Cationic Ring-opening Polymerization Research Articles

Ring-opening polymerization of six-membered cyclic hybrid dimers composed of an oxoester and thioester

AbstractThree cyclic oxoester-thioester hybrid monomers, 1 (3-methyl-1,4-oxathiane-2,5-dione), 2 (6-methyl-1,4-oxathiane-2,5-dione), and 3 (3,6-dimethyl-1,4-oxathiane-2,5-dione), were studied for anionic and cationic ring-opening polymerizations. These monomers are six-membered cyclic cross-dimers corresponding to combinations of glycolic and lactic acids with their thiol analogs. Anionic polymerizations using thiol as the initiator and 2,6-lutidine as the base catalyst were successful for the chemoselective cleavage of the thioester with the thiol propagating end. The polymerizability increased in the order of 3 < 1 < 2, which was in good agreement with the increasing ring strain order evaluated by Density Functional Theory calculations. The living character, to some extent, was suggested by the postpolymerization reactions, which involved a two-stage feed of the monomers and a thiol-ene terminal coupling reaction to form a block copolymer with PEG. Additionally, it was found that the polymerization took place in 2,6-lutidine without a thiol initiator and produced macrocyclic polymers. The cationic polymerizations took place with the aid of CF3SO3H and benzyl alcohol but involved side reactions with low chemoselective ring cleavage. The thioester unit caused the polymers to exhibit a lower Tg with greater thermal and photo degradability.

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.

One-pot in situ bulk polymerization of triblock copolymer PCL-b-PA6-b-PCL based on PA6 by using tetraphenyl tin

The preparation of polyesteramides (PEAs) with regular block structure has long been a challenge, owing to the occurrence of the ester-amide exchange during the polymerization at high tempereture. In this work, we report a simple and green method to prepare polyamide 6 (PA6) based triblock PEA whose structure is poly(ε-caprolactone)-block-PA6-block-poly(ε-caprolactone) (PCL-b-PA6-b-PCL) using tetraphenyl tin as catalyst without solvent. The first segment PA6 was synthesized via cationic ring-opening polymerization (ROP) of ε-caprolactam (CPL), later, ROP of ε-caprolactone (CL) was involved to extend polycaprolactone (PCL) segments by using tetraphenyl tin as catalyst. By varying the proportion of CPL and CL, series PA6-based PEAs was prepared and characterized by gel permeation chromatography, 1H, DOSY, and 13C NMR. The results showed that PEA containing more proportion of CL presented a higher molecular weight with narrow polydisperse index. In particular, the lower water absorption and lower oligomers content infers that the PEA could have a prospective application in processing precision electronic devices.

Precision Cellulose from Living Cationic Polymerization of Glucose 1,2,4-Orthopivalates.

Cellulose serves as a sustainable biomaterial for a wide range of applications in biotechnology and materials science. While chemical and enzymatic glycan assembly methods have been developed to access modest quantities of synthetic cellulose for structure-property studies, chemical polymerization strategies for scalable and well-controlled syntheses of cellulose remain underdeveloped. Here, we report the synthesis of precision cellulose via living cationic ring-opening polymerization (CROP) of glucose 1,2,4-orthopivalates. In the presence of dibutyl phosphate as an initiator and triflic acid as a catalyst, precision cellulose with well-controlled molecular weights, defined chain-end groups, and excellent regio- and stereospecificity was readily prepared. We further demonstrated the utility of this method through the synthesis of precision native d-cellulose and rare precision l-cellulose.

Synthesis of fully bio-based poly (3-hydroxybutyrate)-oligo-2-ethyl oxazoline conjugates.

This work refers to the synthesis and characterization of poly (3-hydroxybutyrate)-b-oligo (2-ethyl oxazoline) (oligoEtOx). Cationic ring-opening polymerization of 2-ethyl oxazoline yielded poly (2-ethyl oxazoline) (oligoEtOx) with a hydroxyl end. Carboxylic acid-terminated PHB was reacted with oligoEtOx via dicyclohexylcarbodiimide chemistry to obtain PHB-b-oligoEtOx conjugates. The obtained PHB-b-oligoEtOx conjugates were successfully characterized by 1H- and 13C NMR, FTIR, DSC, and size exclusion chromatography. PHB-b-oligoEtOx conjugates can be promising biologic active materials.

Cryogels Based on Poly(2-oxazoline)s through Development of Bi- and Trifunctional Cross-Linkers Incorporating End Groups with Adjustable Stability.

