Cationic Ring-opening Polymerization Of 2-oxazolines Research Articles

Photoactivatable poly(2-oxazoline)s enable antifouling hydrogel membrane coatings

In recent years, hydrophilic poly(2-oxazoline)s have been examined as an alternative polymer to poly(ethylene glycol) (PEG) with added stability and chemical versatility. They show excellent anti-fouling properties and, like PEG, are of interest in many fields where bio-inertness or anti-adhesiveness are important such as for medical devices, tissue engineering, biosensors, or drug delivery carriers. Surprisingly few studies have utilized this polymer class for the preparation of anti-fouling membranes. Here we describe a new termination reaction of the living cationic ring-opening polymerization of 2-oxazolines in which a benzophenone group is introduced as a photoactivatable terminal head group of poly(2-oxazoline)s. Using 4-vinylbenzyl chloride as initiator, heterotelechelic poly(2-methyl-2-oxazoline) (PMOXA) macromonomers were synthesized and copolymerized with hydroxyl-functional PMOXA macromonomers to yield photoactivatable PMOXA-based prepolymer bottle brushes. Likewise, star-PMOXAs were synthesized by using a trifunctional initiator. Photoactivatable PMOXA prepolymers allowed us to readily form anti-fouling hydrogels by a simple and versatile CH-insertion cross-linking (CHic) process. Hydrogels were applied to poly(ether sulfone) ultrafiltration membranes as pore coating and the pure water permeance (PWP) was investigated as a function of hydrogel architecture and cross-link density. Anti-fouling hydrogel pore-coated membranes having PWPs on the order of hundred kg·m−2·h−1·bar−1 were obtained.

Cationic ring-opening polymerization of 2-oxazolines and 2-oxazines in biomass-derived solvents

In this study, we investigate the cationic ring-opening polymerization (CROP) of 2-oxazolines and 2-oxazines in two biomass-derived solvents, namely isosorbide dimethyl ether (DMI) and dihydrolevoglucosenone (Cyrene), environmentally friendly alternatives to conventional, fossil-based solvents such as acetonitrile and chlorobenzene. DMI matched the CROP kinetics and control of acetonitrile, with the advantage of being a non-toxic and biodegradable solvent. Furthermore, the living character of the polymerization process in DMI enabled us to synthesize well-defined, low-dispersity polymers with precise length and chain-end functionality. Furthermore, the potential of DMI is showcased by the successful synthesis of a triblock copolymer comprising three different monomers, as well as poly(2-ethyl-2-oxazoline) with a wide range of chain lengths. In turn, the polymerization rates were faster in Cyrene, but its application is limited by potential chain transfer reactions. Overall, this study may help to evaluate greener solvents and methods for polymer production, thereby mitigating the environmental impact of the increasing demand for polymer materials.

Investigation of cationic ring-opening polymerization of 2-oxazolines in the "green" solvent dihydrolevoglucosenone.

For about the last ten years, poly(2-oxazoline)s have attracted significant attention as potential material for biomedical applications in, e.g., drug delivery systems, tissue engineering and more. Commonly, the synthesis of poly(2-oxazoline)s involves problematic organic solvents that are not ideal from a safety and sustainability point of view. In this study, we investigated the cationic ring-opening polymerization of 2-ethyl-2-oxazoline and 2-butyl-2-oxazoline using a variety of initiators in the recently commercialized "green" solvent dihydrolevoglucosenone (DLG). Detailed 1H NMR spectroscopic analysis was performed to understand the influence of the temperature and concentration on the polymerization process. Size exclusion chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry were performed to determine the molar mass of the resulting polymers. Our work shows clearly that the solvent is not inert under the conditions typically used for the cationic ring-opening polymerization, as evidenced by side products and limited control over the polymerization. However, we could establish that the use of the 2-ethyl-3-methyl-2-oxazolinium triflate salt as an initiator at 60 °C results in polymers with a relatively narrow molar mass distribution and a reasonable control over the polymerization process. Further work will be necessary to establish whether a living polymerization can be achieved by additional adjustments.

Differences and similarities between mono-, bi- or tetrafunctional initiated cationic ring-opening polymerization of 2-oxazolines

Cationic ring-opening polymerization (CROP) is an interesting synthesis technique to obtain well-defined polymers with narrow molar mass distribution (MMD).

UV-Induced Cationic Ring-Opening Polymerization of 2-Oxazolines for Hot Lithography.

