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Visible-Light Emulsion Photopolymerization of Styrene
Abstract
The photopolymerization of styrene in emulsion is achieved in a conventional double wall reactor equipped with a LED ribbon coiled around the external glass wall. Styrene mixed to acridine orange is added to the water phase containing sodium dodecyl sulfate, a water-soluble N-heterocyclic carbene-borane and disulfide, and irradiated. Highly stable latexes are obtained, with particles up to a diameter of 300 nm. The ability to reach such large particles sizes via a photochemical process in a dispersed medium is due to the use of visible light: the photons in the visible range are less scattered by larger objects and thus penetrate and initiate better the polymerizations. They are also greener and cheaper to produce via LEDs, and much safer than UVs. The method presented does not require any specific glassware; it works at lower temperature and delivers larger particles compared to thermal processes at similar solids contents and surfactant concentrations.
... As part of our exploration of light-driven reactions [29][30][31][32][33] we considered photocatalysis. UV photons were used to form nitrenes en route to the azimines. ...
We report the photocatalytic formation of azimines upon visible light irradiation of mixtures of sulfonyl azides and electron‐rich azo reagents in the presence of IrIII complexes. The method was applied to a series of alkyl‐ and arylsulfonyl azides and azo reagents. The azimines were previously prepared using toxic reagents such as lead‐tetraacetate, or harsher UV photons. The reactions can be carried out either in batch or flow conditions. The observation that the azo reagent equilibrates under the reaction conditions and the high energy of the azide triplet state suggests that the reaction may proceed through a photocatalytic cycloaddition, rather than via a nitrene.
... Los iniciadores más comunes en polimerización en emulsión son de tipo térmico, los cuales, a temperaturas relativamente altas se descomponen generando radicales libres, y los de tipo rédox, los cuales generan radicales libres por reacciones de reducción-oxidación a temperaturas inferiores de 50 °C 38,40 . Los fotoiniciadores no son comúnmente empleados en polimerización en emulsión, no obstante, ha habido estudios académicos en esta área [44][45][46][47] . Los iniciadores de radicales libres también pueden ser clasificados como solubles en agua o aceite. ...
El presente trabajo tuvo como objetivo diseñar nanopartículas tipo core-shell por copolimerización en emulsión basados en copolímeros metacrílicos para determinar su potencial aplicación en la encapsulación y liberación de limoneno como componente mayoritario en algunos aceites esenciales. Para lo cual, la investigación se fraccionó en cuatro etapas. La primera fue establecer las condiciones experimentales para la copolimerización en emulsión tipo batch del metacrilato de laurilo (LMA) que es un monómero muy insoluble en medio acuoso, con un comonómero parcialmente soluble como metacrilato de metilo (MMA), en relación molar 1:1. Por tal motivo, se siguió un diseño experimental variable a variable mediante la modificación independiente de los siguientes parámetros: variación de la temperatura de copolimerización en un intervalo de 60 a 80 °C; adición de metanol como cosolvente en relaciones en masa respecto al agua de 0.8/15.2, 1.6/14.4, 2.4/13.6 y 3.2/12.8; modificación de la concentración en 6.2, 12.5 y 31.2 g/L de beta ciclodextrina (β-CD) como catalizador de transporte para monómeros insolubles; y la variación de la concentración del dodecilsulfato de sodio (SDS) en 0.7, 1.4, 2.9 y 4.4 veces la concentración micelar critica (CMC) en presencia de β-CD, empleado como surfactante y estabilizador coloidal. Con las condiciones experimentales establecidas para la copolimerización tipo batch del MMA y el LMA en relación 1:1, la segunda etapa consistió en sintetizar látex semillas compuestos de estos dos monómeros con relaciones molares 1:1, 2:1 y 1:2 para obtener el P(LMA-co-MMA), asimismo, sus respectivos homopolímeros (PMMA y PLMA). Obteniendo rendimientos de látex (RL) superiores al 90% con variaciones inferiores al 7 %, tamaños de partícula inferiores a 61 nm, con dispersiones en el tamaño de partícula bastante bajas (PdI ˂ 0.043), y una composición muy cercana a la nominal. De las semillas sintetizadas se produjeron seis nanopartículas core-shell (CSNs) de [P(MMA)/PMPS], [P(LMA)/PMPS], y [P(LMA-co-MMA)/PMPS] en relaciones semilla 1:1, 2:1 y 1:2 de los monómeros LMA:MMA por polimerización en emulsión vía semi-batch seed emulsion polymerization, mediante la lenta adición y el recubrimiento de γ-metacriloxipropil-trimetoxisilano (MPS). Obteniendo un rendimiento para las CSNs (R. CSNs) superiores al 78% con errores de medida menores al 6%, tamaños nanométricos en el rango de 54 a 100 nm, valores de potencial zeta inferiores a -30.0 mV, estableciendo la fijación del PMPS por espectroscopia infrarroja, calorimetría diferencial de barrido y la alta estabilidad térmica por análisis termogravimétrico. La cuarta etapa consistió en determinar la capacidad de encapsulación y liberación de las CSNs al emplear limoneno en tres relaciones de carga, mediante el uso de técnicas de análisis térmico para establecer la desorción de la sustancia, la capacidad de hinchamiento por dispersión de luz dinámica (DLS) y método gravimétrico para determinar la capacidad de carga y eficiencia de encapsulación, además de los perfiles de liberación. Donde se estableció que las CSNs presentan una capacidad máxima de hinchamiento por DLS, los análisis térmicos permitieron establecer la liberación acelerada del limoneno, favorecida por núcleos más ricos en LMA y por método gravimétrico se determinó la capacidad de carga y la eficiencia de encapsulación que también fue alta para núcleos ricos en LMA, demostrando que el limoneno también se encuentra adsorbido en las CSNs y la cinética de liberación acelerada a 50 °C.
