Cationic Polymerization Research Articles

Tuning Polymer Molecular Weight Distribution in Cationic RAFT Polymerization by Mixing Chain Transfer Agents†

Comprehensive SummaryPolymer dispersity (Đ) or molecular weight distribution (MWD) is a basic but vital parameter for the properties of polymeric materials. Developing new methodologies for controlling polymer MWD is emerging as a research hotspot. However, the methods to tune polymer MWD in cationic polymerization are still not well explored. Herein, we present a simple method to control the dispersity of poly(isobutyl vinyl ether) (PIBVE) by mixing two different chain transfer agents in batch visible light induced cationic RAFT polymerization. A broad dispersity range (Đ ≈ 1.16—1.80) was successfully achieved while maintaining monomodal MWD. Moreover, chain extension of PIBVE through both cationic polymerization and radical polymerization has been studied, which also provides a method to tune polymer MWD in mechanism transformation polymerization.

Study of a Novel Fluorine-Containing Polyether Waterborne Polyurethane with POSS as a Cross-Linking Agent.

Waterborne polyurethane are more eco-friendly materials due to lower volatile organic compounds (VOCs, mainly isocyanates) content than the alternative materials. However, these rich hydrophilic groups polymers have not yet reached good mechanical properties, durability and hydrophobicity behaviors. Therefore, hydrophobic waterborne polyurethane has become a research hotspot, attracting significant attention. In this work, firstly, a novel fluorine-containing polyether P(FPO/THF) was synthesized by cationic ring-opening polymerization of 2-(2,2,3,3-tetrafluoro-propoxymethyl)-oxirane (FPO) and tetrahydrofuran (THF). Secondly, fluorinated polymer P(FPO/THF), isophorone diisocyanate (IPDI) and hydroxy-terminated polyhedral oligomeric silsesquioxane (POSS-(OH)8) were used to prepare a new fluorinated waterborne polyurethane (FWPU). Hydroxy-terminated POSS-(OH)8 was used as a cross-linking agent, while dimethylolpropionic acid (DMPA) and triethylamine (TEA) were used as a catalyst. Four kinds of waterborne polyurethanes (FWPU0, FWPU1, FWPU3, FWPU5) were obtained by adding different contents of POSS-(OH)8 (0%, 1%, 3%, 5%). The structures of the monomers and polymers were verified by 1H NMR and FT-IR, and the thermal stabilities of various waterborne polyurethanes were analyzed by thermogravimetric analyzer (TGA) and differential scanning calorimetry (DSC). As the results, the thermal analysis showed that the FWPU performed the good thermal stability and the glass transition temperature could reach at about -50 °C. The FWPU1 film exhibited that the elongation at break was 594.4 ± 3.6% and the tensile strength at break was 13.4 ± 0.7 MPa, elucidating that the FWPU1 film developed the excellent mechanical properties relative to the alternative FWPUs. Further, the FWPU5 film performed the promising properties, including the higher surface roughness of FWPU5 film (8.41 nm) obtained by the atomic force microscope (AFM) analysis, and the higher value of water contact angle (WCA) at 104.3 ± 2.7°. Those results illustrated that the novel POSS-based waterborne polyurethane FWPU containing a fluorine element could develop the excellent hydrophobicity and mechanical properties.

Ring-Opening Polymerization of Cyclic Acetals: Strategy for both Recyclable and Degradable Materials.