The cationic ring-opening polymerization (CROP) of 2-oxazolines gives polymers with unique characteristics arising from its polyamide backbones and structural versatility. Up to now, poly(2-oxazoline)s were obtained by classical thermal polymerization methods not aiming for application in bulk curing of structural polymers. We introduce the cationic photopolymerization of 2-oxazolines at elevated temperatures for the direct UV-induced curing of materials with exclusive chemical and structural particularities. After efficient photoinitiation via onium salt photoacid generators (PAGs), the immanent low-rate propagation is crucially promoted by thermal energy input to the ring-opening reaction. In simultaneous thermal analysis (STA), photo-DSC, and (thermo)mechanical analyses we investigated the UV-induced CROP of 2-oxazolines in a temperature range of 100-140 °C and show the exceptional potential of the introduced photopolymers. Furthermore, we applied the photopolymerizable system in Hot Lithography, a stereolithography-based 3D printing technology at elevated temperatures.

One‐Pot Synthesis of Block Copolymers by a Combination of Living Cationic and Controlled Radical Polymerization

Front Cover: The in-situ switch between different controlled/living polymerization methods remains a challenging but powerful tool to expand the wealth of well-defined functional block copolymers. In article number 1800398, Ulrich S. Schubert and co-workers present an approach that bridges the gap between cationic ring-opening polymerization of 2-oxazolines and the reversible addition-fragmentation chain transfer process for the controlled synthesis of vinyl monomers in a straightforward manner and without the need for tedious purification steps.

Bifunctional Initiators as Tools to Track Chain Transfer during the CROP of 2-Oxazolines.

Detailed kinetic studies during the cationic ring-opening polymerization (CROP) of 2-ethyl-2-oxazoline (EtOx) are conducted using four bifunctional bromo-type initiators in N,N-dimethylformamide (DMF) at 140 °C. Serving as models to quantify chain transfer to monomer occurring during the CROP initiated by monofunctional initiators, size exclusion chromatography (SEC) resolves a second molar mass distribution with lower molar mass at initial [monomer] to [initiation site] ratios ([M]0 /[I]0 ) of 25, while the resolution is insufficient at [M]0 /[I]0 of 10. Slightly slow initiation is revealed at [M]0 /[I]0 = 25, which prohibits the derivation of chain transfer rates by fitting of the size exclusion chromatography (SEC) data. Although conventional kinetic plots give no indication of significant amounts of chain transfer, the molar mass distributions resolved by SEC can unambiguously be identified as such by matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) in both the high as well as the low m/z regions of the mass spectra.

Sulfolane as Common Rate Accelerating Solvent for the Cationic Ring-Opening Polymerization of 2-Oxazolines.

The search for alternative solvents for the cationic ring-opening polymerization (CROP) of 2-methyl-2-oxazoline (MeOx) is driven by the poor solubility of P(MeOx) in polymerization solvents such as acetonitrile (CH3CN) and chlorobenzene as well as in MeOx itself. In this study, solvent screening has revealed that especially sulfolane is a good solvent for PMeOx. Unexpectedly, an increased propagation rate constant (kp) was found for the CROP of MeOx in sulfolane. Further extended kinetic studies at different temperatures (60-180 °C), revealed that the acceleration is due to an increase in frequency factor, while the activation energy (Ea) of the reaction is hardly affected. In order to explore the versatility of sulfolane as polymerization solvent for the CROP of 2-oxazolines in general, also the polymerization kinetics of other 2-oxazoline monomers, such as 2-ethyl-2-oxazoline (EtOx) and 2-phenyl-2-oxazoline (PhOx), have been studied, revealing a common acceleration of the CROP of 2-oxazoline monomers in sulfolane. This also enabled more controlled synthesis of PMeOx-block-PPhOx block copolymers that otherwise suffers from solvent incompatibility.

Systematic investigation of alkyl sulfonate initiators for the cationic ring-opening polymerization of 2-oxazolines revealing optimal combinations of monomers and initiators

A systematic kinetic investigation of the living cationic ring-opening polymerization (CROP) involving 2-ethyl-2-oxazoline, 2-methyl-2-oxazoline, and 2-phenyl-2-oxazoline employing a series of alkyl sulfonate initiators with variation of the alkyl initiating fragment (methyl, ethyl, iso-propyl) and the leaving group/counterion (tosylate, nosylate, triflate) is reported. The study reveals that the initiation and propagation reactivity increases in the order tosylate<nosylate<triflate. Slow initiation is observed for EtOTs, while EtONs is a sufficiently fast initiator even for 2-phenyl-2-oxazoline. It is thus recommended to avoid the use of alkyl tosylates, except MeOTs, as initiators for the CROP of 2-alkyl-2-oxazolines. Although triflates are generally the best initiators, the use of the more stable and easier synthesizable nosylates provides a suitable alternative for the design of functional initiators for the CROP of 2-alkyl-2-oxazolines.

