27.03.2014
Flow (micro)reactor technology (MRT) has recently gained increasing attention since its offers a wide range of new possibilities for academic research and industrial production in the polymer field.1–4 Flow reactions feature several advantages over batch processes. Ligation of polyacrylates with other polymer counterparts via copper catalyzed azide–alkyne conjugation (CuAAC) has been demonstrated on a flow chip reactor with high efficiency and comparatively fast reaction times.11 A distinct problem of such reaction, however, is the use of certain copper species, which in this case tend to be only partially soluble. Until recently only a few transition metal mediated, controlled radical polymerization reactions – unlike free-radical polymerization reactions – have been efficiently initiated by UV-light.27–31 Also reversible addition fragmentation radical transfer polymerization (RAFT)32,33 could be initiated by the use of conventional UV-initiators.
Herein, we focus on the photo-induced polymerization of methyl acrylate in polar solvents under reaction conditions typical for a SET-LRP reaction (or SARA-ATRP, supplemental activators and reducing agents-ATRP). Based on the novel photo-polymerization protocol introduced by Haddleton and coworkers, we demonstrate for the case of methyl acrylate, how this polymerization can be optimized for micro- and milliflow with high efficiency and short reaction times. Ethyl 2-bromoisbutyrate (EBiB, Alfa Aesar, 98+%), copper(II) bromide (CuBr2, Sigma-Aldrich, 99%), and dimethyl sulfoxide (DMSO, Merck, pro analysis) were all used as received. 1H NMR spectra were recorded in deuterated chloroform applying a pulse delay of 12 s with two NMR spectrometers (300 and 400 MHz) from Oxford Instruments Ltd. Electrospray ionization mass spectrometry (ESI-MS) was performed on an LCQ Fleet mass spectrometer (ThermoFischer Scientific) equipped with an atmospheric pressure ionization source operating in the nebulizer-assisted electrospray mode. The application of photo-induced copper-mediated polymerization allows for a fundamental struggle with respect to these polymerization types to be directly solved when translating the reaction protocol to a flow system. Three series of polymerization reactions were performed by variation of the target molecular weight, Fig. Generally, conversion and thereby degree of polymerization are in a flow reaction influenced by variation of the residence time.
The molecular weight evolution of the three polymerization reactions can be fitted linearly.
In line with the observation of well-controlled polymerization, a decrease in the dispersity of the polymers is observed (see ESI Fig. To unambiguously demonstrate that the reaction proceeds under high control the first-order kinetic character of the polymerization was determined, Fig. Besides the milli-flow reactor, also a micro-flow reactor was employed to carry out UV SET-LRP polymerization to demonstrate that the reaction may also be performed in a true microfluidic device. The first order kinetic plot in micro- and milliflow reactions shows good linearity, as seen in Fig. Finally, the quality of the polymers were analyzed by mapping the end group fidelity via electrospray ionization mass spectrometry (ESI-MS). Based on pMA obtained from the tubular reactor, block copolymers were synthesized in the micro-flow reactor. In that respect, it is important to test that other monomers can also be polymerized in flow using the same protocol. Photo-initiated copper-mediated radical polymerization offers intriguing features for the design and synthesis of complex materials. Overall, the described reactions give a further example for the increasing number of polymer reactions that benefit from microreactor application and flow chemistry in general. The authors are grateful for funding in the framework of the European Science Foundation – Precision Polymer Materials (P2M) program. ACOMP contributes to a significant progress in unifying polymer and colloid worlds: Simultaneous monitoring of both polymer and particle characteristics in emulsion polymerization (A. Raw data and analysis for free radical polymerization of Methyl methacrylate in emulsion at 70C. Left: The evolution of the specific surface area A (upper) for two BA polymerization reactions. JH Liu, YH Chiu, 2009, A Novel Process Equipped with a Sloped UV-lamp for the Fabrication of Gradient Refractive Index Lens, Optic Letters, 34 (9), 1393-1395. In this investigation, a method for the preparation of radient-refractive-index (GRIN) lenses by UV energy-controlled polymerization has been developed.
Figure 2 shows the dependence of the initiator concentration on the ?n distribution of the GRIN lens, where np and Rp denote the refractive index at the periphery and the radius of the plastic rod, respectively. Figure 3 shows a color (online) image transmitted through a freshly fabricated GRIN lens with a 15 mm diameter and 80 mm length. In conclusion, we have demonstrated that a UVcontrolled polymerization can be used to fabricate GRIN plastic rods.
The development of plastic materials derived from renewable resources for replacement of petroleum-based ones continues gaining growing interest both in industry and in the scientific research community. Scheme 1 Stepwise thiol–ene reaction scheme involving the two unconjugated double-bonds of limonene with: initiation (i), reversible propagation (insertion–elimination) (ii), chain-transfer (hydrogen-abstraction) (iii), and thiyl self-termination (homocoupling) (iv) steps. Significant efforts have been conducted throughout the years to polymerize limonene either by itself or with other monomers using a variety of chemistries.