1,4-Bis(iodomethyl)benzene and 1,3,5-tris(iodomethyl)benzene were used as initiators for the cationic ring-opening polymerization (CROP) of 2-ethyl-2-oxazoline (EtOx) and its copolymerization with tert-butyl (3-(4,5-dihydrooxazol-2-yl)propyl)carbamate (BocOx) or methyl 3-(4,5-dihydrooxazol-2-yl)propanoate (MestOx). Kinetic studies confirmed the applicability of these initiators. Termination with suitable nucleophiles resulted in two- and three-armed cross-linkers featuring acrylate, methacrylate, piperazine-acrylamide, and piperazine-methacrylamide as polymerizable ω-end groups. Matrix-assisted laser desorption/ionization mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy confirmed the successful attachment of the respective ω-end groups at all initiation sites for every prepared cross-linkers. Except for acrylate, each ω-end group remained stable during deprotection of BocOx containing cross-linkers. The cryogels were prepared using EtOx-based cross-linkers, as confirmed by solid-state NMR spectroscopy, scanning electron microscopy, and thermogravimetric analysis. Stability tests revealed a complete dissolution of the acrylate-containing gels at pH = 14, whereas the piperazine-acrylamide-based cryogels featured excellent hydrolytic stability.

Computational evaluation of zirconocene catalysts for ε-caprolactone cationic ring-opening polymerization

This quantum chemical study presents the ligand effect and a structure–property relationship in the cationic ring-opening polymerization (CROP) of ε-caprolactone using zirconocene catalysts. We first examined the effects of catalyst structure on the initiation and chain propagation steps of the CROP process. A total of 54 catalyst structures were investigated to understand the influence of the ligand structure on the stability of the catalyst–monomer complex and polymerization activity. The properties of the catalysts were analyzed in terms of ancillary ligands, ligand substituents, and bridging units. Calculations showed that the polymerization follows a proposed cationic mechanism, with ring opening occurring via alkyl-bond cleavage. A correlation between complex stability and activation energy was also observed, with ligand substituents dominating in both steps. While the ancillary ligands directly affect the HOMO energy level, the bridges are mainly responsible for the catalyst geometries, resulting in reduced complex stability and higher activation energy for the propagation step. This study contributes to a better understanding of the structural characteristics of zirconocene catalysts, which offers guidance for improving CROP activities in lactone polymerization.

Lubricated PDMS-bearing ladderlike polysilsesquioxane coating: Fabrication and factors governing its long-term ice-shedding performance

Reacting an epoxy-bearing ladder-like polysilsesquioxane (LASQ) with mono-carboxy-terminated poly(dimethyl siloxane) (PDMS-COOH) yields LASQ-PDMS, a blend of LASQ and a graft (g) copolymer LASQ-g-PDMS. Photolyzing a cast film of LASQ-PDMS containing a cationic ring-opening polymerization initiator produces a robust and wear-resistant coating. Such a coating presents a liquid-like surface PDMS brush layer that imparts anti-smudge and ice-shedding properties. These properties are significantly enhanced via the application of a silicone oil (SO) as a lubricant. Investigated in this study are the effects of changing the molecular weight and mass fraction of PDMS as well as the molecular weight of a SO or composition of a SO mixture on properties of the lubricated LASQ-PDMS coating. These properties include the coating’s hardness, water repellence, wear resistance, SO retainability, initial ice adhesion strength (τ), variation of τ with the number of abrasions made by steel wool on a coating, and variation of τ with the number of icing/de-icing cycles. This systematic study establishes the exceptional wear resistance of the LASQ-PDMS coatings and reveals SO retainability as the primary factor governing the long-term ice-shedding performance of the coatings.

Kinetic modeling of cationic ring-opening polymerization for the synthesis of biodegradable poly(ε-caprolactone)

Ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) is an efficient way to produce a widely used biodegradable polymer, i.e., poly(ε-caprolactone) (PCL). Compared to coordination ROP, ionic ROP receives great attentions in recent years due to mild reaction conditions and well-defined polymer microstructure. To fully exploit the potential of ionic ROP, in this work, a mathematical kinetic model for activated monomer mechanism (AMM) based cationic ROP of ε-CL was developed based on method of moments (MoM). Using the developed model, we estimated the values of the kinetic coefficients of monomer activation and monomeric units’ protonation reactions. The simulated results are found to be in good agreement with experimental data, as well as the benchmark by kinetic equation. Moreover, the simulations reveal the crucial role of transesterification reaction in molar mass distribution broadening and how the concentrations of acid and alcohol affect the monomer conversion and average molar masses of the polymer. Finally, the model was successfully extended to a newly reported system using trityl tetrafluoroborate (TrBF4) as catalyst, proving the universality of our model. This study provides deeper understanding of the cationic ROP and allows to optimize process conditions to increase the industrial sustainability of PCL-based materials.