Living cationic ring-opening polymerization of 2-oxazolines initiated by rare-earth metal triflates

Lewis acidic rare-earth metal triflates were found to efficiently initiate the cationic ring-opening polymerization of 2-oxazolines in a multi-site propagating fashion.

Poly(2‐oxazoline)s: A Polymer Class with Numerous Potential Applications

The living cationic ring-opening polymerization of 2-oxazolines has been studied in great detail since its discovery in 1966. The versatility of this living polymerization method allows copolymerization of a variety of 2-oxazoline monomers to give a range of tunable polymer properties that enable, for example, hydrophilic, hydrophobic, fluorophilic, as well as hard and soft materials to be obtained. However, this class of polymers was almost forgotten in the 1980s and 1990s because of their long reaction times and limited application possibilities. In the new millennium, a revival of poly(2-oxazoline)s has arisen because of their potential use as biomaterials and thermoresponsive materials, as well as the easy access to defined amphiphilic structures for (hierarchical) self-assembly. Recent developments that illustrate the potential of poly(2-oxazoline)s are discussed in this Review. In addition, the promising combination of poly(2-oxazoline)s and click chemistry is illustrated.

Synthesis and microwave assisted polymerization of fluorinated 2-phenyl-2-oxazolines: the fastest 2-oxazoline monomer to date

The living cationic ring-opening polymerization of 2-oxazolines with fluorinated aromatic substituents was found to be strongly accelerated by o-fluoro substituents.

Synthesis and Aqueous Micellization of Amphiphilic Tetrablock Ter- and Quarterpoly(2-oxazoline)s

The aqueous self-assembly of block copolymer systems is of major interest to prepare well-defined nanosized structures. Recently, the self-assembly of well-defined triblock and tetrablock ter- and quarterpolymers into sophisticated multicompartment micelles was reported. Inspired by these examples, the sequential one-pot synthesis of tetrablock ter- and quarterpolymers via living cationic ring-opening polymerization of 2-oxazolines was investigated under microwave irradiation, leading to well-defined tetrablock ter- and quarterpoly(2-oxazoline)s for the first time. The resulting tetrablock ter- and quarterpolymers were analyzed using both size exclusion chromatography and H-1 NMR spectroscopy, revealing their successful preparation. In addition, the surface energy of the polymers was determined to investigate the effect of block order on the surface properties. The last part of the paper discusses the aqueous self-assembly of the tetrablock ter- and quarterpolymers. Atomic force microscopy, transmission electron microscopy, and dynamic light scattering revealed the presence of spherical micellar aggregates.

Microwave-Assisted Cationic Ring-Opening Polymerization of 2-Oxazolines: A Powerful Method for the Synthesis of Amphiphilic Triblock Copolymers

We report on the first successful example of the preparation of triblock copolymers via a cationic ring-opening polymerization procedure. A library of 30 triblock copolymers was prepared from 2-methyl-, 2-ethyl-, 2-nonyl-, and 2-phenyl-2-oxazoline in a single-mode microwave reactor. The polymers exhibited narrow molecular weight distributions and showed only minor deviations from the targeted monomer ratio of 33:33:33. The glass-transition temperature of the triblock copolymers spanned the range from 50 to 100 °C depending on the incorporated monomers. The micellization behavior was investigated for some amphiphilic triblock copoly(2-oxazoline)s containing two hydrophilic and one hydrophobic blocks. The size of the micelles was larger when the hydrophobic block is located at one end of the triblock copoly(2-oxazoline)s, as measured by atomic force microscopy and dynamic light scattering.

Accelerated pressure synthesis and characterization of 2-oxazoline block copolymers

The cationic ring-opening polymerization of 2-oxazolines in acetonitrile was investigated under pressure conditions utilizing methyl tosylate as initiator of which the single crystal X-ray structure is described as well. The polymerization kinetics were studied and compared with previously reported microwave-assisted pressure polymerizations. Moreover, a series of block copolymers was synthesized in an automated parallel synthesis robot utilizing this pressure polymerization method. The resulting block copolymers were characterized with both differential scanning calorimetry and contact angle measurements to determine the effect of copolymer composition on glass transition temperature, melting point and surface energy. Atomic force microscopy was applied to further investigate the possible phase separation.