... [30][31][32][33][34] They reduce oxygen inhibition [31,35] and-in the case of the water-soluble DiMeTriaz-BH 3 -allow polymerization in water. [32,34,36] Moreover,N HC-boranebased type II co-initiators for radical photopolymerization have alabile BÀHbond, and the resulting radicals have high rate constant for addition onto double bonds.T herefore, photopolymerizations of (meth)acrylate resins with aN HCborane-based photoinitiating system (PIS) are fast, quantitative (high final C = Cconversion), solventless and they can be performed under air or in water. That is,t hey meet many features of click reactions.That is why we thought that NHCboranes not only could replace smelly thiols but, because they offer the possibility of linking more than one chain to the Boron atom, could lead to cross-linking via the heteroatom (as opposed to thiols which have only one site open for that, and necessitate multifunctional molecules for polyadditions). ...
In the development of 3D printing fuels, there is a need for new photoinitiating systems working under mild conditions and/or leading to polymers with new and/or enhanced properties. In this context, we introduce herein N‐heterocyclic carbene–borane complexes as reagents for a new type of photo‐click reaction, the borane–(meth)acrylate click reaction. Remarkably, the higher bond number of boranes relative to thiols induced an increase of the network density associated with faster polymerization kinetics. Solid‐state NMR evidenced the strong participation of the boron centers on the network properties, while DMA and AFM showed that the materials exhibit improved mechanical properties, as well as reduced solvent swelling.
... The authors interpreted this result with photoinitiator end chain fragments (phenacyl groups) incorporated into the polymer backbone (as already mentioned above) generating upon electronic excitation new initiating radicals by hydrogen abstraction and called it a "snowballing radical generation" effect. Very recently, Lacôte and coworkers described the first use of visible light in emulsion photopolymerization using styrene as monomer and a photoinitiating system based on three different water-soluble compounds [120]. Although not studied, the mechanistic details may involve a first step of electron-transfer upon electronic excitation of a dye (acridine orange), leading to the photoreduction of a disulfide compound, further decomposed into thiyl radicals. ...
Zero-VOC technologies combining ecological and economic efficiency are destined to occupy a growing place in the polymer economy. Today, Polymerization in dispersed systems and Photopolymerization are the two major key players. The hybrid technology based on photopolymerization in dispersed systems has emerged as the next technological frontier, not only to make processes even more efficient and eco-friendly, but also to expand the range of polymer products and properties. This review summarizes the current knowledge in research relevant to this field in an exhaustive way. Firstly, fundamentals of photoinitiated polymerization in dispersed systems are given to show the favourable context for developing this emerging technology, its specific features as well as the distinctive equipment and materials necessary for its implementation. Secondly, a state-of-the-art and critical review is provided according to the seven main processing methods in dispersed systems: emulsion, microemulsion, miniemulsion, dispersion, precipitation, suspension, and aerosol.
A readily accessible organic‐inorganic hybrid catalyst is reported for the reductive fragmentation of α‐halocarbonyl compounds. The robust hybrid catalyst is a self‐stabilizing combination of ZnCl2 Lewis acid and acridine orange as the photoactive organic dye. Mechanistic specifics of this hybrid catalyst have been studied in detail using both photophysical and electrochemical experiments. A systematic study enabled the discovery of the appropriate Lewis acid for the effective LUMO stabilization of α‐halocarbonyl compounds and thereby lowering of reduction potential within the range of a standard organic dye. This strategy resolves the issues like dehalogenative hydrogenation or homo‐coupling of alkyl radicals by guiding the photoredox cycle through an oxidative quenching pathway. The cooperativity between the photoactive organic dye and the Lewis acid counterparts empowers functionalization with a wide range of coupling partners through efficient and controlled generation of alkyl radicals and serves as an appropriate alternative to the expensive late transition metal‐based photocatalysts. To demonstrate the application potential of this cooperative catalytic system, four different synthetic transformations of α‐carbonyl bromides were explored with broad substrate scopes.
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The strategy of laser beam expansion was used to rapidly prepare nanogels by photopolymerization at 532 nm under low monomer concentration. According to the unique micellar morphology formed by amphiphilic polyethylene glycol diacrylate (PEGDA) in water, the monomer concentration was largely decreased to increase the distance of micellar aggregates. In this case, the photo-crosslinking could prefer to occur inside the micelles instead of crosslinking between the micellar aggregates. The variations of double bond content with reaction time in different beam expansion experiments were investigated. Finally, nanogels with uniform size could be rapidly prepared by regulating the reaction parameters, including monomer concentration, reaction time and power density.
Citric acid-based carbon dots (CDs) as nano-photoinitiating catalysts for 3D printing.
N-Heterocyclic carbene–borane complexes can lead to a photochemically triggered borane–acrylate click reaction, a new type of photo-click reaction. Because boranes have three substituents they can participate to cross-linking, unlike thiols in thiol–(meth)acrylate reactions. This generates materials with improved mechanical properties.
Abstract
In the development of 3D printing fuels, there is a need for new photoinitiating systems working under mild conditions and/or leading to polymers with new and/or enhanced properties. In this context, we introduce herein N-heterocyclic carbene–borane complexes as reagents for a new type of photo-click reaction, the borane–(meth)acrylate click reaction. Remarkably, the higher bond number of boranes relative to thiols induced an increase of the network density associated with faster polymerization kinetics. Solid-state NMR evidenced the strong participation of the boron centers on the network properties, while DMA and AFM showed that the materials exhibit improved mechanical properties, as well as reduced solvent swelling.
This chapter is devoted to the use of photoinitiators (PIs) in a lot of particular polymerization processes such as thiol–ene reactions, hybrid cure and interpenetrating polymer networks (IPNs) synthesis, photopolymerization in ionic liquids, photografting reactions, photoactivated redox polymerization, and hydrosilylation reactions. The change of thiols for silanes, thereby leading to a silane–ene process, gives an access to a chemistry rather similar to that of thiol–ene. The formation of IPN combining the properties of the two polymer backbones can be achieved using the hybrid cure technique where a blend of a radical and a cationic monomer/oligomer is photopolymerized. The photoinduced copper‐catalyzed azide–alkyne cycloaddition allows the formation of a cross‐linked network from multifunctional azides and alkyne monomers having a triazole linkage. The sol–gel photopolymerization of organic–inorganic hybrid resins leads to the production of hybrid sol–gel materials. Working in particular experimental conditions has an influence on the design of PIs.
