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Curcumin, a natural colorant as initiator for photopolymerization of styrene: Kinetics and mechanism
Abstract
The photopolymerization of styrene in presence of an efficient, eco-friendly, and a cost-effective photoinitiator, curcumin,
which is found in turmeric root, has been reported for the first time. The catalytic concentration (10−6M) of curcumin is effective to photoinitiate the polymerization of styrene. The kinetic data, inhibiting effect of benzoquinone
and electron spin resonance studies, indicate that the polymerization proceeds via a free radical mechanism. The system follows
non-ideal kinetics (R
p ∝ [Cur]0.36 [Sty]1.04) due to both primary radical termination and degradative chain transfer reactions. The broad peaks due to methine and methylene
protons in 1H-NMR (nuclear magnetic resonance [NMR]) spectrum and a band of resonances at 145–146ppm in 13C-NMR indicate atactic nature of the polystyrene formed. The maximum conversion at 30 ± 0.2°C in 17h has been limited to
23% without gelation. The formation of radicals and mechanism of polymerization are also discussed.
... cations or free radicals) that can initiate some polymerization reactions through electron (or energy) transfer between themselves or with other additives. Their reactivities are closely related to their photochemical properties that depend of their chemical structures [15][16][17]. ...
... As the ferrocene-based dyes (11)(12)(13)(14)(15)(16)(17)(18) are mainly used to induce the polymerization of epoxy compounds, the different three-component systems (dyes/Iod/amine, 0.5 %/1.5% w/1.5%, w/w/w) were used as PISs for the CP of EPOX. The function conversions vs. irradiation time curves obtained using RT-FTIR are given in Figure 2 and the final conversions (FCs) of the epoxide functional groups of monomers are summarized in the ...
Chalcones are well-known natural dyes, 18 different derivatives selected from the rational molecular design (i.e. through molecular orbital calculations) were synthesized. If used in conjunction with an amine (i.e. ethyl l,4-(dimethylamino) benzoate, EDB) and an iodonium salt (bis(4-tert-butylphenyl) iodonium hexafluorophosphate, Iod) as photoinitiating systems (PISs), these dyes can initiate both the free radical polymerization of polyethylene glycol diacrylate (PEG-diacrylate) and the cationic polymerization of 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate (EPOX) via oxidation–reduction reaction mechanisms. Furthermore, among the 18 different suggested structures, except for dyes 9 and 10, which have been synthesized and used in other fields, the remaining 16 dyes have never been synthesized before. The photopolymerization efficiency of acrylates carried out under [email protected] nm are excellent both for thin films (the thickness is about 0.2mm) and thick films (the thickness is about 2mm). Furthermore, all of them can also boost the cationic polymerization of EPOX upon LED irradiation at 405 nm, in the presence of an iodonium salt and an amine. More interestingly, stereoscopic 3D patterns were successfully fabricated by the direct laser write (DLW) technique which demonstrated reversible swelling properties and reversible shape-memory effects induced via swelling and dehydration for the access to 4D printing.
... Interest for biosourced photoinitiators is not new since the first report mentioning the use of riboflavin as photoinitiator for the polymerization of acrylic monomers was reported as soon as 1967 [53,54]. If natural dyes were discarded during 40 years in favor of synthetic dyes, in 2005, the first natural product to be revisited as photoinitiator of polymerization was curcumin which showed outstanding photoinitiating properties during the polymerization of styrene, so that the likehood that other natural dyes could do so was newly examined [55]. However, several questions arise concerning natural dyes. ...
... Following these pioneering works devoted to the cationic polymerization of epoxides, another group reported in 2007 the free radical polymerization of styrene with a catalytic concentration for curcumin as low as 10 −6 M [55]. However, the polymerization tests were carried out at 257 nm, despites the strong absorption of curcumin in the visible range. ...
The development of new procedures aiming at reducing the environmental impact of polymerization processes is a major societal issue. In this field, light-assisted polymerization and especially visible light photopolymerization can address this issue by enabling in the future, Sun, to be used as the irradiation source. Presently, numerous visible light photoinitiators (xanthene dyes, porphyrins and phthalocyanines) are used in industry but their toxicities constitute a major issue for future uses of polymers. Therefore, photopolymerization is facing a short-term challenge and the development of visible light photoinitiators of polymerization has become a blooming field of research during the last decade. An ever-growing effort is thus done to develop new structures, effort which is also supported by the recent applications of photopolymerization in 3D-printing. Depending on the applications, use of synthetic photoinitiators can constitute a severe limitation for future applications of photopolymers so that the use of natural products has been identified as a promising alternative to address the toxicity or the biocompatibility issues. In this review, an overview of the different visible-light photoinitiators based on natural products is provided.
... Curcumin, an intensively yellow dye found in the rhizomes of Curcuma longa, has been previously reported as an effective PI for the copolymerization of styrene and methylmethacrylate [7]. Curcumin is also known to be an inexpensive multicolor photoinitiator with panchromatic light absorption [4,8,9]. ...
... After cleaving the double bonds, the I 1 radical can bond with either the primary carbon, leaving the tertiary one with a radical (more stable), or the tertiary carbon, leaving the primary carbon with an unpaired electron (less stable), as seen in Fig. 3. This mechanism incorporates toxic tertiary amine into the polymers' structures [2,[4][5][6][7]19]. The radical left on the structure can then cleave another double bond, further propagating the polymerization reaction (Fig. 4). ...