Microwave-assisted synthesis and properties of a series of poly(2-alkyl-2-oxazoline)s

Microwave-assisted organic synthesis is a quickly expanding field of research. The fast non-contact (super)heating of the reaction mixtures has already resulted in many examples of increased reaction rates and improved yields. However, the number of investigations focusing on microwave-assisted polymerizations, and especially living/controlled polymerizations, is still limited. Following our recent success in accelerating the cationic ring-opening polymerization of 2-oxazolines, we report here our studies on the cationic polymerization of a series of linear 2-alkyl-2-oxazolines under microwave irradiation. The effect of chain length on the polymerization kinetics was investigated. Moreover, the microwave polymerization of 2-ethyl-2-oxazoline was further investigated by 1H-NMR spectroscopy to determine the active species during the polymerization. Besides the microwave polymerization of the linear 2-alkyl-2-oxazoline monomers, the thermal and surface properties of the resulting polymers were investigated by DSC and contact-angle measurements to study the effect of side-chain length on the polymer properties.

Synthesis of α-alkyl ω-trimethoxysilane polyoxazolines and their application as coatings on glass fibres

A novel class of telechelic α-alkyl ω-trimethoxysilane poly(N-propionylethylene imine)s (TMS-PPEls) has been synthesized by living cationic ring-opening polymerization of 2-oxazolines. A new route for the termination reaction of the living polyoxazoline is described. These block copolymers have been tested as coatings for glass fibres. The polymers were applied by means of a solvent dip test. It is supposed that the affinity of the polyoxazoline block for glass surfaces promotes the reaction of the silane with the silanol groups of the glass substrate. Because of the presence of the apolar α-block, the surface tension of the glass surface will be lowered, as was shown by contact angle measurements. The presence of the polymer on the glass surface was also demonstrated by XPS measurements. The interfacial properties of coated glass plates and polymer matrices were determined by performing lap-shear tests.

Block and graft copolymers of 2-oxazolines

The cationic ring-opening polymerization of 2-oxazolines is known to be initiated by alkyl halides, Lewis acids and esters of strong acids. The polymerization proceeds by a living mechanism. Numerous block and graft copolymers have already been described. Recently it was found that chloroformates (R-O-CO-CI) are also useful initiators. The mechanism of the initiation and propagation is discussed. This type of initiators allows the synthesis of different novel two-block and three-block copolymers, star-shape polymers, and a graft copolymer with a polyvinylacetate backbone.

Monolayers and Langmuir-Blodgett Films of Poly(N-acylethylenimines) with Hydrocarbon and Fluorocarbon Side Chains

A series of poly(N-acylethylenimines) with hydrocarbon and fluorocarbon side chains varying in length from 6 to 17 carbon atoms were prepared by cationic ring-opening polymerization of 2-oxazolines. The behavior of monolayers of these materials at the air-water interface was studied in a Langmuir through, and multilayer Langmuir Blodgett (LB) films were prepared. Both monolayer behavior and LB film formation were strongly dependent on the size and nature of the side chains. Polymers with short hydrocarbon side chains (6-11 carbon atoms) form fluid liquil like condensed monolayers in which the polymer backbone is at the water surface and the side chains are tilted toward the air. They can be easily transferred onto solid substrates, and uniform LB films can be prepared

Block and graft copolymers of 2-oxazolines

This paper describes our recent developments of block and graft copolymers prepared by the cationic ringopening polymerization of 2-oxazolines. First, the mechanistic aspects of polymerization are mentioned, in which terms of “electrophilic and nucleophilic polymerizations” are proposed. Then, preparations and properties of poly(N-acylethylenimine) chains from 2-oxazoline monomers are described. They are (i) AB and BAB type block copolymers of 2-oxazolines for nonionic polymer surfactants, (ii) poly[(N-acetylethylenimine)-b-(oxyethylene)] for anti-electrostatic agents, (iii) cellulose diacetate having poly(N-acylethylenimine) graft chain for “compatibilizer” with commodity polymers, and (iv) polyethylene/vinyl acetate copolymer having poly(N-acylethylenimine) graft chain for blood-compatible polymeric materials.