Chemical modifications of limonene and derived compounds to make them suitable for more standard polymerization chemistries can be found throughout the literature. Thiol–ene chemistry, although extensively studied in the past, has witnessed a spiraling development over the last 10–15 years and has recently been recognized as a ‘click’ method due to the high yields and reduced by-products attained.58–62 This reaction (videScheme 1 exemplified with limonene) presents well-established key advantages in thermoset synthesis, including a reduced sensitivity to oxygen inhibition, self-initiation without the need for a photoinitiator (enabling the cure of thick coatings), relatively rapid reaction rates leading to highly crosslinked products, improved curing control, and a free radical step-growth mechanism resulting in polymeric network materials with homogenous properties across all spacial dimensions. The notable features inherent to free-radical thiol–ene addition prompted us to investigate the synthesis of bio-based thermosets using R-(+)-limonene 1, as a renewable diolefinic substrate, together with TMPMP 2 as the primary synthetic thiol crosslinker. Multifunctional macromonomers 4 and 5 depicted in Scheme 2 were synthesized via free-radical thiol–ene coupling between R-(+)-limonene 1 and one of the multifunctional thiol propionate esters (TMPMP, 2 or PETMP, 3) in the presence of a small amount of initiator. Scheme 3 Synthesis of resin 2 (TE-C2) issued from limonene 1 (n = 2) and tetrathiol 3 (m = 4). One particular prerequisite involving typically known thiol–ene thermosets is the need for full stoichiometric control in order to ensure optimal thermo-mechanical properties with very few or no free reactive groups remaining within the material. A series of crosslinked thiol–ene networks were synthesized through photopolymerization of the two multifunctional macromonomer resins with the two polyfunctional thiol monomers. Scheme 4 Multifunctional thiol–ene combinations and nomenclature adopted in the preparation of thermoset films based on limonene. Table 1 Thermo-mechanical properties of the final cured thiol–ene films as a function of combination of functionality and initial ene–thiol stoichiometry.
To gain a first assessment of the curing degree and density of crosslinks within the networks achieved after photopolymerization, sol-content determinations were performed by immersing the films in an appropriate organic solvent. The dynamics of novel ink evolution have been a direct function of the regulations stipulated by governmental and other enforcement agencies.
When the necessity of switching over from solvent systems became imminent, a two-pronged approach was adopted.
It is against this backdrop that the potential of radiation and photochemistry to polymerize monomers and oligomers was thought to be worthy of consideration.
Radiation chemistry is concerned with the interactions of energetic charged particles and high-energy photons with matter.
When electrons collide with matter and interact, two types of processes take place: scattering and nuclear capture. When electrons interact with matter, they collide with the electrons in the atom and transfer some of their energy to them. Some of these secondary electrons may be captured by a positive ion and produce a highly excited state that can dissociate into other products. Inks used in EB curing make use of the polymerizing capability of acrylic monomers and oligomers. CH2=CH-COOHThe double bond in the acrylic moiety opens up during interaction with electrons (initiation) and forms a free radical that acts on other monomers forming a chain (propagation) leading to high-molecular-weight polymers. It may be interesting to introduce the effect of radiation on polymers themselves, even though such a continuous irradiation of ink coating by electrons does not happen. Radiation-induced curing technology is met with some amount of skepticism by the general public since it is linked with exposure to radiation and the fear of radioactivity. It is natural that the industries concerned very often compare the EB technology with UV technology since they compete with each other.2,8 In spite of the downside of the high investment involved in EB technology, it is superior to UV technology in being more acceptable, as it does not leave any initiator fragments.
Ancients worshipped the sun intuitively, probably unaware of the immense potential it embodies in shaping the destiny of humans.
The nexus between ink and light originates from the effect of light energy on ink coatings. UV inks came to light with the need to circumvent the VOC problem, since the inks provided a solventless, nearly 100%-solids system. UV inks and UV curing technology have been successfully applied in many areas: A major sector is the food packaging industry. Many natural and synthetic processes such as photosynthesis, photography, and photopolymerization are examples of photochemistry in action.
The UV curing process is considered to be one of the fast growing applications of industrial photochemistry.
Cationic polymerization methods compete with radical polymerization in UV curing technology. Flash lamps find applications for studying fast photochemical processes that needs high intensity, short duration flashes.
UV curing is carried out by moving the product by conveyer under the UV lamp for 1–4 seconds of exposure. The measurement of light intensities is necessary for controlling the absorbed dose and the photochemistry, both in basic and applied works.
Great advances in photochemistry have been made by the technique of flash photolysis since it brought to light the early events in the photochemical processes subsequent to light absorption that occurs in femtosecond (10-15S) time scales. Our September issue focuses on sustainable solutions, specialty chemicals, architectural coatings and the latest pigment technology. Polymerization reactions proceed rapidly to approximately 90% monomer conversion within 20 minutes of reactor residence time. These inhomogeneities inevitably lead to blockages and fouling in the reactor channels, prohibiting stable flow conditions and eventual failure of the reactor setup. However, these reactions are prone to side reactions due to the RAFT typical dithioester moieties acting as chromophores. Haddleton and coworkers have reported on the successful polymerization of a variety of monomers by this process and have demonstrated an efficient and fast light induced polymerization process. To the best of our knowledge, this is the first report on photo-induced controlled radical polymerization employing microreactors. Tris(2-(dimethylamino)ethyl)amine (Me6TREN) was synthesized according to a literature procedure.53 Methyl acrylate (MA, Acros, 99%) and butyl acrylate (BA, Acros, 99%) were deinhibited over a column of activated basic alumina, prior to use.
Reaction solutions were introduced into the reactor through two 1 mL gas-tight syringes (SGE) capable of delivering two solutions at flow rates between 0.1 and 25 ?L min?1.
Due to the rather low concentrations of copper that are required to mediate polymerization (in the present case about 0.02 eq. 1 depicts the outcome of MA polymerization reactions at 15 °C in the tubular reactor setup.