To cope with the severe plastic waste crisis, massive efforts have been made to develop sustainable polymer materials whose degradation involves a disposing and decomposing to small molecule (DDM) and/or a chemical recycling to monomer (CRM) process. Polyacetals, a type of pH-responsive polymers, are degradable under acidic conditions, while highly stable under neutral and basic circumstances. As for their synthesis, the cationic ring-opening polymerization (CROP) of cyclic acetals is an elegant and promising approach, though suffering from fatal side reactions and polymerization-depolymerization equilibrium. Recent development in CRM restimulates our interests in the long-forgotten CROP method due to its inherent depolymerization characteristics. In terms of the end-of-life options, polyacetals are recyclable materials with both DDM and CRM potentials. They not only expand the scope of materials for closed-loop recycling, but also help to tune the degradation properties of traditional polyesters and polyolefins. This review aims to discuss the synthesis of various polyacetals by CROP and their degradation properties from the perspectives of (1) polymerization of cyclic acetals, dioxepins, and hemiacetal esters, (2) copolymerization of cyclic acetals with heterocyclic or vinyl monomers, and (3) degradation and recycling properties of the related polymers. This article is protected by copyright. All rights reserved.

Kinetic and Chemical Effects of Clays and Other Fillers in the Preparation of Epoxy–Vinyl Ether Composites Using Radical-Induced Cationic Frontal Polymerization

Addition of fillers to formulations can generate composites with improved mechanical properties and lower the overall cost through a reduction of chemicals needed. In this study, fillers were added to resin systems consisting of epoxies and vinyl ethers that frontally polymerized through a radical-induced cationic frontal polymerization (RICFP) mechanism. Different clays, along with inert fumed silica, were added to increase the viscosity and reduce the convection, results of which did not follow many trends present in free-radical frontal polymerization. The clays were found to reduce the front velocity of RICFP systems overall compared to systems with only fumed silica. It is hypothesized that chemical effects and water content produce this reduction when clays are added to the cationic system. Mechanical and thermal properties of composites were studied, along with filler dispersion in the cured material. Drying the clays in an oven increased the front velocity. Comparing thermally insulating wood flour to thermally conducting carbon fibers, we observed that the carbon fibers resulted in an increase in front velocity, while the wood flour reduced the front velocity. Finally, it was shown that acid-treated montmorillonite K10 polymerizes RICFP systems containing vinyl ether even in the absence of an initiator, resulting in a short pot life.

C,N co-doped TiO2 hollow nanofibers coated stainless steel meshes for oil/water separation and visible light-driven degradation of pollutants

Complex pollutants are discharging and accumulating in rivers and oceans, requiring a coupled strategy to resolve pollutants efficiently. A novel method is proposed to treat multiple pollutants with C,N co-doped TiO2 hollow nanofibers coated stainless steel meshes which can realize efficient oil/water separation and visible light-drove dyes photodegradation. The poly(divinylbenzene-co-vinylbenzene chloride), P(DVB-co-VBC), nanofibers are generated by precipitate cationic polymerization on the mesh framework, following with quaternization by triethylamine for N doping. Then, TiO2 is coated on the polymeric nanofibers via in-situ sol–gel process of tetrabutyl titanate. The functional mesh coated with C,N co-doped TiO2 hollow nanofibers is obtained after calcination under nitrogen atmosphere. The resultant mesh demonstrates superhydrophilic/underwater superoleophobic property which is promising in oil/water separation. More importantly, the C,N co-doped TiO2 hollow nanofibers endow the mesh with high photodegradation ability to dyes under visible light. This work draws an affordable but high-performance multifunctional mesh for potential applications in wastewater treatment.

Study on the cationic polymerization of isobutyl vinyl ether initiated by dual active species based on [Ph3C][B(C6F5)4] and its mechanism

AbstractThe cationic polymerization of isobutene vinyl ether(IBVE) contains waters system was achieved by using triphenylmethylium tetrakis‐(pentafluorophenyl)borate [Ph3C][B(C6F5)4], which overcame the harsh experimental requirements of traditional cationic polymerization. The influences of water content, solvent polarity, and reaction temperature on polymerization were studied. The end structure of the polymer was analyzed by 1H‐NMR. Combined with the effect of the proton trapping agent and triethylamine salt which can trap [H+], it was found that there was a reversible equilibrium between [Ph3C+] and [H+] active species during the polymerization process. The cationic polymerizations of isobutyl vinyl ether were initiated by these two active species simultaneously. The controllable cationic polymerization of isobutyl vinyl ether was achieved by the addition of ethyl methyl carbonate and ethyl benzoate. Gaussian 09 W simulation confirms the polymerization mechanism.