Benzylidene piperidones (CZ-PD, TPA-PD, and PTZ-PD) with conjugated structure were synthesized by the aldol condensation of aromatic aldehyde and N-benzyl piperidone. These D-π-A-π-D dyes showed good visible light absorption and photosensitizing properties to iodonium salts in visible light-emitting diode (LED) photopolymerization. The photophysical properties of the synthesized structure were investigated via UV–vis spectroscopy and density functional theory (DFT) calculation. The mechanisms of structure-sensitized iodonium salts were explored through steady-state photolysis experiments. Polymerization experiments proved that these dyes with iodonium salt were an effective photoinitiating system for radical photopolymerization under visible LED light source and showed the ability to initiate deep curing.
Carbon dots (CDs) have been used for the first time as a sensitizer to initiate and activate free radical and controlled radical polymerization, respectively, based on an ATRP protocol with blue LEDs. Consideration of diverse heteroatom‐doped CDs indicated that N‐doped CDs could serve as an effective photocatalyst and photosensitizer in combination with LEDs emitting either at 405 nm or 470 nm. Free radical polymerization was initiated by combining the CDs with an iodonium or sulfonium salt in tri(propylene glycol) diacrylate. Polymerization of methyl methacrylate (MMA) by photo‐induced ATRP was achieved with CDs and ethyl α‐bromophenylacetate using CuII as catalyst in the ppm range. The polymers obtained showed temporal control, narrower dispersity ≲1.5, and chain‐end fidelity. The first‐order kinetics and ON/OFF experiments additionally gave evidence of the constant concentration of polymer radicals. No remarkable cytotoxic activity was observed for the CDs, underlining their biocompatibility.
Punktlandung: Aus Meeresalgen isolierte Kohlenstoff‐Nanopunkte eignen sich als Photokatalysator und Sensibilisator für die kontrollierte und freie radikalische Polymerisation, während ihre geringe zytotoxische Aktivität sie für verschiedene Anwendungen interessant macht.
Abstract
Kohlenstoff‐Nanopunkte (CDs) fanden Verwendung als Sensibilisator, um die freie radikalische und kontrollierte radikalische Polymerisation auf Basis einer ATRP‐Vorschrift mit blauen LEDs zu initiieren und zu aktivieren. N‐dotierte CDs sind wirksame Photokatalysatoren und Photosensibilisatoren in Kombination mit LEDs, die bei 405 oder 470 nm emittieren. Die Initiierung der freien radikalischen Polymerisation erfolgte durch Kombination von CDs mit einem Iodonium‐ oder Sulfoniumsalz in Tripropylenglycoldiacrylat. Die Polymerisation von Methylmethacrylat durch photoinduzierte ATRP gelang mit CDs und α‐Bromphenylessigsäureethylester unter Verwendung von CuII als Katalysator im ppm‐Bereich. Die erhaltenen Polymere zeigten im betrachteten Zeitfenster eine engere Dispersität und eine Funktionalität der Endgruppen bei ATR‐Experimenten. Die Kinetik erster Ordnung bestätigt zusätzlich eine konstante Konzentration von Polymerradikalen. Es wurde keine bemerkenswerte zytotoxische Aktivität der CDs beobachtet, was ihre Biokompatibilität verdeutlicht.
The synthesis and reactivity of mono and bis‐S‐xanthyl NHC‐boranes is reported. The new NHC‐boranes are prepared via nucleophilic exchange at boron from either mono or bis‐triflyl NHC–boranes, themselves obtained by protolysis of the NHC–BH3 starting compounds. The B–H bond of the S‐xanthyl NHC‐boranes can be cleaved both homolytically and heterolytically, albeit the latter is more synthetically useful. The S‐xanthyl NHC‐boranes can reduce both aldehydes and imines. The B–S bond can also be cleaved homolytically. Under UV irradiation, the S‐xanthyl NHC‐boranes generate NHC–boryl radicals that can initiate radical polymerizations of acrylates.
The use of photochemical transformations is a powerful strategy that allows for the formation of a high degree of molecular complexity from relatively simple building blocks in a single step. A central feature of all light-promoted transformations is the involvement of electronically excited states, generated upon absorption of photons. This produces transient reactive intermediates and significantly alters the reactivity of a chemical compound. The input of energy provided by light thus offers a means to produce strained and unique target compounds that cannot be assembled using thermal protocols. This review aims at highlighting photochemical transformations as a tool for rapidly accessing structurally and stereochemically diverse scaffolds. Synthetic designs based on photochemical transformations have the potential to afford complex polycyclic carbon skeletons with impressive efficiency, which are of high value in total synthesis.
A short review of the role of photocatalysts in photopolymerization reactions is presented. A special emphasis is placed on photoredox catalysts leading to i)high-performance initiating systems for polymerization upon low light intensity for the use of light emitting diodes (LEDs) and/or visible light and ii)controlled radical photopolymerization (CRP2) processes with formation/reactivation of dormant species triggered by light.
Although there have been many reports on photoinitiating systems adapted to visible lights for radical photopolymerization, the challenge for the design and development of photoinitiating systems for cationic photopolymerization or concomitant radical/cationic photopolymerization (for interpenetrating polymer network IPN synthesis) with visible lights still remains open. Particularly, the recent development of cheap and easily accessible LEDs operating upon soft visible light irradiations has opened new fields for polymer synthesis. Since 2011, many novel photoinitiating systems based on organic and organometallic compounds with excellent visible light absorption have emerged and exhibited outstanding photoinitiating abilities especially for cationic photopolymerization. In this review, recent progress (mainly from 2011 to early 2014) in applications of photoinitiators and sensitive photoinitiating systems under visible lights are reported. In addition, their relative efficiencies in the photopolymerization of different monomers are exemplified and discussed.