The objectives of this study were to verify the viability of the photoinitiating system using curcumin as a photoinitiator and glycerol as a coinitiator, to analyze the thermal and morphological features of the polymers produced, and finally evaluate the possibility of removing toxic Ethyl-p-dimethylaminobenzoate from the photoinitiating system. Curcumin, an orange–yellow dye extracted from the rhizoids of the plant Curcuma longa, is known to be a common photosensitizer, and the type II photoinitiating system consisting of a dye and an amine is long known to be effective. In recent years, the production of biodiesel has increased, and consequently, so has the generation of its main by-product, glycerol. Thus, it becomes necessary to study ways to incorporate glycerol into products. Using thermoanalytical methods (Thermogravimetry, Differential Thermal Analysis, and Differential Scanning Calorimetry), Middle Infrared Spectroscopy, and Scanning Electronic Microscopy, it was possible to assess the thermal and morphological characteristics of the polymers produced, and compare whether the presence or lack of tertiary amine, as well as the addition of different molar ratios of glycerol, had any significant impact on these characteristics. Results suggested that neither the removal of tertiary amine nor the addition (in different proportions) of glycerol affected the thermal stability of the polymers. Also, removing the tertiary amine enhanced the total conversion degree of the polymers.
... Over the years, several natural compounds have been identified as interesting and efficient scaffolds for radical, cationic and thiol-ene photopolymerization reactions, such as anthraquinones [235,[256][257][258][259], carotenoids [230,231], various derivatives of chlorophyll [260,261], curcuminoids [18,250,251,[262][263][264][265][266], flavonoids [267][268][269][270][271][272][273], naphthoquinones [274,275], riboflavin [276][277][278][279][280][281] and lignin derivatives [282]. ...
Cationic photopolymerization is among the greenest processes used to obtain polymers since light is abundant, inexpensive and allows for rapid and waste-less curing at room temperature. Moreover, in the current search for the reduction of the environmental impacts of polymers, the use of biobased monomers is one of the most crucial stakes. At the crossroads of these domains, biobased monomers photopolymerization offers the best of both. Hence, this review aims at underlying the increasing importance of cationic photopolymerization in combination with bio-based photopolymerizable monomers, and describes the numerous reactive species derived from bioresources that can lead to innovative materials through cationic photopolymerization reactions. This review intends to be a guide to orient academic teams and industries involved in reducing both environmental impacts and toxicity towards the synthesis of environmentally safe materials. The recent advances on the photochemical systems used, the toxicity of the cationic photoinitiating systems, the reactivity of the new epoxy bio-sourced monomers, their thermomechanical properties as well as the applications of the targeted materials will be described.
... It can be excited to a singlet state with a short life and then transform into a triplet excited state characterized by long life and high reactivity, which is a crucial feature for an efficient photoactivator in the metal-free ATRP [49]. Up to now, curcumin was extensively studied in free radical polymerization [49,[51][52][53][54]. It has never been used as a photocatalyst in controlled radical polymerizations, thus it is an excellent direction for the development of naturally-derived photocatalytic systems for RDRP techniques. ...
The article presents an application of curcumin, a widely available and cost-effective organic dye, to control visible-light mediated ATRP. A two-component photocatalytic system composed of curcumin and an amine-based electron donor activates alkyl bromide and control radical polymerizations by a reductive quenching pathway. The polymerizations of various types of (meth)acrylates were performed under blue LED irradiation (λmax = 460 nm, 5 mW cm−2). The effect of electron donor type, solvent, photocatalyst and electron donor concentration, target degree of polymerization (DPtarget) etc. on the metal-free ATRP of methyl methacrylate (MMA) and poly(ethylene glycol) methyl ether methacrylate (OEGMA500) was systematically investigated to optimize the polymerization conditions. Moreover, comestible curcumin of four different brands available in the market as kitchen spices was used, which makes this approach economical (∼0.04 € for 15 g of turmeric). In order to increase the applicability of the curcumin-based polymerization strategy, the concept was extended to the polymerization of other methacrylates – butyl methacrylate (nBMA), di(ethylene glycol) methyl ether methacrylate (DEGMA), 2-(dimethylamino)ethyl methacrylate (DMAEMA), and acrylates i.e. poly(ethylene glycol) methyl ether acrylate (OEGA480), n-butyl acrylate (nBA) and 2-hydoxyethyl acrylate (HEA). In situ chain extension experiment demonstrated the preservation of chain-end functionality, enabling facile synthesis of well-controlled copolymers.
... CUR is an infrequent diketone pigment which is distributed in plants. Because of the low toxicity, CUR is applied as a colorant in the field of foods (Mishra and Daswal, 2007;Epstein et al., 2010;Lüer et al., 2014), which is approved by the World Health Organization (WHO), the Food and Drug Administration (FDA), and many countries. In recent years, CUR is been one of the top-selling natural edible pigments. ...