1 Development of the Mn of UV initiated copper-mediated polymerization of MA, with targeted Mn = 2000 (?), 4000 (?) and 9800 (^) g mol?1, in a tubular milli-flow reactor. With increasing flow rates lower residence times are directly achieved and the data shown in Fig. The slopes of all three reactions are slightly lower than the theoretical value (18, 31 and 67, respectively). 2 First order kinetic plots of the MA polymerization reactions in a milli-flow reactor with targeted Mn = 2000 (?), 4000 (?) and 9800 (^) g mol?1. 3 Molecular weight distributions of pMA with increasing residence time in the microflow reactor synthesized via UV-initiated copper-mediated polymerization and with a targeted Mn of 4000 g mol?1.
4 Comparison of the kinetic first-order plots of UV initiated copper-mediated polymerization of MA in a milli- (?) and a micro- (¦) flow reactor.
Samples of polymers from the polymerization with a target Mn = 2000 g mol?1 after reaction times of 10, 15 and 20 minutes were taken to check for the presence of termination products, which may decrease the livingness of the process. 5 Zoomed in ESI-MS spectra of poly(methyl acrylate) obtained by UV-copper-mediated polymerization in flow reactors. For chain extensions, butyl acrylate (BA) was polymerized with a maximum reaction time of 20 minutes. 6 Evolution of molecular weight distributions for the p(MA)-b-p(BA) block copolymers (solid line) obtained in the flow microreactor and the distribution of the p(MA) macroinitiator (dashed line).
Batch reactions had shown that a relatively large variety of monomers can be controlled by photo-copper-mediated polymerization.
The translation of the batch process to flow chemistry offers to scale up this reaction for the production of significant amounts of complex materials, which in a batch process is not directly possible due to light absorption profiles and insufficient penetration of light at increased optical path lengths.
The methods described herein make use of comparatively simple and versatile flow reactors and thus do not require sophisticated instrumentation. The ‘particle side' included a Malvern Mastersizer2000 detector, employing Mie scattering analysis from multi-angle scattering data.
The volume weighted mean diameter, D[4,3] for two modes in the particle size distribution (lower) for the 14% BA. The results of the refractive index distribution suggest that the concentration of high-refractiveindex dopant decreases from the center axis to the periphery of the gel rod, resulting in a GRIN distribution.
An inverted virtual image was obtained through the plastic rod fabricated in this investigation. This method is a technique that is easy to perform and requires relatively inexpensive equipment and materials.
The high efficiency of this reaction to prepare multifunctional ene-terminated resins, as intermediary macromolecular precursors, for thermosets synthesis was demonstrated under thermal and photoinitiated conditions. Also, by using renewable feedstocks all the biosynthetic capabilities of Nature are fully exploited in a straightforward fashion. For instance, this terpene was originally homopolymerized into poly(D-limonene) by Roberts and Day (1950)13 and Marvel and coworkers (1965)14 using a Friedel–Crafts catalyst and Ziegler–Natta catalyst, respectively.
In the classic example reported by Marvel and Olson (1957), D-limonene was transformed into di-pentene dimercaptan (DD) by free-radical thiol–ene photo-addition with thiolacetic acid followed by hydrolysis of the resulting dithiol ester.34 The terpene-based dithiol was subsequently polymerized with original limonene into poly-alkylene sulfides via thiol–ene chemistry affording low melting, highly viscous sticky materials. Moreover, because of the stepwise nature of radical growth, viscosity builds-up slower throughout the liquid phase resulting in much higher conversions at gel-point than with conventional acrylic formulations.63 This results in decreased property change with time.
This observed value for the selectivity obtained from empirical kinetics was confirmed by the relative reactivity based on the detailed two-route linear cyclic mechanism at steady-state, further revealing that contribution of the chain-transfer routes to the double-bond selectivity is negligible, this being explained mostly by the two reversible propagation steps (addition–elimination) directly involving each unsaturation. Limonene represents an exceptionally versatile monomer to introduce new chemical functional groups which can be directed towards specific polymerization techniques. Ethyl acetate (EtOAc, 50 wt%) was added to the mixtures to ensure complete reactant miscibility since the thiol–ene monomers are originally incompatible at room-temperature which prevents any direct synthesis of macromolecular thiol–ene materials in the bulk based on these compounds. 3 Normalized spectral FT-Raman profiles of finished syntheses products resulting from the thermal thiol–ene reaction with 10 mole excess of limonene. 6 Superimposition of normalized DMF-SEC traces of the two multifunctional resins synthesized via photoinduced thiol–ene coupling. Not only we have observed that the two alkene structures preserve enough reactivity necessary for the thiol–ene coupling process72 but the existence of a slightly rigid cycloaliphatic ring may also turn advantageous at conferring extra mechanical strength to the final thermoset materials. In this case, equimolar mixtures of multifunctional thiol and ene components taking the forms xR1–(SH)m and yR2–(‘ene’)n require that xm = yn, where x and y represent the number of molecules of each monomer and m and n denote the number of thiol and ene functionalities per molecule, respectively. Different cross-over combinations of the monomers, with and without an excess of thiol functional groups at different levels, were prepared leading to a total of 16 different compositions. Generally, the higher the crosslink density of the cured film the lower the fraction of the unbound material occluded within the network; and, therefore, the lower the sol-content. In the United States, the EPA in its policy statement1 recommended drastic reduction in the use of volatile organic compounds (VOC) that might be released to the environment from various products and applications. The first one was the obvious choice of using water as solvent, and accommodating high-solid content. High-energy radiation and photons have a reputation for being clean reagents, in that they do not leave any of their fragments in the reaction medium. Among them, the first two are of low energy and others are considered as high energy radiation.
The radiation chemistry brought out by high-energy electrons is very similar to that induced by photon sources such as X-rays and g-rays.
Scattering is the interaction in which the particle retains its nature or maintains identity.