Determination of ω-end functionalities in tailored poly(2-alkyl-2-oxazoline)s by liquid chromatography and mass spectrometry.

The in-depth analytical characterization of polymers, in particular regarding intended biomedical applications, is becoming increasingly important to elucidate their structure-property relationships. Specifically, end group analysis of e.g. polymers featuring a 'stealth effect' towards the immune system is of particular importance because of their use in coupling reactions to bioactive compounds. Herein, we established a liquid chromatography (LC) protocol to analyse bicyclo[6.1.0]nonyne-functionalized poly(2-alkyl-2-oxazoline)s (POx)s as promising functional polymers that can be applied in strain-promoted click reactions. This work involved the synthesis of poly(2-methyl-2-oxazoline) (PMeOx) and poly(2-ethyl-2-oxazoline) (PEtOx) by living cationic ring-opening polymerization (CROP) with different molar masses ranging from 2 up to 17.5 kDa and, to our knowledge, the first liquid chromatographic analysis of PMeOx. The developed analytical protocol enables the quantitative determination of post-polymerization reaction sequences with respect to the conversion of the ω-end groups. All synthesized polymers were straightforwardly analysed on a C18-derivatized silica monolithic column under reversed-phase chromatographic conditions with a binary mobile phase gradient comprising a mixture of acetonitrile and water. Subsequent mass spectrometry of collected elution fractions enabled the confirmation of the desired ω-end group functionalities and the identification of synthetic by-products.

Amphiphilic Heterograft Copolymers Bearing Biocompatible/Biodegradable Grafts.

The amphiphilic heterograft copolymers bearing biocompatible/biodegradable grafts [poly(2-methyl-2-oxazoline-co-2-pentyl-2-oxazoline)-g-poly(d-l-lactic acid)/poly(2-ethyl-2-oxazoline)] were synthesized successfully by the combination of cationic ring-opening polymerization and click chemistry via the ⟨"grafting to"⟩ approach. The challenge of this synthesis was to graft together hydrophobic and hydrophilic chains on a hydrophilic platform based on PMeOx. The efficiency of grafting depends on the chemical nature of the grafts and of the length of the macromolecular chains. The self-assembly of these polymers in aqueous media was investigated by DLS, cryo-TEM, and SANS. The results demonstrated that different morphologies were obtained from nanospheres and vesicles to filaments depending on the hydrophilic weight ratio in the heterograft copolymer varying from 0.38 until 0.84. As poly(2-ethyl-2-oxazoline) is known to be thermoresponsive, the influence of temperature rise on the nanoassembly stability was studied in water and in a physiological medium. SANS and DLS measurements during a temperature ramp allowed to show that nanoassemblies start to self-assemble in "raspberry like" primary structures at 50 °C, and these structures grow and get denser as the temperature is increased further. These amphiphilic heterograft copolymers may include hydrophobic drugs and should find important applications for biomedical applications which require stealth properties.

Synthesis of Amphiphilic Block Copolymer and Its Application in Pigment-Based Ink.