A novel polymer of poloxamer188-b-PCL was synthesized via a ring-opening polymerization. Fourier transform infrared spectroscopy (FTIR), Raman, and ¹H nuclear magnetic resonance (¹H NMR) spectra were used to study the structures of obtained poloxamer188-b-PCL. The thermo-stability of poloxamer188 -b-PCL was carried out with a thermal gravimetric analyzer (TGA), and cytotoxicity was obtained using the CCK8 method. Cargo-free and curcumin (CUR)-loaded poloxamer188-b-PCL NPs were fabricated via the solvent evaporation method. The morphology, particle size distribution, and stability of cargo-free NPs were studied with a scanning electron microscope (SEM) and laser particle analyzer. The in vitro radioprotection activity of CUR-loaded NPs was performed. FTIR, Raman, and ¹H NMR spectra confirmed that poloxamer188-b-PCL was obtained. TGA curves suggested poloxamer188-b-PCL had better thermo-stability than original poloxamer188. Cell tests suggested that the cargo-free NPs had no cytotoxicity. SEM image showed that the cargo-free NPs were spherical with a diameter of 100 nm. Free radical scavenging experiments proved that CUR-loaded NPs had better antioxidant activity than CUR solutions. CUR-loaded NPs could be detected in all tissues, including liver, kidneys and lung. In summary, this work demonstrated a feasibility of developing an injective formulation of CUR and provided a protection agent in caner radiotherapy.
... The absorption spectrum of the dye and the anchorage of the dye to the surface of TiO 2 are important parameters determining the efficiency of the cell 9 . In recent several years it has become a great attention to use DSSC based on natural dyes due to the high extinction coefficient for natural dye, and simply procedure to extract it, as well as its environmental friendliness [10][11][12] . ...
Dye sensitized solar cells (DSSC) have been constructed by using diferuloylmethane dye as natural photosensitizers for
anatase nanostructure TiO2 thin film coated on FTO conductive glass substrate. Structural and morphological properties of the
semiconductor films have been characterized by X-ray diffractometer and scanning electron microscopy, respectively, as well as,
the wavelength of the maximum absorbance of the dye anchored on TiO2 film has been studied by using UV-Vis
spectrophotometer. The XRD pattern of TiO2 thin films made by Doctor-blading deposition technique has revealed that the grain
size of TiO2 is equal to 40 nm. The photovoltaic performance of the cells have been investigated in terms of power conversion
efficiency, furthermore the effect of different spectroscopic parameters on the PV performance of constructed DSSC has been
studied.
... The absorption spectrum of the dye and the anchorage of the dye to the surface of TiO 2 are important parameters determining the efficiency of the cell 9 . In recent several years it has become a great attention to use DSSC based on natural dyes due to the high extinction coefficient for natural dye, and simply procedure to extract it, as well as its environmental friendliness [10][11][12] . ...
Dye sensitized solar cells (DSSC) were constructed by using diferuloylmethane dye as natural photosensitizers for anatase nanostructure TiO 2 thin film coated on FTO conductive glass substrate. Structural and morphological properties of the semiconductor films were characterized by X-ray diffractometer and Scanning electron microscopy respectively, as well as, the wavelength of the maximum absorbance of the dye anchored on TiO 2 film has been studied by using UV-VIS spectrophotometer. The XRD pattern of TiO 2 thin films made by Doctor-Blading deposition technique revealed that the grain size of TiO 2 was equal to 40 nm. The photovoltaic performance of the cells were investigated in terms of power conversion efficiency, furthermore the effect of different spectroscopic parameters on the PV performance of constructed DSSC have been studied .
... ANNs can act as onecomponent hydrogen abstraction photoinitiators due to the presence of both the chromophore and the tertiary amine substituent in the same compound. 36,37 The radicals are expected to be generated as follows ((R1)-(R2)): ...
Four N-[2-(dimethylamino)ethyl]-1,8-naphthalimide derivatives (ANNs) with different substituents (bromo group, primary amine, secondary amine, and tertiary amine) in the naphthalimide skeleton have been synthesized and can be used as one-component free radical photoinitiators or incorporated into multi-component photoinitiating systems for free radical or cationic photopolymerization under the irradiation of various LEDs. ANN2 (with secondary amine substituent) or ANN3 (with tertiary amine substituent) alone is capable of initiating the free radical polymerization of acrylates under LED at 405 nm while ANN0 (with bromo substituent) or ANN1 (with primary amine substituent) does not work. When combined with various additives (e.g. iodonium salt, N-vinylcarbazole, chloro triazine, or tertiary amine), most of the ANN1-3 based multi-component photoinitiating systems are efficient in free radical photopolymerization at 385 nm, 405 nm, 455 nm, 470 nm and even under a low-intensity polychromatic visible light (from halogen lamp) with final conversions higher than 60% (even more efficient than commercial photoinitiators bisacylphosphine oxide and camphorquinone). In addition, ANN1-3 based photoinitiating systems also exhibit a high efficiency for cationic photopolymerization of epoxides upon diverse LEDs. The ANN2 based photoinitiating system can also be used for the synthesis of interpenetrated polymer networks and adapted for 3D printing. The photochemical mechanisms of the ANNs based systems are comprehensively investigated and discussed in detail.
... The absorption spectrum of the dye and the anchorage of the dye to the surface of TiO 2 are important parameters determining the efficiency of the cell 9 . In recent several years it has become a great attention to use DSSC based on natural dyes due to the high extinction coefficient for natural dye, and simply procedure to extract it, as well as its environmental friendliness [10][11][12] . ...