These electrons can be excited to higher energy levels and at appropriate electron beam energies, the atom can lose that electron producing a positive ion. Acrylic chemistry has a special significance in modern day inks.6 The structure of the simplest acrylic compound, acrylic acid, is shown in the equation below. It may be noted that in radiation induced polymerization, no external initiator is needed since radiation itself generates free radicals with the result that no initiating species will be left in the coating unlike in a UV cured coating. Furthermore, its penetration is dependent only on the density of the coating and not restricted by specific absorption ranges as is the case with UV that may be masked by certain pigments. Primarily, the lightfastness of colored prints depends on the interaction of the UV components of light with the pigments. Properties such as uniform consistency also contribute to maintaining uniformity in prints, thus eliminating subjective variation since the variability from operator to operator and press run to press run is minimal. The central process in UV curing is the UV light induced polymerization of monomers and oligomers through the mediation of photoinitiators.
More specifically, it is concerned with studying a type of chemical reaction that is dependent on the action of visible or UV light. In medicine, the photodynamic therapy is a course of treatment for benign and malignant growth of tissue in which a photosensitizer medicine is administered intravenously and activated with laser light. Since chemical reactions are about bond breaking and bond making, let us see what happens when a bond breaks.


Among other industrial applications, the conversion of cyclohexane into Nylon 6 by photoreaction with nitrosyl chloride, transformation of 7-dehydrocholesterol into vitamin D3, synthesis of gammexane (an insecticide and a rodenticide) by the chlorination of benzene, and synthesis of surfactants by the photosulfochlorination of hydrocarbons are significant. Cationic photoinitiators are effective in polymerizing cycloaliphatic and aliphatic epoxides. Oxygen retards the polymerization reaction by removing radicals forming peroxy compounds on one hand, and quenches triplet excited state of photoinitiators forming singlet oxygen.
Xenon flash lamps use gas discharge technique at a moderate pressure of about 6–20 cm, the energy for the flash being derived from a bank of condensers.
This is generally done by the method called actinometry, which is parallel to dosimetry in radiation chemistry.
Stringent safety regulations should be followed since the UV light is extremely harmful to eyes and skin. Control of reactions is high as evidenced by ideal polymerization kinetics, low dispersities of the obtained polymers (in the range of 1.1) and linear evolution of number average molecular weights during polymerization reactions. In recent years, the advantages of flow (micro)reactor technology were demonstrated by different research groups, with most examples originating from the realm of organic and pharmaceutical research.9 With respect to polymerization reactions, MRT also features distinct advantages. While these problems can be overcome by careful choice of reaction conditions and by adjusting the physical parameters of the employed reactors, fewer possibilities exist to translate traditional copper-catalyzed reactions to flow. Thus, while the focus is herein put on a specific polymerization protocol, adaption of the described reactors to other (photo)polymerization methodologies is expected. Calibration was performed using linear narrow polystyrene (PS) standards from PSS Laboratories in the range of 470–7.5 ? 106 g mol?1. A constant spray voltage of 5 kV was used, and nitrogen at a dimensionless auxiliary gas flow rate of 3 and a dimensionless sheath gas flow rate of 3 was purged. The flow rates were controlled via a syringe pump (Chemyx) and the reactor temperature was controlled via a thermoelectric cooler temperature controller MTTC1410 (Melcor Thermal Solutions, temperature range ?15 to 195 °C).
The mixture was poured into a 250 mL brown laboratory bottle with a GL-45 screw cap and purged with nitrogen for approximately 45 min before starting the polymerization process.
Prior to use the mixture was purged with nitrogen for 3 min and then transferred into two 1 mL gas tight syringes and inserted into the syringe pump. The dotted lines are best fits of the data whereby the axis intercept was set to the molar mass of the initiator.
1 can simply be constructed from continuous polymerization by variation of the pump flow rate.
All reactions show good linearity indicating that the concentrations of the growing radicals remained approximately constant over the course of reactions with only very small deviations from first order kinetics seen for the highest target molecular weight reaction. A maximum conversion of 80% was reached after a residence time of 20 minutes in this specific reactor and light source combinations, thus relatively comparable yields with the tubular flow reactor.
However, there is a somewhat lower slope, indicating that radical concentrations in both reactions are slightly different. The 15 and 20 min samples show higher relative intensities for double charged species due to their higher Mn which lies above the scan range of 2000 g mol?1.
However, in practically all cases, fast polymerizing acrylates were used to stay within reasonable polymerization times to reach high conversions.
The good applicability of photo-copper-mediated polymerization of MA is demonstrated for both a commercial glass-chip microreactor (volume 19.5 ?L) as well as a simple tubular milli-flow reactor (11 mL).
Both the microflow and the tubular reactors are comparatively cheap and easy to set-up and are thus of potential interest to the whole polymer community as a novel way to produce materials for a broad range of research projects. On the left side of figure, polymer Mw and hr are determined as functions of conversion, and on the right side, particle size distribution and specific surface area from the Mie scattering analysis are shown. Methyl methacrylate and diphenyl sulfide were used as the reactive monomer and nonreactive dopant, respectively. Schematic representation of the equipment for UV-controlled polymerization of plastic rods. Although an excess of terpene favors formation of well-defined macromonomers in organic solution, the characteristic low-vapor pressure of limonene hinders its simple removal (or recycling) via evaporation after synthesis.
However, low average molecular weights in the order of 1200 g mol?1 were obtained.13 Later, Doiuchi et al. To the best of our knowledge, and from a thermosetting film-formation perspective, very little has been published regarding limonene as a direct building-block co-monomer to obtain permanent networks.30,67 Only a few cases presented in the literature make use of either its dioxide analog or DD, as already mentioned.