Amphiphilic block copolymers-based aqueous color inks show great potential in the field of visual communication design. However, the conventional step-by-step chemistry employed to synthesize the amphiphilic block copolymers is intricate, with low yield and high economic and environmental costs. In this work, we present a novel method for preparing an amphiphilic AB di-block copolymer of PCL-b-PAA by employing a combined polymerization strategy that involves both cationic ring-opening polymerization (ROP) of the ε-caprolactone monomer and the reversible addition-fragmentation chain-transfer (RAFT) polymerization on the acrylic acid monomer simultaneously. The corresponding polycaprolactone (PCL) and polyacrylic acid (PAA) serve as the hydrophobic and hydrophilic units, respectively. The effectiveness of the amphiphilic AB di-block copolymer as the polymeric pigment dispersant for water-based color inks is evaluated. The amphiphilic AB di-block copolymer of PCL-b-PAA exhibits a molecular weight of 1400 g mol-1, which is consistent with the theoretical value and suitable for polymeric dispersant application. The high surface excess (Γmax) of the PCL-b-PAA in water indicates a densely packed molecular morphology at the water/air interface. Additionally, micelles can be stably formed in the aqueous PCL-b-PAA solution at very low concentrations by demonstrating a low CMC value of 10-4 wt% and a micelle dimension of approximately 30 nm. The model ink dispersion is prepared using organic dyes (Disperse Yellow 232) and the amphiphilic block copolymer of PCL-b-PAA. The dispersion demonstrates near-Newtonian behavior, which is highly favorable for the application as inkjet ink. Furthermore, the ink dispersion displays a low viscosity, making it particularly suitable for visual communication design and printing purposes. Moreover, the ink dispersion demonstrates an unimodal distribution of the particle size, with an average diameter of approximately 500 nm. It retains exceptional stability of dispersion and even conducts a thermal aging treatment at 60 °C for 5 days. This work presents a facile and efficient synthetic strategy and molecular design of AB di-block copolymer-based dispersants for dye dispersions.

Characteristics of novel cycloaliphatic epoxides based on UV-induced cationic ring-opening polymerization for plastic coatings

Cationic ring-opening polymerization (CROP)-capable cycloaliphatic epoxide monomers bearing a benzyl ether group (CE-BEs) were developed as glass-replacement plastic coatings for lightweight automobiles. Three different monomers of bis(((7-oxabicyclo[4,1,0]heptan-3-yl)methoxy)methyl)benzene isomers were synthesized to provide the cycloaliphatic epoxide functional groups. 1H and 13C nuclear magnetic resonance and Fourier transform infrared (FT-IR) spectroscopic analyses confirmed that the monomers were successfully synthesized. The shear viscosity and real-time crosslinking behavior of the CROP coatings were investigated using a rheometer. In particular, the release of thermal energy via CROP under UV irradiation was monitored using differential scanning photocalorimetry. The reaction conversion of cycloaliphatic epoxides by CROP before and after UV irradiation was verified using FT-IR spectroscopy. The transparency of the CROP coating films cured on a polycarbonate substrate was measured using an ultraviolet–visible spectrophotometer, and their adhesion was confirmed using cross-cut tests. For characterization of the surface mechanical properties, a nanoindentation tester and a nanoscratch tester were used in conjunction with an atomic force microscope to evaluate the surface hardness and scratch resistance, respectively. In addition, the thermomechanical properties of the cured CROP coating films were investigated using a rigid-body pendulum tester operated in dynamic physical mode. The ternary coating systems with CE-BEs were found to exhibit high photoreactivity, a dense crosslinking density, a transparent appearance, and enhanced adhesion and mechanical properties, making it suitable for glass-replacement plastic coatings.

Amino-modified 2-oxazoline copolymers for complexation with DNA

Novel amino-functionalized 2-oxazoline-based copolymers able to form complexes with DNA were obtained. Their efficiency of DNA condensation into polyplexes of scales appropriate for effective cellular internalization was studied.

Mechanism of cationic ring-opening polymerisation of ε-caprolactone using metallocene/borate catalytic systems: a DFT and NCI study on chain initiation, propagation and termination.

We present a comprehensive DFT investigation on the cationic ring-opening polymerisation (CROP) of ε-caprolactone (CL) using zirconocene/borate catalyst systems. All possible pathways of the interaction between cationic species [Cp2ZrMe+] and counteranions, [A-] = [MeB(C6F5)3]- and [B(C6F5)4]-, were examined during chain initiation, propagation, and termination steps. The calculations reveal an active chain-end mechanism with O-alkyl bond cleavage of the polymerisation. The catalytic performance of the two counteranions is found to be identical, and they influence the initial process through stabilisation of the cationic species via non-covalent interactions (NCI), with the [MeB(C6F5)3]- anion stabilising the catalyst-monomer complex more effectively than the [B(C6F5)4]- anion by 24.3 kJ mol-1. The first two propagations are likely the rate-determining step, with calculated free-energy barriers of 61.4-71.2 and 73.9-80.6 kJ mol-1 with and without the anions (A-), respectively. The presence of the counteranion significantly affects the third propagation rate, lowering the barriers up to 20 kJ mol-1. Comparison of the first termination and the third propagation shows that they are not competitive, with the termination being less facile. We also studied the initiation and propagation steps for the hafnocene catalyst and found that the Hf catalyst slightly favours the CL CROP in comparison to the Zr catalyst. Analysis of solvent and dispersion interaction demonstrates that both factors play an important role in the process. NCI analysis reveals weak (van der Waals) interactions at the contacts between the cationic species and the counteranions during the reaction course. Overall, our results offer insights into the structures and interactions involved in the polymerisation.