Cadmium sulphide (CdS) and CdS doped with PVA semiconductor thin films were prepared by chemical spray pyrolysis technique. The thickness of the films was varied in the range (1.281, 1.367, 1.421, 1.485) μm and (1.722, 1.776, 1.873, 1.938) μm for CdS and PVA doped films respectively. This parameter has affected the physical properties of the specimens. The films were characterized by X-ray diffraction (XRD). XRD patterns were indicated the presence of hexagonal phase of CdS, and with the increase of film's thickness the grain size has increased. The optical absorption spectra showed that the CdS films have a direct band gap value of (2.4-2.55) eV and (2.425-2.75) eV for doped samples with PVA.
... van den Oever et al. (2004) studied the application of several food grade colorants for use in coloration of thermoplastics and suggested some of best performing natural colorants such as indigo and anthraquinone-based colorants can be used in appliances where moderate properties concerning heat resistance and (UV) light stability are required, such as indoor PP applications and PVC water drainage pipes. Use of curcumin as sensitizer for photopolymerization of styrene is also reported (Mishra and Daswal, 2007). Siva et al. (2008) reported an easy, practical and reliable procedure for using Bixa orellana L. natural dye as an alternative tracking dye for gel electrophoresis in place of regularly used dye marker bromophenol blue (BPB). ...
The use of photopolymerization is expanding across a multitude of biomedical applications, from drug delivery to bioprinting. Many of these current and emerging photopolymerization systems employ visible light, as motivated by safety and energy efficiency considerations. However, the “library” of visible light initiators is limited compared with the wealth of options available for UV polymerization. Furthermore, the synthesis of traditional photoinitiators relies on diminishing raw materials, and several traditional photoinitiators are considered emerging environmental contaminants. As such, there has been recent focus on identifying and characterizing biologically sourced, visible light‐based photoinitiator systems that can be effectively used in photopolymerization applications. In this regard, several bio‐sourced molecules have been shown to act as photoinitiators, primarily through Type II photoinitiation mechanisms. However, whether bio‐sourced molecules can also act as effective synergists in these reactions remains unknown. In this study, we evaluated the effectiveness of bio‐sourced synergist candidates, with a focus on amino acids, due to their amine functional groups, in combination with two bio‐sourced photoinitiator molecules: riboflavin and curcumin. We tested the effectiveness of these photoinitiator systems under both violet (405 nm) and blue (460–475 nm) light using photo‐rheology. We found that several synergist candidates, namely lysine, arginine, and histidine, increased the polymerization effectiveness of riboflavin when used with both violet and blue light. With curcumin, we found that almost all tested synergist candidates slightly decreased the polymerization effectiveness compared with curcumin alone under both light sources. These results show that bio‐sourced molecules have the potential to be used as synergists with bio‐sourced photoinitiators in visible light photopolymerization. However, more work must be done to fully characterize these reactions and to investigate more synergist candidates. Ultimately, this information is expected to expand the range of available visible light‐based photoinitiator systems and increase their sustainability.
The design of photoinitiating systems activable in the visible range and under low light intensity is an active research field supported by the vast applications of photopolymerization. In the search for new dyes that can exhibit panchromatic absorptions, anthraquinone has been identified as a promising scaffold. Over the years, numerous modifications have been carried out on this scaffold by molecular engineering, enabling to design water-soluble, panchromatic or photobleachable dyes. In this review, an overview of the recent advances concerning photopolymerization with anthraquinone-based photoinitiating systems is given.
Photoinitiating systems (PISs) are key components in photocurable formulations (inks) for three-dimensional (3D) printing. Natural PISs are promising and more environmentally friendly alternatives to conventional photoinitiators. These systems offer enhanced features such as less toxicity and therefore, higher biocompatibility. Furthermore, they can be found in a large “color palette” variety (absorbing in the different regions of the UV-visible spectrum) enabling new possibilities in the field of 3D printing. In this review, we outline the current efforts to implement natural and naturally derived PISs in light-based 3D printing. First, recently applied natural PISs and their performance for light-based 3D printing are discussed. In a second part, an overview of naturally derived PISs including the most common synthetic strategies are provided. Last, current challenges as well as future perspectives of the field are discussed.
Introduction
Natural Photoinitiating Systems
Naturally Derived Photoinitiating Systems
Conclusions and Outlook
During the past decades, visible light photoinitiators have been extensively studied due to the development of cheap and energy-saving irradiation sources, namely light-emitting diodes (LEDs). With aim at developing chromophores with absorption perfectly fitting the emission of LEDs, triphenylamine has been identified as a versatile building block for designing Type I and Type II photoinitiators of polymerization. In this review, an overview of different photoinitiators comprising a triphenylamine unit is presented. To evidence the interest of these structures, comparisons with benchmark photoinitiators is presented.
Visible light photoinitiators of polymerization are actively researched as these molecules can advantageously address the light penetration issue within the photocurable resins. With aim at developing dyes using a scaffold that can be easily modified, phenothiazine is one of those. In this review, an overview of the recent developments concerning phenothiazine-based photoinitiators of polymerization is provided. Over the years, structures of photoinitiators have greatly evolved and phenothiazine is now exclusively included in photoinitiators of more complex structures and not used alone as during the pioneering works. Using this unprecedented approach, Type I and Type II photoinitiators have been designed and synthesized. To evidence the pertinence of the approach, comparisons with reference photoinitiating systems will be provided.