The difference in double-bond reactivity was ascribed predominantly to a higher relative energy of the tertiary ?-thioether carbon-radical intermediate resulting from thiyl-radical insertion across the endocyclic double-bond; and, partially to steric hindrance effects controlling thiyl radical addition onto the two double bonds.
In spite of the obvious attributes of functionalization, this is an approach that generally entails multistep synthesis reactions, involving numerous precursors and different chemistries, some of them hardly following the principles of green chemistry, alongside with laborious intermediate purifications, also depending on the route of synthesis and physical–chemical features of the desired polymers.
Preliminary syntheses of 4 and 5 were performed under thermal conditions in an inert atmosphere by means of addition of AIBN as an initiator, for a period of 24 hours, using two different starting thiol–ene stoichiometries between co-reactants. Non-ideal synthesis of resins comprises a mixture of all possible combinations (complete or partial).
The synthesis reaction was conducted for a period of 6 hours at room temperature in EtOAc solution. The second one, which was more imaginative, centered on manipulating the molecular weight problem of the polymer.
These special reagents, though expensive, were unique in their effect and occupied a special position in the arena of chemical reactions.
Added to this is the decreasing trend in the cost of the irradiation facility and improvement in the performance of constituent chemicals.
His experiments on cathode rays led to the revolutionary concept of the existence of subatomic particles. Since these sources cause ionization of the medium in which they traverse, they are also known as ionizing radiation.
However, radical scavengers like oxygen should be eliminated from the molecular neighborhood to prevent the removal of the crucial initiating alkyl radicals as alkyl hydroperoxide radicals. However, caution must be exercised with these sources by protecting them with appropriate shields that absorb stray particles and harmful X-rays emanating as secondary radiation. Plants absorb solar radiation, converting it into carbohydrate by combining carbon dioxide and water through photosynthesis. They improved the color density since all the coated material remained on the substrate after curing. It is in this context that the importance of photochemistry becomes significant to ink chemists.
Contributions in the last century have developed photochemistry into a mature science for which it has heavily drawn from other branches like spectroscopy and quantum mechanics. When a covalent bond connecting two atoms, A and B, breaks, the cleavage can take place either by sharing the bond electrons equally or unequally.
In the ground state, oxygen is an exceptional triplet designated by the spectroscopic state 3Greek sigmag- . Finally lasers are also used increasingly in advanced photochemical studies since they offer a convenient, highly collimated, monochromatic, and coherent beam of light.
The UV dose needed is a function of the color of the ink since the pigments may absorb the UV component to different extents. In sensitive cases such as printing on disposable cigarette lamps, the UV flash curing method14 can be adopted. In flash photolysis, an ultrafast light pulse generated from a flash lamp or a laser intercepts the sample that produces transient species, which can be interrogated by another light pulse to analyze the transients. Poly(MA) with average molecular weights between a few hundred and ?5000 g mol?1 was obtained under retention of pristine end group fidelity. Application of (micro or milli) flow conditions to polymer reactions intrinsically allows for simple upscaling of processes and likewise acceleration of reactions in general. However, even though this is favorable from an economic point of view as UV-irradiation is cheaper than thermal heating, UV-induced polymerization is not unproblematic. The capillary voltage, the tube lens offset voltage, and the capillary temperatures were set to 25 V, 120 V, and 275 °C, respectively. Furthermore the irradiation source and reactor were kept in a sealed wooden box for safety reasons.
Polymerization reactions with other target molecular weights were carried out with appropriate adjustment of concentrations. Thus the reactor channel widths play no important role and the polymerization can be applied to micro- and milliflow conditions. Polymerization rates would be significantly higher at slightly elevated temperatures (and due to heat transfer issues and the high exothermicity of the polymerization reactions also most thermal SET-LRP reactions in non-aqueous solvents are carried out significantly above ambient temperature), nevertheless we opted for low reaction temperatures to underpin the photoinitiated character of the reactions. No higher molecular weights have been targeted since this range covers already a broad range of materials typically synthesized via controlled polymerization reactions. Deviation from ideal behavior in controlled polymerization is often observed and the outcome of the reactions can be regarded to be within the usual limits of deviation. Also in good agreement with expectations, slightly higher initial dispersities are observed for increasing monomer to initiator ratios. It may thus be assumed that radical concentrations in all three polymerization reactions were of a similar order. Overall, the polymerization reactions in the micro-flow reactor feature the same characteristics as the in milli-flow reactor. Since the reactor volume and reactor type had been changed and a light source of different intensity was used, this is not surprising. At the same time the novel photo-copper-mediated polymerization process solves the general problem of inhomogeneities that are commonly observed in thermal copper-mediated polymerization reactions in flow devices, thus making the photo-initiated process also from that point of view the first choice for flow synthesis of materials from controlled polymerization. Right: BA polymerization reactions - hr,w as functions of time for reactions with different amount of transfer agent. By contrast, nonreactive DS was fixed more easily in the polymer matrix, leading to a decrease in refractive index at the central axis.
The quality of these results suggests that GRIN lenses can be successfully fabricated via UVcontrolled polymerization to produce GRIN lenses with excellent optical properties.
Chromium-chloride based catalysts provided the best results without any major drop in catalytic activity.
Furthermore, documented work of its combined utilization with free-radical thiol–ene chemistry is practically inexistent within the context of thermosetting polymers. Instead, the reaction system proposed in this work offers the possibility to synthesize partially bio-based (‘semi-synthetic’) crosslinked materials in a simple and environmentally benign fashion via a two-step reaction procedure using only limonene and TMPMP 2 as multifunctional comonomers in the presence of minute amounts of a radical initiator to speed-up the reaction.
