Earth Metal-mediated Group Transfer Polymerization Research Articles

Poly(2-isopropenyl-2-oxazoline) as a Versatile Functional Polymer for Biomedical Applications.

Functional polymers play an important role in various biomedical applications. From many choices, poly(2-isopropenyl-2-oxazoline) (PIPOx) represents a promising reactive polymer with great potential in various biomedical applications. PIPOx, with pendant reactive 2-oxazoline groups, can be readily prepared in a controllable manner via several controlled/living polymerization methods, such as living anionic polymerization, atom transfer radical polymerization (ATRP), reversible addition-fragmentation transfer (RAFT) or rare earth metal-mediated group transfer polymerization. The reactivity of pendant 2-oxazoline allows selective reactions with thiol and carboxylic group-containing compounds without the presence of any catalyst. Moreover, PIPOx has been demonstrated to be a non-cytotoxic polymer with immunomodulative properties. Post-polymerization functionalization of PIPOx has been used for the preparation of thermosensitive or cationic polymers, drug conjugates, hydrogels, brush-like materials, and polymer coatings available for drug and gene delivery, tissue engineering, blood-like materials, antimicrobial materials, and many others. This mini-review covers new achievements in PIPOx synthesis, reactivity, and use in biomedical applications.

Synthesis and Application of Functional Group-Bearing Pyridyl-Based Initiators in Rare Earth Metal-Mediated Group Transfer Polymerization

The polymer class of poly(vinylphosphonates) offers a wide array of attractive features such as high biocompatibility, thermoresponsive behavior, and the option for the directed introduction of sma...

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.

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...

Ligand Induced Steric Crowding in Rare Earth Metal-Mediated Group Transfer Polymerization of Vinylphosphonates: Does Enthalpy Matter?

This contribution presents the first rare earth metal-mediated group transfer polymerization of vinylphosphonates from substituted cyclopentadienyl rare earth catalysts. Several initiators with increased steric demand of the coordinated ligand sphere and decreased size of the catalytically active metal center are prepared using salt metathesis. The effects of the modifications on molecular mass, propagation rate, initiation delay, and molecular mass distribution are monitored via activity measurements using a normalization method for living polymerizations, and the correlation between steric crowding and activity is presented. Temperature-dependent kinetic analyses are performed for several methyl-, trimethylsilyl-, and tetramethylcyclopentadienyl-substituted complexes to determine the activation enthalpies ΔH‡ and entropies ΔS‡ according to the Eyring equation. For (C5Me4H)3Ln complexes (Ln = Sm, Tb, Y), a change in the reaction enthalpy ΔH‡ is observed compared to un- and monosubstituted compounds. The ...

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 ...

C–H Bond Activation by σ-Bond Metathesis as a Versatile Route toward Highly Efficient Initiators for the Catalytic Precision Polymerization of Polar Monomers

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.

Stereospecific catalytic precision polymerization of 2-vinylpyridine via rare earth metal-mediated group transfer polymerization with 2-methoxyethylamino-bis(phenolate)-yttrium complexes

C 1-Symmetric 2-methoxyethylamino-bis(phenolate)-yttrium complexes for the stereospecific polymerization of 2-vinylpyridine.

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.

Poly(vinylphosphonate)s functionalized polymer microspheres via rare earth metal-mediated group transfer polymerization

We demonstrate a facile, yet efficient method for the functionalization of crosslinked polystyrene (PS) microspheres with biocompatible poly(vinylphosphonate)s via the combination of a UV grafting polymerization and a surface-initiated group transfer polymerization. Self-initiated photografting and photopolymerization of ethylene glycol dimethacrylate results in direct photografting of poly(ethylene glycol dimethacrylate) on the PS microspheres with dangling methacrylate functionalities, which are used to immobilize ytterbocene complexes to form the surface-bound rare-earth metal catalyst system. The surface-initiated GTP of dialkyl vinylphosphonates from the initiator system leads to the functionalization of PS microspheres with poly(vinylphosphonate) brushes. Polymerization kinetic investigation indicates that surface-initiated GTP leads to a constant and remarkably rapid weight gain of the microsphere (a microsphere weight increase of 600% within 3 min), owing to the highly living and efficient character of GTP. The surface-initiated GTP occurring inside the microsphere causes an accumulation of the tension between the polymer chains in the microsphere, which eventually induces fracture of the microsphere for longer polymerization time. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 2919–2925

Mechanistic Studies on Initiation and Propagation of Rare Earth Metal-Mediated Group Transfer Polymerization of Vinylphosphonates

Initiation of rare earth metal-mediated vinylphosphonate polymerization with unbridged rare earth metallocenes (Cp2LnX) follows a complex reaction pathway. Depending on the nature of X, initiation can proceed either via abstraction of the acidic α-CH of the vinylphosphonate (e.g., for X = Me, CH2TMS), via nucleophilic transfer of X to a coordinated monomer (e.g., for X = Cp, SR) or via a monomer (i.e., donor)-induced ligand-exchange reaction forming Cp3Ln in equilibrium (e.g., for X = Cl, OR), which serves as the active initiating species. As determined by mass spectrometric end group analysis, different initiations may also occur simultaneously (e.g., for X = N(SiMe2H)2). A general differential approach for the kinetic analysis of living polymerizations with fast propagation and comparatively slow initiation is presented. Time-resolved analysis of monomer conversion and molecular weights of the formed polymers allow the determination of the initiator efficiency throughout the whole reaction. Using this normalization method, rare earth metal-mediated vinylphosphonate GTP is shown to follow a Yasuda-type monometallic propagation mechanism, with an SN2-type associative displacement of the polymer phosphonate ester by a monomer as the rate-determining step. The propagation rate of vinylphosphonate GTP is mainly determined by the activation entropy, i.e. the change of rotational and vibrational restrictions within the eight-membered metallacycle in the rate-determining step as a function of the steric demand of the metallacycle side chains and the steric crowding at the metal center.

Poly(vinylphosphonate)s with Widely Tunable LCST: A Promising Alternative to Conventional Thermoresponsive Polymers

Novel statistic copolymers of dialkyl vinylphosphonates have been synthesized via rare earth metal-mediated group transfer polymerization using easily accessible tris(cyclopentadienyl)ytterbium. The copolymerization parameters have been determined by activity measurements showing the formation of almost perfectly random copolymers (r1, r2 ∼ 1). Thus, the polymerization rate of vinylphosphonate GTP is mainly limited by the steric demand of growing polymer chain end. The obtained copolymers of diethyl vinylphosphonate and dimethyl or di-n-propyl vinylphosphonate show thermoresponsive properties, i.e., exhibit a tunable lower critical solution temperature following a coil–globule transition mechanism, with cloud points between 5 and 92 °C. Hereby, the LCST can be precisely adjusted by varying the comonomer composition and correlates linearly with the content of hydrophilic/hydrophobic comonomer. These thermoresponsive poly(vinylphosphonate)s, exhibiting a sharp and reversible phase transition, and minor envi...

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).