We report the synthesis of new visible-light photosensitizers derived from purpurin (mono-allyl- and triallyl- purpurin), and used as type II photoinitiating systems when associated with N-methyldiethanolamine, bis(4-methylphenyl) iodonium hexafluorophosphate or...
This study reports for the first time the use of β-carotene as a natural photosensitizer for both the cationic photopolymerization and thiol-ene click reactions under visible light irradiation. In an all-green synthesis approach, this performing dye-based system leads to the cationic and thiol-ene polymerization of bio-based monomers, mono- and di-epoxy limonene, with very high final conversions, and appears as efficient as the common visible-light photosensitizers used in free-radical or cationic photopolymerization, i.e. thioxanthone derivatives or camphorquinone. Effective antibacterial and tack-free coatings have been synthesized by the incorporation of a natural antibacterial agent (eugenol) in the limonene-derived polymer network. The antibacterial assays have demonstrated a tremendous effect of the eugenol-containing coatings against the adhesion of Escherichia coli (E. coli, Gram negative) and Staphylococcus aureus (S. aureus, Gram positive).
The photochemical mechanism on the radical generation from 1, 8-naphthalimide derivative (ANDs; with tertiary amine moities) based photoinitiating systems are investigated using various approaches including cyclic voltammetry, steady state photolysis, and electron spin resonance spin trapping techniques. It reveals that radicals can be produced from ANDs alone or ANDs/additive additives (e.g. iodonium salt or chloro triazine) combinations. The generated radicals and cations can initiate both free radical and cationic photopolymerization reactions under irradiation with various LEDs or halogen lamp. For most of the ANDs based photoinitiating systems, more than 60% of acrylate or epoxide conversions were attained and even higher than those got by using commercial photoinitiators e.g. bisacylphosphine oxide or camphorquinone. More interestingly, the AND1 based photoinitiating system can also initiate concomitant cationic/radical polymerizations and be used for the synthesis of interpenetrated polymer networks.
The natural dye, curcumin, was extracted from Curcuma longa L. using a simple extraction technique.
The dye was then used as a sensitizer in a dye-sensitized solar cell (DSSC) without further purification,
and its characteristics were studied. The dye was treated with acetic, nitric and hydrochloric acid for
stabilization and increasing the conversion efficiency. The current conversion efficiency was 0.60%
for the acetic acid-treated curcumin dye, which was double that of the nitric and hydrochloric acids
treated dyes. The short-circuit current (Jsc), open-circuit voltage (Voc) and fill factor of the DSSCs
made from the acetic acid treated dye were 1.6544 mA cm-2
, 0.5511 V and 0.6646 respectively.
Therefore, the addition of acetic acid to the curcumin dye results in slightly higher conversion
efficiency than the bare dye, whereas the conversion efficiency is suppressed when the dye is treated
with nitric and hydrochloric acid. This paper briefly discusses the effects of naturally obtained
curcumin dye on the performance in DSSCs.
The nonsurfactant templated sol–gel route has been demonstrated to be a cost effective, green and biofriendly pathway to obtain mesoporous materials with an interconnected network of wormhole–like pores. It involves the formation of a metal–oxide network around an inert organic molecule (e.g., sugar molecules) which functions as a template and can be later removed by simple extraction with water or other solvents. This research describes the preparation of mesoporous zirconia and organo–functionalized silica by the acid–catalyzed nonsurfactant templated sol–gel route and also describes the use of sublimation as a method for the template removal. Mesoporous silica nanospheres with tunable particle size were also prepared by modifying the base–catalyzed Stober process with the addition of various sugar molecules as templates. The materials were characterized using TGA, nitrogen adsorption–desorption, SEM, TEM, XRD and FTIR. The application of nonsurfactant templated mesoporous materials in the area of enzyme encapsulation and stabilization is explored in this research. A novel ‘double encapsulation’ approach that enables a sol–gel encapsulated protease to retain 60 % of its original activity after 4 weeks in harsh environments, such as high pH buffer and laundry detergent, is demonstrated. The need for aesthetic, as well as durable, dental restorations has led to extensive research in the area of dental composites. This study examines the use of nonsurfactant templated mesoporous materials as fillers in dental composites. Mesoporous materials of irregular morphology prepared by acid–catalyzed sol–gel routes as well as mesoporous silica spheres prepared via base–catalyzed sol–gel reactions were both employed as fillers in experimental dental composites. This research led to the development of novel ‘dental monomer templated mesoporous materials’, which were also evaluated as fillers in dental composites. Various approaches, such as dense packing, addition of nanosilica and spherical fillers, etc., were employed to fabricate nanocomposites with improved mechanical properties. The composites were evaluated using flexural and compression testing. Curcumin, the ground rhizome of Curcuma longa, a common South Asian herb has attracted much attention due to its chemopreventive and anti–inflammatory properties. This research describes the synthesis of a series of poly [(arylenedioxy)(diorganylsilylene)]s via polycondensation between curcumin and various diorganodichlorosilanes. These novel polymers incorporate the β–diketone unit of curcumin as well as the Si–O bond in the backbone. The polymer structure was characterized by means of 1HNMR, FTIR and elemental analysis, while GPC results showed high molecular weights. Preliminary cell culture results suggested lack of cytotoxicity, which is important for potential applications, such as implants and tissue engineering scaffold materials. The unique and interesting thermal behavior of these polymers was studied by DSC. The technology of enzymatically degrading biomass into simple sugars, such as glucose, is a critical step towards viable production of bio–based ethanol from non–food related sources. This research demonstrates the use of a biosensor–based diabetic blood glucose monitor as a rapid glucose detector and compares it to time consuming UV assays that are currently employed in research laboratories. Pretreatment of wood shavings with ferric chloride, followed by treatment in cold NaOH/urea solution was found to significantly enhance glucose production upon enzymatic hydrolysis. Finally, accounts of exploratory experiments in the areas of thermally crosslinkable high temperature elastomers and inorganic–organic hybrid materials are provided in the appendix sections of this thesis.