Successive peaks in the direction of lower retention volumes represent dimers, trimers, tetramers and other high molecular weight oligomers (in the same order) of the respective multifunctional macromonomers. The dotted line refers to the reference mixture, the solid line is from the TE-A1 product, and the dashed line the spectrum obtained from the TE-A2 product.
The dotted line refers to the reference mixture, the solid line is from the TE-B1 product, and the dashed line the spectrum obtained from the TE-B2 product.
Integration value of the ethyl ester proton signal (c) was used as a reference peak for the calculation of double-bond conversion.
The dotted line refers to the initial reference mixture, the solid line is from the TE-C1 product (resin 1), and the dashed line the spectrum obtained from the TE-C2 product (resin 2). Network formation was accomplished by UV-irradiating liquid thiol–ene samples spread onto glass slides. During this period, chemists depended heavily on the molecular weight aspect of the polymeric materials to fine-tune the performance characteristics of the system.
In this approach, monomers and low-molecular-weight polymers (oligomers) were substituted for the high-molecular-weight ones. This article pertains to the special chemistry induced by radiation and photons, respectively, in the context of inks that have relevance to paints and coatings also. A favored approach to radiation technology is reflected in the popularity of RadTech conferences. The modern understanding of electrons considers it to be a member of the class of atomic particles called leptons, the other important particle being quarks.
Conversely, UV and visible light that do not cause ionization are called nonionizing radiation. These low-energy electrons will dissipate energy over a small volume in the medium that results in little clusters of ionizations and excitations. Otherwise, an induction period will be induced in the polymerization reaction and longer irradiation time (higher doses) will be required for the sustenance of polymerization. The former type of radicals can respond by eliminating a hydrogen atom from a neighboring CH2 group forming a double bond. This is the first link in the great chain involving animals that consume plants, both of them contributing to fossil fuel on their decay. They imparted superior image quality and print quality due to reduced color bleed and improved dot gain. UV inks do not dry in air; hence, they do not tend to plug the cells of the anilox rolls in flexography.
More esoteric applications like printing on CD, and, of late, printing on DVD are also in vogue. As an example, the photodegradation of the laser dye coumarin 120 (C120, 7-amino-4-methyl coumarin) to the hydroxy derivative, 3-hydroxy-7-amino-4-methyl coumarin, proceeds through the participation of singlet oxygen formed by the action of coumarin triplets on ground state oxygen triplets. This state quenches the other triplet excited states producing the two singlet states designated as 1Greek deltag and 1Greek sigmag+.
Usually, one or two flashes from high-intensity flash lamps with a flash duration of 5–10 milliseconds cure the ink to a point to which it can be overprinted. Frontiers of photochemistry are exploring attosecond (10-18S) time scale events with pulsed lasers. Besides homopolymers, block copolymers can also be successfully synthesized and poly(methyl acrylate)-b-poly(butyl acrylate) block copolymers with a similar low dispersity are obtained. At the same time – and this is a distinct advantage of chain reactions – also materials of higher quality and overall yields can be obtained from MRT due to the higher stability that is offered by flow systems combined with the more defined reaction conditions that can be achieved. Both ATRP and SET-LRP yield polymers of a similar structure and the difference between both processes mostly stems from the choice of reaction conditions such as solvent polarity and temperature.
Reaction efficiencies depend largely on the nature of the light source and also on the reactor geometry.
Pictures of the described reactor can be found in the supporting information in the publication of Conradi et al.52 which used a similar reactor setup. Additionally, this presents an even higher motivation, low temperatures help to avoid the formation of midchain radicals, as usually observed in any acrylate polymerization reaction.54,55 Midchain radicals inevitably lead to short chain and long chain polymer branching. Higher molecular weights will lead to increased viscosities, which was avoided in the current study in order to cancel out complex viscosity effects. Thus, increasing the reactor volume by a factor of two with concomitant increase of the flow rate by the same factor leaves the reaction product unaffected, but increases the overall yield of polymer twofold. Additionally, in combination with the linear increase in the average molecular weight and low dispersity of the polymers obtained it may be assumed that only very little transfer and termination events disturb the livingness of the reactions.
Molecular weight evolution and dispersities are in the same range as in the polymerization reactions discussed above (see ESI† for detailed information on molecular weight and dispersity evolution and molecular weight distributions of the milliflow reactions), as shown in Fig.
It must be noted that the fit of the microreactor data indicates that at zero minutes of residence time some polymerization had taken place already (the reactor is fed by light-transparent syringes, thus polymerization could already occur in the feed). The targeted Mn of the block copolymer was 7700 g mol?1 (at full BA conversion) and good control over the second block is also achieved, see Fig. Styrene does not show significant polymerization of the timescale of the flow reactions (we chose 20 minutes as the highest residence time as for the other reactions described in this work). Reactions follow first order kinetics, number average molecular weight increases linearly with monomer conversion and low dispersities are reached for all polymers obtained.
The effects of initiator concentration, the addition of acrylic polymers, and the preparation conditions on the optical characteristics of the GRIN lenses produced by this method were also investigated.
The highest energy exists at the bottom and decreases gradually from the bottom to the top, as indicated by the dotted lines. UV-curing of these polyfunctional resins combined with equal mole compositions of multifunctional alkyl ester 3-mercapto propionates yields highly sticky, amorphous and flexible elastomers with different thermo-mechanical properties.
Our major interest in this simple unmodified monoterpene stems from the idea that the two naturally occurring alkene groups can easily couple to sulfur compounds via a free-radical thiol–ene mechanism and this way be used, for the first time, in the formation of multi-chiral crosslinked poly(thioether) networks which could be of relevance to the field of polymer coatings.9,10 The second reason is related to the rigid structure and bulkiness of the alicyclic ring which may help introduce stiffness to the final network structure in a way similar to well-known stiff aromatics.
