymers, and Polymer Brushes

Recent studies have shown that rare earth metal‐mediated group transfer polymerization (REM‐GTP) can be applied to a variety of polar monomers, besides the classic (meth)­acrylates or (meth)acrylamides, giving access to new polymeric materials. This article highlights progress in this new field and gives a current overview of the initiation and propagation of vinylphosphonates, the extension to Michael‐type nitrogen‐coordinating monomers, and application to block copolymers, statistical copolymers, and 2D and 3D structures. Based on molecular understanding and high‐precision synthesis, REM‐GTP represents a versatile method for the preparation of new materials. image

Rare Earth Metal‐Mediated Group Transfer Polymerization of Vinylphosphonates

Recent studies have shown that poly(vinylphosphonate)s are readily accessible by rare earth metal-mediated group transfer polymerization (GTP). This article highlights the progress in this new field and advantages of GTP in comparison to classical anionic and radical polymerization approaches. Late lanthanide metallocenes proved to be efficient initiators and highly active catalysts for vinylphosphonate polymerization yielding polymers of precise molecular weight and low polydispersity. Using this method, our group has developed a surface-initiated GTP to prepare poly(vinylphosphonate) brushes. In combination with different ester cleavage strategies, rare earth metal-mediated GTP is an efficient way to create well-defined high-molecular-weight poly(vinylphosphonic acid).