An efficient continuous synthesis of nanolatex was achieved in water using a single-lane photochemical microreactor combined with an energy-saving and safe UV fluorescent lamp. Acrylate and thiol-ene miniemulsions were polymerized in high yields at low irradiance (3 mW cm-2) upon controlling droplet size, temperature and residence time.
A mixture of vinyl acetate, water and polyoxyethylene nonyl phenyl ether sodium sulfate (LWZ; Kao Co.) was irradiated with a high pressure mercury lamp in the absence of initiator at 0°C and high molecular weight poly(vinyl acetate) (PVAc) which gave poly(vinyl alcohol) (PVA) of Pn (average degree of polymerization)=12800 after hydrolysis was obtained in good yield. Initiation by a radical from LWZ decomposed by photoirradiation was deduced. The charateristics of the PVA were similar to those of usual PVA.
Over decades of carefully designed kinetic experiments and the development of complementary theory, a more or less complete picture of the mechanisms that govern emulsion polymerization systems has been established. This required means of determining the rate coefficients for the individual processes as functions of controllable variables such as initiator concentration and particle size, means of interpreting the data with a minimum of model-based assumptions, and the need to perform experiments that had the potential to actually refute a given mechanistic hypothesis. Significant advances have been made within the area of understanding interfacial processes such as radical entry and exit into and out of an emulsion polymerization particle, for electrostatic, steric and electrosteric stabilizers (the latter two being poorly understood until recently). The mechanism for radical exit is chain transfer to monomer within the particle interior to form a monomeric radical which can either diffuse into the water phase or propagate to form a more hydrophobic species which cannot exit. Entry is through aqueous-phase propagation of a radical derived directly from initiator, until a critical degree of polymerization z is reached; the value of z is such that the z-meric species is sufficiently surface active so that its only fate is to enter, whereas smaller aqueous-phase radical species can either be terminated in the aqueous phase or undergo further propagation. For both entry and exit, in the presence of (electro)steric stabilizers, two additional events are significant: transfer involving a labile hydrogen atom within the stabilizing layer to form a mid-chain radical which is slow to propagate and quick to terminate, and which may also undergo β-scission to form a water-soluble species. Proper consideration of the fates of the various aqueous-phase radicals is essential for understanding the overall kinetic behaviour. Intra-particle termination is explained in terms of diffusion-controlled chain-length-dependent events. A knowledge of the events controlling entry and exit, including the recent discoveries of the additional mechanisms operating with (electro)steric stabilizers, provides an extension to the micellar and homogeneous nucleation models which enables particle number to be predicted with acceptable reliability, and also quantifies the amount of secondary nucleation occurring during seeded growth. This knowledge provides tools to understand the kinetics of emulsion polymerization, in both conventional and controlled/living polymerization systems, and to optimize reaction conditions to synthesize better polymer products.
This tutorial review is designed to dispel the myth, still believed by many synthetic organic chemists, that radical-based syntheses are free from significant solvent effects. However, many synthetically valuable radical reactions do exhibit large kinetic solvent effects. It is therefore important to select the solvent for any proposed radical synthesis with considerable care if good product yields are to be achieved.
The use of visible light as a catalyst in organic reactions has developed greatly in the last decade as a result of the increasing urge to implement energy-efficient and green processes and through the availability of light-emitting diodes as convenient sources of visible light. Although photoredox reactions with visible light have been used in a broad range of transformations, the mechanism behind these reactions needs to be understood to design better catalytic systems.
Photoredox catalysis has experienced a revitalized interest from the synthesis community during the past decade. For example, photoredox/Ni dual catalysis protocols have been developed to overcome several inherent limitations of palladium-catalyzed cross-couplings by invoking a single-electron transmetalation pathway. This Perspective highlights advances made by our laboratory since the inception of the photoredox/Ni cross-coupling of benzyltrifluoroborates with aryl bromides. In addition to broadening the scope of trifluoroborate coupling partners, research using readily oxidized hypervalent silicates as radical precursors that demonstrate functional group compatibility is highlighted. The pursuit of electrophilic coupling partners beyond (hetero)aryl bromides has also led to the incorporation of several new classes of C(sp²)-hybridized substrates into light-mediated cross-coupling. Advances to expand the radical toolbox by utilizing feedstock chemicals (e.g., aldehydes) to access radicals that were previously inaccessible from trifluoroborates and silicates are also emphasized. Additionally, several organic photocatalysts have been investigated as replacements for their expensive iridium- and ruthenium-based counterparts. Lastly, the net C–H functionalization of the radical partner in an effort to improve atom economy is presented. An underlying theme in all of these studies is the value of generating radicals in a catalytic manner, rather than stoichiometrically.
The design of efficient radical photoinitiating systems requires a systematic and detailed evaluation of their photochemical characteristics. Correlating absorbance and the corresponding electronic transitions of a photoinitiator is critical for understanding its photoinduced reaction pathways. In the current contribution, we provide an in-depth investigation into the photochemistry and photophysics of two oxime ester derivatives (O-benzoyl-alpha-oxooxime, OXE01, and O-acetyloxime, OXE02), known for their excellent performance in pigmented formulations. In particular, we shed light on their wavelength-dependent photopolymerization properties. We utilized a combination of UV-vis spectroscopy, density functional theory (DFT) calculations, photochemically induced dynamic nuclear polarization spectroscopy (photo-CIDNP), and pulsed-laser polymerization with a wavelength-tunable laser with subsequent size exclusion chromatography coupled to high-resolution electrospray ionization mass spectrometry (PLP-SEC-ESI-MS) for obtaining detailed insights. Both photoinitiators have high molar extinction coefficients (epsilon) of greater than 1.75 X 10(4) L mol(-1) cm(-1) at close to 330 nm, with the n-pi* and pi-pi* transitions, relevant for cleavage of the N-O bond, at approximately 335 nm according to DFT calculations. We have probed the wavelength-dependent initiation behavior of both OXE01 and OXE02 in the presence of methyl methacrylate (MMA) via PLP with a wavelength-tunable laser between 285 and 435 nm at constant photon counts. Surprisingly, the highest conversions of MMA were found at a wavelength of 405 nm, even though the molar extinction coefficients of the photoinitiators are low (8405 of 45 and 2 L mol(-1) cm(-1) for OXE01 and OXE02, respectively) compared with shorter wavelengths. Accordingly, the absorption spectrum of a photoinitiator is not a straightforward guide for selecting the most efficient excitation wavelength.