Finally, the thermal and viscoelastic properties of the semi-synthetic photocured materials are reported and discussed thoroughly. We also attempt to increase the crosslink density within the final material(s) by introducing a tetra-functional thiol monomer, PETMP 3, and compare the final properties with those obtained from the standard thiol–ene formulation involving the tri-functional thiol. 1–3 obtained from DMF-SEC and FT-Raman measurements illustrate well the kinetic effect of initial thiol–ene stoichiometry on the overall outcome of the reaction. The carbonyl band (CO, 1735 cm?1) was used as an internal reference in the spectral normalization process. The carbonyl moiety (CO, 1735 cm?1) was used as an internal reference band in the spectral normalization process. The carbonyl group (CO, 1735 cm?1) was used as an internal reference band in the spectral normalization process. All photocured thiol–ene films were obtained as clear (amorphous) homogeneous and flexible materials without any discernible thiol odor. Thiol–ene films with incremental mole amounts of thiol functional groups resulted in higher fractions of soluble material trapped inside because once the network is chemically locked at the gel point the remaining unreacted portion becomes occluded into the network. What was needed was a mechanism to initiate the in-situ polymerization of these species that would cure the coating. Two of them may combine, forming a crosslink that results in a high-molecular-weight polymer.


Since the inks are devoid of solvent, there is no need to clean them between press runs; cell can even be left overnight or even through the weekend without cleaning. A variety of substrates such as metal, paper, glass, wood and plastics have been printed with UV ink.
Light absorption also helps in crossing the activation energy barriers associated with chemical reactions. While oxygen is harmful due to its depletive action on the excited state, the reactivity of the singlet oxygen formed from is itself another concern since they can act on many organic compounds. Also, the curing UV dose is dependent on the print thickness: the higher the thickness, the higher the dose needed. The capacitors recharge in less than a second, and the total operating time is about 1 second per flash. Light intensity gradients throughout a batch reactor are unavoidable due to absorption of light.
The OMNICURE system was equipped with a 100 W high pressure mercury vapor short arc lamp (320–500 nm) at an iris setting of 50%. Choosing low reaction temperatures thus allows for synthesis of primarily linear chains with minimum branch points giving additional value to this photo-polymerization process. All polymerization reactions show a linear increase in Mn with respect to conversion and very high conversions were reached in all cases in a relatively short reaction time (maximum 20 minutes of residence time, see discussion below). For the present reactor setup, a production of pMA of ?60 g per day can be realized at a residence time of 20 min.
Additionally, it can be observed that no inhibition period occurs for the polymerization and that already at very low residence times significant polymerization occurs. All ESI-MS spectra display only one single-charged product species, this being the sodium adduct of the expected structure with the initiator group in the ? position and a bromine atom at the ? site, Fig. In contrast, methyl methacrylate could well be polymerized by the protocol, reached, however, only ?35% conversion after 20 minutes (Mn = 2100 g mol?1, D = 1.45). Reactions are particularly fast and conversions in the range of 90% are reached within 20 minutes of reactor residence time. Refractive index distributions and image transmission properties were estimated for all GRIN lenses prepared. These can be further modulated by varying the amount of unreacted thiol occluded within the networks working as a plasticizer. Among the several olefins tested, including styrene, indene, and 5-ethylidene-2-norbornene, limonene resulted to be the most active chain-transfer agent, although an increase in the initial feed concentration resulted in low molecular weight oligomers with reduced polydispersities and exhibiting low Tg's.24 Following a different line, Nakatani et al. Finally, the physicomechanical properties of the film coatings obtained from crossed over thiol–ene resins with the multifunctional thiols, mixed in different stoichiometric levels, are evaluated. However, films with low functionality (M1 + T3) resulted to be much stickier to the glass surface than those with higher functionality (M2 + T4) which were also tack-free. Upon swelling the soluble fraction is released and a reduction in mass is detected gravimetrically. In an atom, the negatively charged electrons are positioned outside the nucleus in orbitals of increasing energies.
Purging the reaction environment with an inert gas like nitrogen would eliminate this problem.
On the other hand, radicals generated from the C-C bond breaking undergo elimination reactions, producing low-molecular-weight fractions.
Other positive attributes are improved rub resistance, chemical resistance and superior gloss. Initially, the photoinitiator absorbs light and generates the initiating species, which acts on the acrylic monomer or oligomer to initiate and propagate the addition polymerization sequences. The printing speed in UV curing may be manipulated by the efficient choice of photoinitiators and their concentrations. Oxygen inhibition problems may be tackled by purging the reaction medium and the immediate vicinity with an inert gas like nitrogen or argon. Due to the abundance in the radical concentration produced in view of the high light intensity discharged by the flash lamp, oxygen quenching is not a problem in this case.
But chemical actinometers are more commonly used in the UV region, the most important one being the ferrioxalate actinometer in which light absorption converts Fe(III) into Fe(II), and the Fe(II) is estimated by a colorimetric method. But, it is going to be an unquestionable alternative for many specific applications where other printing technologies fail to be effective. This feature allows the reaction to be carried out in milli- and also in microflow devices. As a consequence, such reactions can be difficult to scale up and reaction kinetics vary widely with the type of reaction vessel chosen.
The microflow setup is best suited for kinetic studies under very economic conditions, and – due to the very small channel width – can be used with UV-light sources of intermediate power.
The reaction in flow is thus significantly faster compared to all previously reported batch photo-polymerization reactions. Upscaling of the reaction is, as described, simple and can be realized by using longer PFA tubing being wrapped around the light source.