The dye-sensitized photopolymerization of N,N′-methylenebisacrylamide (MBA) in aqueous medium initiated by eosin-ascorbic
acid (AH2) was studied at 25°C. The polymerization takes place only at higher concentration of AH2 (> 10−3 mol/L) and its rate increases rapidly as [AH2] is increased. The rate of polymerization is proportional to [MBA] and [AH2]. Kinetic results indicate that semi-quinone (DH) dye radical and semi-oxidized form (
$$\mathop {\mathop A\limits^ \bullet }\limits^ - $$
) of ascorbic acid are unable to initiate polymerization. Formation of H2O2 is proposed and confirmed. A suitable mechanism is proposed in accordance with experimental results obtained
The series of new dyes, which structures are based on 6H-indolo[2,3-b]quinoxaline skeleton that possess characteristic electronic absorption band at a boundary of UV and visible light were tested as potential light absorbing chromophores for photoinitiated polymerization.The studied dyes can be classified into two different groups. The first is the group, so called ‘the branched dyes’, which structures possess the part of molecule that can rotate without restraints and are characterized by low photoinitiation ability. The second, planar and rigid group of molecules provides another chromophores, which possess quite different properties in comparison to that observed for the branched dyes. Their photoinitiation ability is comparable to that observed for many commercially available photoinitiating systems.The location of electronic absorption spectra at a boundary of UV and visible light makes the tested dyes the good candidates for the photoinitiating system applied in dental restorative materials. Their high molar absorption coefficient allows to decrease the dyes concentration in dental formulation in comparison to commonly used camphorquinone.
Si relaziona circa esperimenti di polimerizzazione con stirene in benzene a 60°, iniziata con benzoilperossido, per una larga gamma di concentrazioni sia del monomero che dell'iniziatore; i risultati non si possono spiegare in termini del classico rapporto di velocità con . Quando la concentrazione dell'iniziatore viene aumentata e quella del monomero diminuita, si sono avute deviazioni in ordine inaspettato, del monomero in aumento fino a [M]1,4 e dell'iniziatore in diminuzione fino a [I]0,42. In base al concetto della terminazione radicalica primaria, si propone la seguente equazione:
R
p
2
2
R
p
2
k
prt
k
i
k
p
f
k
k
prt
k
i
k
p
f
k
f
k
2
R
p
Summary
ReBr(CO)3(3,3'-disubstituted-2,2'-biquinoline) complexes modified with diphenylzinc (Ph2Zn) and methylaluminoxane (MAO) were
employed in the polymerization of styrene (St). The results show that both the initiating activity and the characteristics
of the polystyrenes (PSts) are independent of the different conformational rhenium complexes. Studies made using UV-vis spectroscopy
agree with a dissociation of the biquinoline ligand when MAO is added to the reaction system.
A method is proposed for analysing the problems associated with non-ideal polymerizations reflected mainly in the variability of Rp/[I]0·5[M] where Rp is the rate of polymerization and [I] and [M] are the initiator and monomer concentrations, respectively. Primary radical termination and degradative chain transfer are treated jointly and the entirely different mathematical natures of the two processes are described. The method could dispense with the need to use the uninhibited rate of polymerization which does not lend itself to reliable measurements for many systems. It is found to be efficient in detecting the active species in a polymerization system that leads to non-ideality due to degradative chain transfer.This method is applied to vinyl chloride polymerization data from the literature, the values of constants obtained therefrom are found to agree well with the existing values.
The behavior of the stable free radical moderated polymerization of styrene in the presence of 2-fluoro-1-methylpyridinium p-toluenesulfonate (FMPTS) has been examined. The rate enhancement has been found to be the result of the decrease of free nitroxide (TEMPO) concentration that shifts the equilibrium between capped and growing chains toward the growing free radical form. The decrease in TEMPO concentration was shown to be coupled to the presence of FMPTS. The system is sensitive to the initiator employed and the timing of the addition of FMPTS. If the polymerization rate change due to the change in TEMPO concentration is accounted for, a secondary polymerization retarding influence of FMPTS was observed.