Stereoregular poly(2-phenylthiirane) via cationic ring-opening polymerization.

Herein, we describe an effective strategy for synthesizing polythioethers with a well-defined structure through the cationic polymerization of thiirane with electron-withdrawing substituents. This strategy allows for precisely controlling the regio- and stereochemistry of the ring-opening polymerization of 2-phenylthiirane, thus allowing for producing poly(2-phenylthiirane) with high stereoregularity using enantiomeric pure thiirane.

Solving fundamental concepts in supramolecular science with functionality-tolerant living polymerizations of self-assembling monomers and dendronized monomers

The evolution and development of several examples of functionality-tolerant living polymerizations to the level of perfection that they could be employed to solve fundamental concepts in supramolecular science by living polymerization of self-assembling monomers and self-organizable dendronized monomers is briefly reviewed. The review focuses on the development of the living polymerization methodology with brief examples on how they impacted the field of self-assembly and self-organization. The first polymerization reaction discussed is the stereoselective polymerization of phenylacetylene and of other arylacetylenes to generate helical poly(arylacetylene) stereoisomers. The transition from functionality non-tolerant to functionality-tolerant stereoselective polymerizations and the evolution to functionality-tolerant living stereoselective polymerization of phenylacetylene is used as the main example of this brief review. This living polymerization methodology is followed by living polymerization of vinyl ethers, oxazolines, of group transfer polymerization, of cationic ring-opening polymerization of cyclic siloxanes combined with hydrosilylation, of ROMP of cylooxanorbornene. This brief review is concluded by a brief discussion of the capabilities of the above-mentioned living methodologies with living radical polymerizations. The numerous applications in the field of supramolecular science of these living methodologies are not reviewed. However, the reader is directed to numerous comprehensive review articles discussing these applications. The challenges that remain to be solved in all these functionality-tolerant living polymerizations are presented.

Designing Polymer Electrolytes via Ring‐Opening Polymerization for Advanced Lithium Batteries

AbstractReplacing liquid electrolytes with solid‐state polymer electrolytes (SPEs) can solve the safety hazards of Li metal batteries (LMBs) while increasing their energy density. However, there has been limited success so far in preparing advanced SPEs with controllable molecular structure and chemical composition, posing great obstacles to further promoting its application in LMBs. Recently, ring‐opening polymerization (ROP), including cationic ROP, anionic ROP, and ring‐opening metathesis polymerization, has become a dazzling new star in achieving SPEs due to its mild polymerization conditions and controllable chemical composition (molecular structure, functional group), etc. Besides, there is no small molecule released during the polymerization process, which means reduced interfacial side reaction. Hence, in this review, the merits of ROP in preparing SPEs and its mechanism as well as interfering factors, etc are evaluated from the perspective of synthetic chemistry. Furthermore, the review focuses on outlining the existing cases related to ROP as much as possible and summarize them from different ring structures (from triple ring to multivariate ring) and polymerization methods, hoping to provide a comprehensive understanding and serve as strategic guidance for designing high‐performance SPEs. Challenges and opportunities regarding this burgeoning field are also discussed at the end.

Synthesis and cure kinetics of bisoxazoline monomers during cationic ring‐opening polymerization

AbstractIn this work, the synthesis and characterization of highly aromatic bisoxazoline (BOX) monomers containing diphenyl ether, furan, and thiophene backbones are presented. Preliminary investigations of the cure and reaction kinetics are performed to understand the impact of these moieties on polymerization and their glass transition temperatures. To do this, two catalysts are used, methyl triflate (MT) and dodecyl benzene sulfonic acid (DBS) to explore their role in controlling the cationic ring‐opening polymerization (CROP) mechanism. Polymerization was monitored using non‐isothermal differential scanning calorimetry and characterized according to the Kissinger and Ozawa models. Kinetic modeling using the Malek and Sestak‐Berggren methods showed excellent agreement with experimental values and was used to determine reaction rate constants. The polymerization of BOX monomers in the presence of both MT and DBS is shown to be highly latent, being stable until the melting point is reached, after which reaction occurs rapidly.

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.