Organocatalytic stereoselective cationic polymerization of alkenyl vinyl ethers and post-functionalization via click chemistry

Vinyl polymer tacticity is critical for regulating the thermal and physical properties of materials. Stereoselective cationic polymerization of vinyl ethers has gained significant progress in recent years, while monomer scope beyond simple alkyl vinyl ethers to access functionalized isotactic polymer materials remains to be explored but highly desirable. Herein, we report the successful development of a stereoselective cationic polymerization of alkenyl vinyl ethers by employing confined Brønsted acid organocatalysis. This organocatalytic system shows high efficiency and stereoselectivity in the polymerization of several alkenyl vinyl ethers, as well as good compatibility to the diolefinic monomers without touching the C=C bonds in the alkenyl part during the cationic polymerization, thus allowing for further modification on these C-C double bonds after polymerization to construct functionalized, isotactic polymer materials. Further, a series of thermoplastic copolymers with tunable properties and variable and predictable contents of side-chain C=C bonds can be synthesized through the corresponding stereoselective copolymerization. Sequential post-polymerization modification of both isotactic homopolymers and copolymers by combining thiol-ene and sulfur(VI) fluoride exchange (SuFEx) click reactions is also demonstrated.

Photoinitiation Mechanisms of Novel Phenothiazine‐Based Oxime and Oxime Esters Acting as Visible Light Sensitive Type I and Multicomponent Photoinitiators

AbstractIn this work, three new photoinitiators, based on the phenothiazine scaffold as a chromophore and potentially bearing the oxime ester functionality as an initiating group are designed and synthesized for the free radical polymerization of acrylates, the cationic polymerization of epoxides, and the formation of interpenetrated polymer networks upon irradiation with a light emitting diode emitting at 405 nm. These phenothiazine‐based oxime and oxime esters revealed impressive photoinitiation ability manifested by excellent polymerization rates and high final reactive function conversions. Significantly, they can be used as both; one‐component (Type I) and two‐component photoinitiating systems. Photoinitiation mechanisms through which reactive species are produced are investigated by means of different complementary techniques including real‐time Fourier transform infrared spectroscopy, UV–visible absorption spectroscopy, electron spin resonance spectroscopy, fluorescence (steady state and time resolved), cyclic voltammetry, and molecular modeling calculations. Thermal initiation behavior of the different oxime esters is also studied by using differential scanning calorimetry, highlighting their dual thermal/photochemical initiation ability. Finally, 3D printed objects are successfully fabricated by conducting both direct laser writing and 3D printing experiments.

Green Synthesis of Functionalized Poly(3-Glycidoxypropyl-Trimethoxysilane) Using an Eco-Catalyst (Treated Montmorillonite)

Telechelic poly(3-glycidoxypropyltrimetho¬xysilane) (PGPTMS) with acetate and methacrylate end groups was successfully synthesized by an efficient and solvent-free approach, with anhydrides (acetic anhydride (AA) and methacrylic anhydride (MA)), by cationic ring-opening polymerization of 3 glycidoxypropyl¬trimetho-xysilane (GPTMS), using an ecologic solid catalyst Maghnite-H+ (Mag-H+), instead of electrophilic catalysts, such as, Bronsted and Lewis acids which are very noxious and corrosive. Mag-H+ is a montmorillonite sheet silicate clay exchanged with protons. The structure of the obta¬ined macromonomers was confirmed by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic reso¬nance (NMR) and UV-visible spectroscopy. The presence of the methacrylate end groups of bis-macromonomers was determined by UV-visible spectroscopy. In order to find the optimal reaction conditions, effects of reaction time and the amount of anhydrides (AA and MA) on the yield of macromonomers were investigated.