The benzoate groups incorporated into polystyrene prepared with benzoyl-carbonyl-13C peroxide initiator have been evaluated by NMR spectroscopy. This provides, for the first time, a direct measure of the amounts of initiation by head and tail addition of benzoyloxy radical to styrene as well as a means of determining the extent of transfer to benzoyl peroxide and primary radical termination. For example, polystyrene prepared by bulk polymerization at 60°C using 0.04 M benzoyl peroxide contains ca. 1.7 benzoate end groups per molecule, 87% of which derive from initiation by tail addition to styrene, 5% by head addition, and 8% by termination through transfer to initiator or primary radical termination. The method also shows the dependence of these processes on reaction conditions.
Polymerization of MMA was carried out in presence of visible light (440 nm), quinoline-bromine charge-transfer complex being used as the photoinitiator. The initiator exponent was observed to be 0.5 up to 0.014 M initiator concentration; when chloroform was used as the solvent, the monomer exponent was found to be unity. The polymerization was inhibited in presence of hydroquinone but little inhibitory effect was observed in the presence of air. An average value of k2p/kt for this photopolymerization system was found to be (1.08 ± 0.22) × 10-2. Kinetic and other evidence indicates that the overall polymerization takes place by a radical mechanism.
Photopolymerization of MMA was carried out at 40°C in diluted systems by use of quinolinebromine (Q–Br2) charge-transfer complex as the initiator and chloroform, carbon tetrachloride, chlorobenzene, dioxane, THF, acetone, benzene, toluene, quinoline, and pyridine as solvents. The results showed variable monomer exponents ranging from 1 to 3. For chloroform, carbon tetrachloride, and chlorobenzene, the monomer exponent observed was unity; for other solvents used, the value of the same exponent was much higher (between 2 and 3). Initiation of polymerization is considered to take place through radicals generated in the polymerization systems by the photodecomposition of (Q–Br2)–monomer complex (C) formed instantaneously in situ on addition of the Q–Br2 complex in monomer. The kinetic feature of high monomer exponent is considered to be due to higher order of stabilization of the initiating complex (C) in presence of the respective solvents. In the presence of the retarding solvents, very low or zero initiator exponents were also observed, depending on the nature and concentration of the solvents used. The deviation from the square-root dependence of rate on initiator concentration becomes higher at high solvent and initiator concentrations in general. This novel deviation is explained on the basis of initiator termination, probably via degradative chain transfer involving the solvent-modified initiating complexes and the propagating radicals.
A new highly active rare earth coordination catalyst composed of rare earth phosphonate, di-n-butylmagnesium (MgBu), and hexamethyl phosphoramide (HMPA) for the polymerization of styrene has been developed for the first time. High molecular weight polystyrene (M̄ν = 50–70 × 104) in 100% conversion could be prepared at following conditions: [Nd] = 6–8 × 10−4 mol/L, [St] = 3.0 mol/L, Mg/Nd = 11, and HMPA/Mg = 1–1.5 (molar ratio). The catalytic activity of this new catalyst is 3530 g PSt/g Nd. Kinetics study shows that the polymerization rate is of first order with respect to both monomer concentration and catalyst concentration, and activation energy of the polymerization is 40.1 kJ/mol. © 1996 John Wiley & Sons, Inc.
Dye-sensitized photopolymerizations of low concentrations (1–5%) of acrylic monomers proceed at 40°C., if fibrous proteins (keratin, fibroin or collagen) are suspended in solution. In the absence of dye (riboflavin or fluorescein) or fibrous protein no appreciable photopolymerization of monomer is observed at the short irradiation times (120 min.). Graft products containing 0.5–23% polymer are found. The quantity of grafted polymer introduced into the protein depends on the chemical nature and concentration of monomer, dye, and fibrous protein. Homopolymerization of monomer is observed to some extent in each graft polymerization. In the presence of oxygen an induction period ia found, but removal of oxygen from solution greatly retards these photo-polymerizations. Chemical and physical characterization of the protein–polymer products suggests that only a small number of initiation and grafting sites are present in the protein and that the grafted polymer partially resides within the protein matrix. We believe these photopolymerizations proceed via a free-radical pathway involving radicalabstraction of hydrogen from the protein by dye intermediates.
Copolymers of styrene and methyl methacrylate prepared with lithium dispersion as initiator do not contain random sequences of both monomers. Fractionation of the copolymers with acetonitrile and the NMR spectra of the insoluble fractions show that these are block copolymers which consist of a polystyrene portion and a poly(methyl methacrylate) portion. When the copolymerization is stopped at low conversion the copolymer has a high styrene content, which sometimes exceeds the value expected for radical copolymerization. This fact would indicate that styrene is preferentially polymerized at the early stages of chain propagation. When the copolymerization is carried to high conversion some crosslinked polymer is formed which contains more styrene than the soluble part of the same experiment. When a piece of metallic lithium is used as initiator, it is found that the crosslinked polymer is formed on the surface of the metal. The addition of lithium phenoxide or β-naphthoxide to the system eliminates the formation of crosslinked polymer. A possible mechanism is proposed.