Herein we report an aqueous photoinitiated polymerization-induced self-assembly (photo-PISA) for the preparation of a remarkably diverse set of complex polymer nanoparticle morphologies (e.g., spheres, worms, and vesicles) at room temperature. Ultrafast polymerization rates were achieved, with near quantitative monomer conversion within 15 min of visible light irradiation. An important feature of the photo-PISA is that diblock copolymer vesicles can be prepared under mild conditions (room temperature, aqueous medium, visible light), which will be important for the preparation of functional vesicles loaded with biorelated species (e.g., proteins). As a proof of concept, silica nanoparticles and bovine serum albumin (BSA) were encapsulated in situ within vesicles via the photo-PISA process.
The implementation of photopolymerization processes in aqueous dispersed systems has the potential to afford greener approaches to the preparation of polymeric materials and has motivated researchers to perform photopolymerization in emulsion. However, these previous works have employed UV irradiation to induce photodegradation of a photoinitiator in addition to specialized equipment setups, thus limiting widespread use of these approaches. In this work, we aim to remedy these drawbacks via the implementation of photoredox catalysis in the regulation of a controlled/living radical polymerization under visible light. Utilizing the photoinduced electron transfer–reversible addition–fragmentation chain transfer (PET-RAFT) process, we report the miniemulsion polymerization of styrene mediated by a household grade blue LED (λmax = 460 nm, 0.73 mW/cm2). The polymerization rate can be easily manipulated by light intensity and catalyst concentration. Finally, temporal control was demonstrated via ON/OFF experiments, which shows that no significant detriment is caused by prolonged interruptions to the light exposure.
A new method for visible-induced emulsion polymerization is successfully demonstrated. Photoinitiators are used upon irradiation with visible light to undergo sequential photodecomposition, demonstrating an effective way to generate radicals with octadecylamine ethoxylates N-oxide (AO-1805, EO = 5). AO-1805, acting both as a surfactant and mediator simultaneously, is proposed for free radical generation through synergistic effects with the photoinitiator, which differs greatly from organoboron compounds, especially in emulsions. Comparative experiments are carried out with nonylphenol ethoxylates (NP-10, EO = 5) upon irradiation with UV and visible light in order to examine the nature of the photoinitiated emulsion polymerization with AO-1805. The laser flash photolysis method is then used to study the interaction between AO-1805 and free radicals. The emulsion photopolymerization of styrene may be successfully mediated by AO-1805, with good control over molecular weight. A synergistic effect between radicals and AO-1805 is observed in the UV irradiation process that can further mediate radical polymerization. In visible light-induced emulsion polymerization, AO-1805 also has a novel stabilizing role because of the interaction between D1 and AO-1805. Moreover, it is hard to see such a trend with the NP-10/D1 combination, indicating that the functional group (N-oxide) of AO-1805 plays an important role in photoinitiation and has an influence on the overall polymerization rate.
The ruthenium-based photoredox catalyst, Ru(bpy)3Cl2, was employed to activate reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization via a photoinduced electron transfer (PET) process under visible light (λ = 460 nm, 0.7 mW/cm2). Poly(oligo(ethylene glycol) methyl ether methacrylate) was chain extended with benzyl methacrylate to afford in situ self-assembled polymeric nanoparticles with various morphologies. The effect of different intrinsic reaction parameters, such as catalyst concentration, total solids content, and cosolvent addition was investigated with respect to the formation of different nanoparticle morphologies, including spherical micelles, worm-like micelles, and vesicles. Importantly, highly pure worm-like micelles were readily isolated due to the in situ formation of highly viscous gels. Finally, “ON/OFF” control over the dispersion polymerization was demonstrated by online Fourier transform near-infrared (FTNIR) spectroscopy, allowing for temporal control over the nanoparticle morphology.
The LED technology knows a fast development that holds great promise. Advanced capabilities, operating economics and environmental advantages are the main driving factors. Therefore, this paper is focused on i) the recent development of the LED technology allowing the design of novel and efficient light sources for the free radical and cationic photopolymerization of various monomers, the synthesis of interpenetrating polymer networks (IPNs) or the thiol-ene photopolymerization, ii) the existing and emerging applications in these areas and iii) the development of novel photoinitiators and photoinitiating systems specifically adapted for LED excitations.
Rate constants for important elementary reactions of 2,4-dimethyl-1,2,4-triazol-3-ylidene borane (1) and its derived boryl radical have been measured in the presence and absence of water. Reactions including H-abstraction by tBuO•, and onward reactions of the derived radical with methyl acrylate (addition), iodopropane (halogen abstraction), and an iodonium salt (oxidation) are unaffected by the presence of a high concentration (1.5 M) of water. Extending these results, borane (1) has been found to be an efficient new co-initiator for the visible light photopolymerization of monomers in the presence of both air and water.
Miniemulsion polymerization facilitates the synthesis complex materials that cannot be produced otherwise. These materials have a broad range of potential applications including among others adhesives, coatings, anticounterfeiting, textile pigments, bio-based polymer dispersions, gene and drug delivery, anti-viral therapy, tissue engineering, catalyst supports, polymeric photoresists, energy storage and self-healing agents. However, 40 years after the pioneering work of Ugelstad, El-Aasser and Vanderhoff the promises have not been fulfilled and the presence of miniemulsion polymerization in commercial products is scarce. This article reviews the advances in the field, discusses the reasons for this delay and analyzes the challenges that have to be overcome in order to fully use this process in commercial practice.
A procedure for the production of N-heterocyclic carbene–boryl radicals (NHC-BH2•) upon visible light irradiation under soft conditions is presented. New acridine orange (dye)/diphenyl disulfide/NHC–BH3 and dye/sulfonium salt/NHC–BH3 three-component initiating systems are introduced for the efficient visible light photopolymerization of trimethylolpropane triacrylate. The new systems could be extendend to polymerization reactions in water (hydroxyethyl acrylate and hydroxyethyl methyl acrylate), which proceeded with strongly improved polydispersity. The chemical mechanisms are investigated through EPR and photolysis experiments.