Preparation and performance of silicone-acrylic latex for exterior wall coating of dirt pickup resistance

A self-cross-linkable core-shell silicone-acrylic latex and coatings with excellent dirt pickup resistance were studied to solve the pollution problem for the exterior wall. This article focused on adding two silicon compounds with CC in the core and shell at the influence on latex and coating performance, respectively. First, the octamethylcyclotetrasiloxane (D4) and tetramethyltetraethylcyclotetrasiloxaneand (D4V) were used to synthesize silicone copolymer (PVMS) by cationic ring-opening polymerization as the core. Next, in this silicone latex, butyl acrylate (BA), methyl methacrylate (MMA), methacrylic acid (MAA), and γ-methacryloxypropyltrimethoxysilane (A174) were copolymerized to form the silicone-acrylic latex (PVMS/ASI). Fourier Transform Infrared Spectrometers proved that PVMS was successfully grafted onto acrylic copolymer (ASI). The morphology of particles exhibited core-shell structure, which was clearly shown in TEM. Finally, a comprehensive study of the effect of D4V in the PVMS/ASI was conducted on the properties of such latexes, films, and coatings. It was found that the D4V content influenced quite significantly the properties of latexes and films (water absorption, contact angle, crosslinking degree) as well as the properties of coatings (dirt pickup resistance, water resistance, hardness, adhesion, water scrub resistance). Ultimately, it found that D4V had a positive effect on dirt pick-up. When D4V content was 23 wt% in PVMS/ASI, the coating had an excellent dirt pickup resistance with extremely low ΔR (2.65%).

Thermal management in radical induced cationic frontal polymerisation for optimised processing of fibre reinforced polymers

Radical induced cationic frontal polymerisation (RICFP) is considered a promising low energy method for processing of fibre reinforced polymers (FRPs). Optimisation of the local heat balance between reinforcement, epoxy resin and the surrounding mould is required to pave the way for its adaptation to an industrial processing method for high volume fraction structural fibre reinforced composites. In this work, we investigate several methods to control the governing heat balance in RICFP-processing of FRPs. Heat generation was controlled by tuning the initiator concentration while limitation of heat losses using highly insulating moulds was found beneficial to the front characteristics and resulting curing degrees. An optimised mould configuration allowed for self-sustaining RICFP in FRPs with fibre volume fractions (Vfs) up to 45.8%, exceeding previously reported maxima of similar systems. A process window was moreover established relating the Vf and required heat generation to the potential formation of a self-sustaining or supported front.

Photochemically generated ionic species for cationic and step-growth polymerizations

Developing innovative photochemical methods for the purpose of attaining green chemistry and mitigating global warming has recently become a very important concept. While we have to move towards a more sustainable world, daily life is becoming increasingly dependent on the use of polymeric materials. Conventional thermal polymerization systems fail to fulfill the requirements of sustainable chemistry, resulting in an increase in energy demand. This article outlines cationic and step-growth polymerizations of various monomers initiated by different photochemically generated ionic species and their advantages over conventional polymerization systems. Typical examples of photoinduced cationic and step-growth polymerizations performed under UV, visible and near-infrared (NIR) light are summarized under two main categories; direct and indirect initiating systems. In direct initiating systems the use of commercially available and structurally modified onium salts as one-component photoinitiators for both polymerizations are summarized. Indirect initiating systems, proceeding through i) photoinduced electron/energy transfer (PET), ii) charge-transfer complexation and iii) free-radical promoted cationic polymerization (FRPCP), are summarized with a focus on the recent step-growth methods developed in our laboratory.