Syndiospecific polymerization of styrene was catalyzed by monocyclopentadienyltributoxy titanium/methylaluminoxane [CpTi (OBu)3/MAO]. The atactic and syndiotactic polystyrenes were separated by extracting the former with refluxing 2-butanone. The activity and syndiospecificity of the catalyst were affected by changes in catalyst concentration and composition, polymerization temperature, and monomer concentration. Extremely high activity of 5 × 107 g PS (mol Ti mol S h)−1 with 99% yield of the syndiotactic product were achieved. The concentration of active species, [C*], has been determined by radiolabeling. The amount of the syndiospecific and nonspecific catalytic species, [C] and [C] respectively, correspond to 79 and 13% of the CpTi(OBu)3. The rate constants of propagation for C and C at 45°C are 10.8 and 2.0 (M s)−1, respectively, the corresponding rate constants for chain transfer to MAO are 6.2 × 10−4 and 4.3 × 10−4s−1. There was no deactivation of the catalytic species during a batch polymerization. The rate constant of chain transfer with monomer is 6.7 × 10−2 (M s)−1; the spontaneous β-hydride transfer rate constant is 4.7 × 10−2 s−1. The polymerization activity and stereospecificity of the catalyst are highest at 45°C, both decreasing with either higher or lower temperature. The stereoregular polymer have broad MW distributions, M̄w/M̄n = 2.8–5.7, and up to three crystalline modifications. The Tm of the s-PS polymerized at 0–90°C decreased from 261.8 to 241°C indicating thermally activated monomer insertion errors. The styrene polymerization behaviors were essentially insensitive to the dielectric constant of the medium.
In the present work we report on the photochemistry and photopolymerization activity of a new bimolecular photoinitiator system, which exhibits maximum sensitivity at 518 nm, consists of a sensitizer dye, pyrromethene 567 (PM567), and a radical generating reagent 3,3′,4,4′‐tetra(tert‐butylperoxycarbonyl)benzophenone (BTTB). The photosensitization of BTTB through the excitation of PM567 induces high polymerization rates, while different experiences have revealed that PM567 or BTTB, separately, are unable of initiating the polymerization of 2‐hydroxyethyl methacrylate (HEMA) over extensive periods of time at 40 °C, either in the dark or under irradiation. The polymerization efficiency of this photoinitiator system, PM567/BTTB, was analyzed following the bulk polymerization kinetics of monomer HEMA by differential scanning photo‐calorimetry. Photopolymerization rates and quantum yields were observed under isothermal conditions (40 °C) for continuous illumination polymerizations (at 518 nm) at different incident light intensities and different BTTB concentrations.
Scheme of the photosensitization process of the bimolecular photoinitiator system of PM567/BTTB.
magnified image Scheme of the photosensitization process of the bimolecular photoinitiator system of PM567/BTTB.
Several dyes containing benzylideneimidazopyridine moiety (BIPDs) were synthesized and evaluated as photoinitiators for free-radical polymerization induced with the visible emission of an argon-ion laser. One method of dye structure change was applied in our study. The modification was based on the character of the substituent introduced into both the imidazopyridine skeleton and phenyl ring. Several different groups were tested including heavy atoms (CI, Br) as well as electron-accepting (NO2), and electron-donating groups [N(CH3)2, OCH3]. Analysis of the dye properties demonstrated that there is a significant heavy atom effect on the photoinitiation ability of the novel dyes in both cases, for example, when a heavy atom is introduced into the phenyl ring as well as into the imidazopyridine part of the molecule. The introduction of an electron-acceptor or electron-donor group into the phenyl part of the dye caused a dramatic decrease in its photosensitivity. The type of applied counterion had no effect on the overall sensitivity of a dye. BIPDs are not particularly good photoinitiators. Further modification of the dye structure involved the elimination of the motion of a CC bond by the coplanarization of the styrylium residue with other parts of the dye. This approach decreased the degree of branching of the dye, which stabilized the molecule in its excited state. The formed dye, quinoline[2,3-b]-2,3-dihydro-1H-imidazo[1,2-a]pyridinium bromide (QDIPB), exhibited dramatically enhanced sensitivity. QDIPB possessed broad structured spectra with a long-wavelength part shifted to the blue as compared to other BIPD dyes. The change of the absorption spectra and its high photoinitiation ability makes QDIPB a good candidate for the photoinitiating system applied in dental restorative materials. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3048–3055, 2003
The photodecomposition of curcumin when exposed to UV/visible radiation is studied. The main degradation products are identified. The reaction mechanisms are investigated and the order of the over-all degradation reactions and the half-lives of curcumin in different solvents and in the solid state are determined.
The comparative photostability of curcumin 1, and two non-phenolic curcuminoids: 1,7-diphenyl-1,6-heptadiene-3,5-dione 2 (unsubstituted curcumin) and dimethylcurcumin 3 in non-degassed dilute solutions (approximately 3-5 x 10(-5) mol l(-1)) has been established by UV-visible absorption spectroscopy; disappearance quantum yields were measured. The similar behavior of the three studied curcuminoids is indicative of only a moderate role of phenol groups in the photodegradation process. Structural analysis of the photodegradation products of compound 2 in more concentrated solution (approximately 3.6 x 10(-3) mol l(-1)) shows formation of benzaldehyde, cinnamaldehyde, 2'-hydroxy-5',6'-benzochalcone 4, flavanone 5 and some other unidentified photoproducts. Flavanone 5 is formed by irradiation of chalcone 4. It represents a unique example of photochemical conversion of a diarylheptanoid molecule into a flavonoid, another very important class of natural products.
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