A visible light-induced surfactant-free emulsion polymerization (SFEP) was developed as an alternative pathway for the preparation of amine-functionalized nanoparticles by using the photo-initiating system consisting of camphorquinone coupled with tertiary amine (CQ/3°-amine). Water-soluble macromolecules containing 3°-amines were used as the sources of 3°-amine species, which not only function with CQ to generate initiating free radicals, but also provide colloidal stabilization to the resulting colloidal products. The prepared nanoparticles showed uniformed size distribution and good colloidal stability with positively charged surface. For SFEP induced by CQ/polyethyleneimine (PEI) photo-redox couple, the polymerization of methyl methacrylate (MMA) was affected by both light intensity and initiator concentration. In addition, to obtain higher solid content products, the weight ratio of PEI:MMA=1:4 was employed. Finally, an opportunity for immobilizing various amine containing polymers by our photo-induced SFEP was evaluated. The achievement of this SFEP also depended on 3°-amine content of the macromolecules used.
In monomer miniemulsions, droplet size is a key parameter impacting the optical properties, and consequently their photopolymerizability under UV light. Three simple spectrophotometric methodologies are developed, based on an integrating sphere, to evaluate precisely the effect of droplet size on radiation absorption and scattering, which both contribute to light attenuation. Using a series of diluted acrylate monomer miniemulsions ranging from 40 to 300 nm, all the methods converge toward a constant absorption coefficient, comparable with that in solution. In contrast, a droplet-size decrease causes a significant and progressive reduction of the scattering coefficient, with a positive effect on light penetration.
Eight N-heterocyclic carbene-boranes (NHC-boranes) are proposed as new efficient co-initiators for acrylate photopolymerization reactions. They are particularly interesting in aerated conditions, where they help overcome the classical oxygen inhibition. The carbene boryl radicals that are the initiating species have been characterized by their transient absorption spectra obtained in laser flash photolysis(LFP) experiments. Rate constants for the generation of the carbene boryl radicals by hydrogen
abstraction with t-butoxyl radical and triplet benzophenone as well as the reactions with oxygen, electron rich and electron poor alkenes, two alkyl halides (CHCl3 and iodopropane) and diphenyliodonium hexafluorophosphate have been measured. The reactivity of N-heterocyclic carbene
borane radicals is clearly affected by the NHC substituent.
The kinetics of the inverse emulsion photopolymerization of acrylamide using the water soluble photoinitiator (triplet radical generator) α-ketoglutaric acid (α-KGA), sorbitan monooleate (Span 80) as the emulsifier and toluene as the oil phase has been studied. The rate of polymerization (Rp) can be represented by: Rp ∝ I0.5[C]0.5[M]1.28[E]−0.42, where I, C, M and E represent the light intensity (313 nm u.v. light), photoinitiator, monomer and emulsifier, respectively. The kinetic results suggest that the polymerization of acrylamide is essentially a microsuspension one. The emulsifier apparently acts as a retarder. The overall activation energy is 10.20 ± 1.58 kJ mol−1. The presence of the gel effect is evident from the increase of molecular weight with conversion and also from the nature of the percentage conversion versus time curves. A useful linear relation between high conversion molecular weight and maximum Rp was found to exist. The relation yields the upper limit of the viscosity average molecular weight [(7.25 ± 0.48) × 106] for 4.5 mol dm−3 acrylamide, 6.3% (w/v) Span 80, toluene:water = 53:47 (v/v), a stirring rate of 425 rev min−1 and a temperature of 30°C.
Methyl methacrylate and butyl methacrylate were polymerized in oil-in-water microemulsions that were stabilized by sodium dodecyl sulphate (SDS). A poly(N-acetylethylenimine) (PNAEI) macromer was also included in the recipe, as a cosurfactant and a comonomer. Polymerizations were initiated by UV-irradiation. The average diameters of latex particles, obtained by STM, were in the range of 17–200 nm. The experimental data evidenced that the particle size was mainly dependent on the SDS/PNAEI ratio. Polymerization yields were around 75–85%. The synthesized copolymers have viscosity average molecular weights in the range of 2.1–2.4×106 and glass transition temperatures of 38.0–43.5°C, lower than those obtained without using PNAEI. The investigation by means of FTIR and 1H-NMR techniques revealed that PNAEI was incorporated into the nanoparticles.
Transparent polystyrene (PS) latexes were prepared by photo-polymerization of a routine styrene (St) emulsion with a surfactant concentration of 0.4wt% and a monomer concentration of 10wt%, by using UV light as well as a hydrophilic photo-initiator. The entire polymerization could be performed within 1–2h and display a conversion higher than 90%. The particle sizes could be tuned in the range of 20–40nm. The formation mechanism for these of nano-sized latex particles was attributed to an in situ formation of PS chains with terminal hydroxyl groups. The presence of the hydroxyl-functionalized polymer chains seemed to promote a colloidal stability of the small latex particles and prevent coagulation even at low surfactant concentrations.
Newly discovered N-heterocyclic carbene-boryl radicals (NHC-BH2•) derived from readily available N-heterocyclic carbene-boranes are found to be efficient initiators for acrylate photopolymerization. Laser flash photolysis (LFP) experiments were used to generate three carbene-boryl radicals, which were characterized by their transient absorption spectra with the aid of DFT calculations. Rate constants were measured for the generation of the carbene-boryl radicals by hydrogen abstraction with tert
-butoxyl radical, a ketophosphonyl radical, and triplet benzophenone. Rate constants were also measured for the reactions of the carbene-boryl radicals with oxygen, three alkenes, two alkyl chlorides, and diphenyliodonium hexa-
fluorophosphate. The observed trends were interpreted with the aid of measured oxidation potentials of the carbene
-boranes and calculated ionization potentials of the carbene-boryls. N-Heterocyclic carbene-boranes show excellent potential as both photopolymerization co-initiators and mediators of small molecule radical reactions, and these results will help guide further development in both fields.