O-Nitrobenzyl Ether-Enabled Polymer Photo-Degradation

Key words polymer photo-degradation - o-nitrobenzyl ether - photo cleavage - alternating copolymer - cationic polymerization

Effect of Excessive Iodonium Salts on the Properties of Radical-Induced Cationic Frontal Polymerization (RICFP) of Epoxy Resin

Effect of Excessive Iodonium Salts on the Properties of Radical-Induced Cationic Frontal Polymerization (RICFP) of Epoxy Resin

The interaction of triglycidyl phosphate with europium nitrate and properties of obtained metal-containing polymer

The interaction of phosphorus-containing epoxy trifunctional monomer – triglycidyl phosphate (TGPhT), with europium (III) nitrate hexahydrate is studied. A dissolution of europium salt added into TGPhT is occurred due to the interaction of the cation with phosphoryl groups and oxygen atoms of the epoxy ring, that is accompanied by the opening reaction and led to the formation of a polymer. The interaction of triepoxide with salt is investigated by FTIR-spectroscopy and differential scanning calorimetry (DSC). The thermal stability of the samples is determined by combined method of thermogravimetry and differential scanning calorimetry (TG/DSC), coupled with a quadrupole mass spectrometry to identify the gaseous products evolved during the thermal analysis of the samples. The kinetic parameters of the curing reaction are computed by using Thermokinetics software. The interaction of TGPhT with europium nitrate is suggested to occur through opening of epoxy rings in the presence of europium ions via the mechanism of cationic polymerization. The cured epoxy polymer exhibits luminescent properties.

Highly Efficient Cationic Polymerization of β-Pinene, a Bio-Based, Renewable Olefin, with TiCl4 Catalyst from Cryogenic to Energy-Saving Room Temperature Conditions

Polymers based on renewable monomers are projected to have a significant role in the sustainable economy, even in the near future. Undoubtedly, the cationically polymerizable β-pinene, available in considerable quantities, is one of the most promising bio-based monomers for such purposes. In the course of our systematic investigations related to the catalytic activity of TiCl4 on the cationic polymerization of this natural olefin, it was found that the 2-chloro-2,4,4-trimethylpentane (TMPCl)/TiCl4/N,N,N',N'-tetramethylethylenediamine (TMEDA) initiating system induced efficient polymerization in dichloromethane (DCM)/hexane (Hx) mixture at both -78 °C and room temperature. At -78 °C, 100% monomer conversion was observed within 40 min, resulting in poly(β-pinene) with relatively high Mn (5500 g/mol). The molecular weight distributions (MWD) were uniformly shifted towards higher molecular weights (MW) in these polymerizations as long as monomer was present in the reaction mixture. However, chain-chain coupling took place after reaching 100% conversion, i.e., under monomer-starved conditions, resulting in considerable molecular weight increase and MWD broadening at -78 °C. At room temperature, the polymerization rate was lower, but chain coupling did not occur. The addition of a second feed of monomer in the polymerization system led to increasing conversion and polymers with higher MWs at both temperatures. 1H NMR spectra of the formed polymers indicated high in-chain double-bond contents. To overcome the polarity decrease by raising the temperature, polymerizations were also carried out in pure DCM at room temperature and at -20 °C. In both cases, rapid polymerization occurred with nearly quantitative yields, leading to poly(β-pinene)s with Mns in the range of 2000 g/mol. Strikingly, polymerization by TiCl4 alone, i.e., without any additive, also occurred with near complete conversion at room temperature within a few minutes, attributed to initiation by adventitious protic impurities. These results convincingly prove that highly efficient carbocationic polymerization of the renewable β-pinene can be accomplished with TiCl4 as catalyst under both cryogenic conditions, applied widely for carbocationic polymerizations, and the environmentally benign, energy-saving room temperature, i.e., without any additive and cooling or heating. These findings enable TiCl4-catalyzed eco-friendly manufacturing of poly(β-pinene)s, which can be utilized in various applications, and in addition, subsequent derivatizations could result in a range of high-added-value products.

PIB-based block copolymer with a segment having alternating sequence of leucine and alanine side-chain pendants

A family of block copolymers with a hydrophobic polyisobutylene segment and another segment with an alternating sequence of tert-butoxy carbonyl (Boc)-leucine containing maleimide unit and Boc-alanine containing styrene unit was synthesized using a combination of living cationic and reversible addition-fragmentation chain transfer polymerizations. After successful deprotection of the Boc-groups from the pendant amino acids, the block copolymers become amphiphilic with free –NH3 + and –COOH functionalities in an alternating fashion in the poly(maleimide-alt-styrene) segment. The alternating sequence of leucine and alanine pendants throughout the second segment of the block copolymer chain was investigated systematically using NMR spectroscopy. In the pH range of 2–9, the deprotected copolymers showed pH-induced transformation of polymeric self-assemblies with critical aggregation concentrations ∼28 and ∼30 mg/L at pH 2 and pH 9, respectively. The pH-induced morphological transformation was well evidenced by 1H NMR spectroscopy, transmission electron microscopy, and field emission scanning electron microscopy analysis. Additionally, these block copolymeric self-assemblies were studied for delivery applications at different pH, using Nile red as a model hydrophobic dye molecule.

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.

Detailed Understanding of Solvent Effects for the Cationic Ring-Opening Polymerization of 2-Ethyl-2-oxazoline

Polymerization of 2-ethyl-2-oxazoline (EtOx) has often been in the spotlight for fundamental studies of poly(2-alkyl/aryl-2-oxazoline)s (PAOx) polymerization, especially initiator screening, solvent screening, and copolymerization trends. In this work, we build on previous observations of solvent effects on the cationic ring-opening polymerization (CROP) of EtOx, with additional experimental observations of previously unreported solvents to expand the explored parameter space. Our objective is to find solvents with the lowest activation energy (Ea) and higher Arrhenius preexponential factor (A), which will allow us to produce narrow molar mass distributions at higher molecular weights, in the least time. To achieve this, we examined the various single factors like Dimroth ET(30) values, the Kamlet–Abraham–Taft (KAT) linear free-energy relationship (LFER) equation(s), and the Catalan LFER equations. Only one of Catalan’s equations sufficiently disentangled dipolarity and polarizability to give a good fit due to contradictory effects. It was found that solvent nucleophilicity, electrophilicity, and polarizability affected the Ea, but not dipolarity. All four factors affected the A. This indicates that the Ea is minimized in solvents that do not solvate ions well (i.e. force ion-pairing), and A was minimized in more dipolar solvents that solvate the polymer chains well. A strongly negative activation entropy (ΔS‡) shows that the propagation reaction is associative. The Catalan LFER allows for the prediction of Ea, A, ΔH‡, and ΔS‡, and the derived kp, across a broad range of solvents.

Crystallinity of side-chain fatty acid containing block copolymers with polyisobutylene segment

Block copolymers of polyisobutylene (PIB) with side-chain fatty acid-containing polymethacrylate segment with varying n-alkyl side-chains were synthesized by the combination of living cationic and reversible addition-fragmentation chain transfer (RAFT) polymerizations. Hydroxyl-terminated PIB (PIB-OH) was converted into macro-chain transfer agent (PIB-CDP macro-CTA) and used during the RAFT polymerization of fatty ester methacrylate (FEMA) monomers to obtain the block copolymers. Block copolymers were characterized by 1H NMR spectroscopy, size exclusion chromatography (SEC), and thermogravimetric analysis (TGA). The block copolymers showed varied crystalline behaviour depending on the chain length of the fatty acids in the polymethacrylate segment, confirmed by differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), polarized optical microscopy (POM), and transmission electron microscopy (TEM). SAXS and POM analysis above the crystalline melting temperature (Tm) of the block copolymers showed crystalline to amorphous phase transformation, which upon cooling below their Tm regenerated crystalline morphology (lamella-like structure). Density functional theory (DFT) calculations were performed to support interlamellar distances determined from the SAXS study. Overall the modification in the orderliness of the lamellar orientation due to the amorphous PIB segment within the fatty acid-containing block copolymer leads to a new direction toward materials with modified physical properties.