A comparative experimental study of bulk and emulsion polymerization of styrene with bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (BAPO) or bis(4-methoxy benzoyl)diethylgermanium (BAG) as photoinitiator or photoinitiator-free reveals astonishing similarities and anticipated differences. Expectedly, both the rate of polymerization and the molecular weight is higher for emulsion than for bulk polymerization. Unexpectedly, the polymerizations continue even if the photoinitiators have been used up and moreover, the average molecular weights increases with monomer conversion. Consideration of all experimental data gives rise to the proposal that the common feature of these polymerizations is photo electron transfer between styrene monomer and a repeating unit in the polystyrene chain.
Styrene is the classical monomer obeying zero-one kinetics in radical emulsion polymerization. Accordingly, particles that are less than 100 nm in diameter contain either one or no growing radical(s). We describe a unique photoinitiated polymerization reaction accelerated by snowballing radical generation in a continuous flow reactor. Even in comparison to classical emulsion polymerization, these unprecedented snowballing reactions are rapid and high-yielding, with each particle simultaneously containing more than one growing radical. This is a consequence of photoinitiator incorporation into the nascent polymer backbone and repeated radical generation upon photo-irradiation.
The kinetics of the photoinitiated polymerization of acrylamide in toluene/AOT/water inverse microemulsions have been examined for systems initiated by AIBN or a dye: triethanolamine redox reaction. The rates of polymerization were determined by dilatometry, the data being corrected for the effect of reaction exotermicity. For both the oil-soluble and the water-soluble initiating systems, the rate of polymerization was found to be proportional to the first power of the incident light intensity. For AIBN-initiated systems, the rate of polymerization was also found to be proportional to the first power of the initiator concentration. The molecular weights of the polymers produced were independent of the rates of polymerization and initiation. These results suggest that exclusively monoradical termination, involving a degradative chain transfer, is occurring in these systems
Microlatices of acrylic monomers, in the particle size range of 10–30 nm, were prepared by photopolymerization of micellar systems (monomer-surfactant) and of microemulsions based on monomer-surfactant-cosurfactant. Formation of monomer containing microemulsions is shown for various surfactants in combination with alcohols as cosurfactants and the corresponding phase diagrams are given. Photopolymerization, in the presence of oil-soluble initiators, was investigated for such systems based on cyclohexyl methacrylate (CHMA) and on tetramethylene dimethacrylate (BDMA). Crosslinked and non-crosslinked colloidal polymer particles could be obtained, with a 5–10% solid content of polymer. The variation of particle size, and the molecular weight for non-crosslinked systems, was studies as a function of initiator concentration, and monomer/surfactant ratio. The smallest particles (10–15 nm) are obtained for the system BDMA/lauryl sulfate (LS).
Poly(vinyl alcohol) (PVA) having a number-average degree of polymerization of 7000 was obtained from the poly(vinyl acetate) (PVAc) having a number-average degree of polymerization 9000, a product of photo-induced emulsion polymerization of vinyl acetate (VAc), carried out at 0°C, using poly(oxyethylene)10 nonyl phenyl ether ammonium sulfate as an anionic emulsifier. It was found that 100% conversion is always attained in the whole range of the investigation and the emulsifier plays an important role in the initiation process. The applicability of the photo-induced emulsion polymerization system to a relatively large-scale production was tested by using an apparatus equipped with an internal high-pressure Hg lamp with a capacity of several hundred grams per batch under nitrogen atmosphere. It was found that both the rate of polymerization and the degree of polymerization of resulting polymers are slightly lower than those obtained from corresponding small-scale polymerizations prepared on a high vacuum system because of the presence of oxygen. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2425–2431, 2002
This paper deals with an investigation of the primary processes involved during the initiation step of methyl methacrylate (MMA) polymerization in miscelles in the presence of thioxanthone derivatives, which were used as photoinitiators on account of their expected 'solubility' in the aggregate environment. Special emphasis is laid on the determination of the rate constants of the different processes and the absorption spectra of the transients. Relevant data are shown and discussed with reference to the effectiveness of these molecules as photopolymerization initiators.
The efficiency of several xanthene dyes as photoinitiators of the free radical polymerization in aqueous medium was evaluated. These results show that dyes with triplet quantum yield higher than 0.1 present similar efficiencies, independently of their different chemical structure. A detailed study of the photophysics of the dyes under the polymerization conditions was carried out using laser time-resolved spectroscopies. These studies show that the active radicals are those which formed in the interaction of excited triplet state of the dye with the amine through an electron transfer process. In spite of this, the photoinitiation efficiency is not correlated with the triplet quantum yield. Also, the photophysics studies show that the quantum yield of the different pathways of the decomposition of the charge transfer intermediate is an important parameter to predict the efficiency of these photoinitiator systems. The experimentally measured active radical formation is well correlated with that calculated from the polymerization rate. The presence of heavy atoms in the xanthene ring increases the triplet quantum yield, but decreases the active radical yield, and then the polymerization rate.Graphical abstract
Carboxylic acid mono functional poly(methyl methacrylate) was prepared by emulsion photopolymerization using 4-diethylthiocarbamoylsulfanylmethyl benzoic acid (DTBA) as sur-iniferter agent. DTBA is efficient to obtain mono functional polymers with a linear evolution of Mn vs. monomer conversion up to 60%. Beyond 60% monomer conversion, Mn decreases drastically and addition of a supplementary surfactant seems to disturb the effect of sur-iniferter. Secondly, an α-functional polymer presenting a a polydispersity index Ip=2.67 and a functionality fCOOH=98% was grafted in the molten state onto a statistical copolymer of ethylene and glycidyl methacrylate and the amount of PMMA grafts was about 10 wt%. In a final step, the graft copolymer was used as emulsifier in LDPE/PVDF blends. Blends including the graft copolymer present a better interfacial adhesion and a reduction of the size of the dispersed phase in comparison with blends without additive.
Solubilization of butyl acrylate in water using sodium dodecyl sulfate (SDS) and 1-pentanol produced transparent oil-in-water microemulsions stable at room temperature. These microemulsions were polymerized using a two-component water-soluble initiator system comprising of Rose Bengal (RB) and methyldiethanolamine (MDEA). These compounds undergo photo-induced electron transfer upon absorption of visible light to produce amine radicals active in free radical polymerization. Analysis of the NMR spectra of polymers revealed only monomer peaks and amine fragments which were incorporated as the polymer ends. Effect of initiator composition on polymer length was also investigated using GPC. When RB concentration was increased with constant concentration of excess MDEA, the average chain length decreased up to a specific concentration where after the polymer length reached an asymptotic value. Experiments were performed to identify this asymptotic value and it was observed that as the MDEA concentration was increased, the asymptotic values of number average degree of polymerization decreased.
The kinetics of photoinitiated, inverse emulsion polymerization of acrylamide with 2,2-dimethoxy-2-phenylacetophenone (DMPA) as a photoinitiator was investigated under three different cases. First, in a quartz reactor transparent to full UV light, the polymerization rate (Rp) increased and then decreased with the change of initiator order from 0.27 to a negative value when the DMPA concentration was increased, and it was particularly unusual that monomer orders at different DMPA concentrations were lower than the first. Second, for polymerization without DMPA in a quartz reactor, the dependence of Rp on monomer concentration was similar to that of Rp on initiator concentration in the aforementioned case. Third, when polymerization was carried out in a Pyrex reactor where the far UV light was filtered, a peak rate was also observed, and initiator orders varied from 0.24 to a negative value; however, under this case monomer orders at different initiator concentrations were greater than the first. These results indicated that the effect of absorbance often observed in bulk or solution photopolymerization also existed in this system, and the self-initiation of monomer had some influence on polymerization, and the role of primary radical termination could not be neglected, as evidenced by kinetic analysis. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 846–852, 2004
This study describes the radical photopolymerization of acrylate monomers in miniemulsion. Starting from nanosized acrylate
droplets (<100nm) which encapsulate a type I radical photoinitiator (BAPO), UV irradiation led after a few minutes to the
formation of polymer nanoparticles of similar size. The present study deals with the kinetics aspects of this reaction and
the colloidal properties of the resulting polymer dispersions. Real-time Fourier transform near-infrared spectroscopy in transmission
was implemented to follow continuously the fast photopolymerization process. In addition, the spatial resolution of the photoinduced
process allowed the online monitoring of the evolution of the miniemulsion size during the UV irradiation through dynamic
light scattering.
KeywordsMiniemulsion-Photopolymerization-Latex
To protect and serve: The efficiency of Type I radical photoinitiators for acrylate polymerization is greatly increased by the presence of NHC-boranes. Conversion of the initially photogenerated radicals, R(.) , into strongly nucleophilic NHC-boryl radicals, through hydrogen-atom transfer, gives a better matched radical for subsequent reaction with monomer, ensuring better initiation.
Minimalist N-heterocyclic carbene boranes 1,3-dimethylimidazol-2-ylideneborane and 2,4-dimethyl-1,2,4-triazol-3-ylideneborane are readily available and have low molecular weights. They exhibit superior performance to first-generation NHC-boranes, providing improved yields in reductions of xanthates and related functional groups.
The rates of formal abstraction of phenolic hydrogen atoms by free radicals, Y* + ArOH --> YH + ArO*, are profoundly influenced by the hydrogen-bond-accepting and anion-solvation abilities of solvents, by the electron affinities and reactivities (Y-H bond dissociation enthalpies) of radicals, and by the phenol's ring substituents. These apparently simple reactions can occur by at least three different, nonexclusive mechanisms: hydrogen atom transfer, proton-coupled electron transfer, and sequential proton-loss electron transfer. The delicate balance among these mechanisms depends on both the environment and the reactants. The main features of these mechanisms are described, together with some interesting kinetic consequences.
Quenching of the 2-benzoylthiophene pi,pi* triplet, (BT)-B-3*, by phenol yields the corresponding ketyl and phenoxyl radicals. Reaction rates were measured in 10 solvents having a range of hydrogen-bond acceptor strengths (beta(H)(2) values). There appear to be two mechanisms: (i) a bimolecular reaction of (BT)-B-3* with "free" (i.e., not H-bonded) phenol in which the (BT)-B-3* accepts both a proton and an electron from the phenol, the rate decreasing as beta(H)(2) increases; (ii) a trimolecular reaction of (BT)-B-3* with phenol that is H-bonded to a solvent molecule, PhO-H center dot center dot center dot S, in which the proton goes to the S and the electron to the (BT)-B-3*, the rate increasing as beta(H)(2) increases.
- Fouassier
Photoinitiators for Polymer Synthesis
- J. P. Fouassier
- J. Lalevée
Encyclopedia of Polymer Science and Technology
- C.-S. Chern
Dyes and Chromophores in Polymer Science
- J. Lalevée
- J.-P. Fouassier
- M Takeishi
- H Yoshida
- S Niino
- S Hayama
For two pioneering studies, see: a) M. Takeishi, H. Yoshida, S.
Niino, S. Hayama, Makromol. Chem. 1978, 179, 1387-1391;
b) J.-P. Fouassier, D.-J. Lougnot, J. Appl. Polym. Sci. 1987, 34,
477-488.
- C Schauber
- G Riess
- Makromol Chem
- M T Carver
- U Dreyer
- R Knoesel
- F Candau
- R M Fitch
C. Schauber, G. Riess, Makromol. Chem. 1989, 190, 725-735;
b) M. T. Carver, U. Dreyer, R. Knoesel, F. Candau, R. M. Fitch,
J. Polym. Sci. Part A 1989, 27, 2161-2177; c) G. David, F. Özer,
B. C. Simionescu, H. Zareie, E. Pis¸kinPis¸kin, Eur. Polym. J. 2002, 38,
73-78; d) K. Jain, J. Klier, A. B. Scranton, Polymer 2005, 46,
11273-11278.