It must be mentioned at this point that the preparation of the reaction solution plays an important role.
Thus, polymerization of such monomers might not be most favorable to produce homopolymers in an efficient manner in the current flow setups. All polymers feature excellent endgroup fidelity and allow for efficient block copolymerization reactions, as demonstrated by the synthesis of a series of pMA-b-pBA materials in the microreactor.
Introduction of a renewable cycloaliphatic structure into the materials offers a convenient way to enhance the glass-transition temperature and stiffness of traditional thiol–ene networks. The numbering terminology of each nucleus used for the assignment of 1H-NMR signals was adopted from a previous reference.9 Characterization by FT-Raman spectroscopy represented qualitatively in Fig. This difference in adhesion capacity may be ascribed to different crosslink densities achieved for the two materials. This was particularly evident from networks resulting from the two intermediate cross-combinations of high and low functionality components, M1 + 4T and M2 + T3, respectively. The simplest means of using heat (thermal curing) did not become popular since relatively high temperatures were necessary to trigger the polymerization process, even in the presence of chemical initiators, which could be detrimental to many substrates. Removal of oxygen is also advantageous since radiation could convert oxygen into hazardous triatomic ozone. Still, properties such as lightfastness and opacity could be retained by choosing the appropriate pigments.
The radical generating step may be exemplified from the instances of free radical formation from benzophenone and a benzoin ether on light absorption. Other parameters include the power of UV lamps, the nature of substrates, and the chemistry of resin components. A molecular approach of incorporating a singlet oxygen scavenger such as tertiary amines covalently bonded to the oligomer backbone has also been attempted. Phototubes calibrated against a thermopile galvanometer system can also be used to measure the absolute light intensity of monochromatic light.
In both cases, equally good control is achieved with only minimal adaptation of the reaction protocol, underpinning the simplicity and fast adaptability of the protocol to different flow reactors. Here we applied a UV-light source with 100 W power, which was, however, not used to full extent and usually attenuated to lower intensities. It should be noted that the rate of reaction may be influenced by the choice of light source and that the rate increase may not only be due to the enhanced light efficiency of the flow reactor, but also due to usage of a more powerful UV-lamp. Of course, natural light is also able to trigger polymerization reactions, thus in principle, a reaction could occur before the solution is injected into the reactor.
3 displays the molecular weight distributions as obtained from SEC, scaled to monomer conversion of the respective samples. With increasing reaction time, more double-charged polymers can be observed, which is associated with the increase in overall chain length of the samples.
For example, at a BA conversion of 51%, a pMA-b-pBA polymer with an Mn = 4990 g mol?1 and a dispersity index of 1.16 is obtained (theoretical Mn at this conversion is 5400 g mol?1).
However, for block copolymerization reactions, where reactions are often stopped prematurely to preserve end group fidelity, such reactions could for the future also be of significant interest. The materials synthesized may be considered potentially useful as sealants and adhesives in a wide variety of applications including organic coatings.
The epoxy polymer, composed of a mixture of stereoisomers and geometric isomers, was cured thermally with AIBN affording a much higher Tg and enhanced physical properties when compared to that of diglycidyl ether of bisphenol A type epoxy resin.26 Mathers et al.
Moreover, from a macromolecular materials synthesis standpoint, it is not absolutely necessary to have the two macromonomers obtained in pure form since the aliphatic ring structure of limonene mediating the two sulfur groups works as a crosslinker unit between propionate ester arms once the two thioether linkages are effectively formed creating a thiol–ene network. 5 indicates a significant reduction of the exocyclic ene band over that of the endocyclic ene with practically no vestigial amounts of thiol detected within the system after synthesis. In general, the higher the crosslink density, the greater the stress build-up during cure,79 even if this occurs via a stepwise growth mechanism (after gelation), resulting in higher contraction energies developed at the film–substrate interface and peeling is facilitated.
Conversely, the low (M1 + T3) and high (M2 + T4) functionality formulations afforded materials with relatively high and very low (or none) sol-content, respectively.
Chemists regard electrons very highly, since their participation is responsible for controlling the chemical reactions.
At relatively high electron energies and large scattering angles, an energy loss mechanism referred to as Bremsstrahlung occurs, which is the light emission due to the deceleration of electrons. Moreover, radiation has the effect of producing crosslinks that can add to the mechanical properties of the coating.
At times, photosensitization reaction can be adopted to suit the requirement of initiators that do not absorb light directly. Another negative aspect of oxygen in the vicinity is the possibility of producing obnoxious ozone by the action of UV light.
UV flow reactors have been shown to be highly efficient and to accelerate several reactions to reach full conversion from reaction times of days to minutes by increasing the light intensity and efficiency.49–52 While gaining more and more popularity in the field of organic chemistry synthesis, no polymerization reactions have, however, to the best of our knowledge, been reported for such lab-scale devices in homogeneous phase, even though reactors are comparatively cheap and easy to handle. Conversely, the tubular reactor is less sophisticated (the reactor itself consists of transparent PFA tubing wrapped around an immersion well photo-reactor), but allows synthesis of materials at a significant scale due to the much higher internal volume. For the reactions described herein this is prevented by using only dark containers, minimizing light contact of the substances as much as possible.
A clear shift of the distributions – also on the low-molecular weight side – is observed, which nicely underpins that the reaction is easily applicable to different reactor setups and is thus highly interesting also for larger flow reactor systems in which kilogram (or higher) production of polymers can be achieved.
Even though traces of side products may be seen in the baseline of the spectra, the absence of any significant amount of termination products (as given in the table for one of the disproportionation species) is clearly seen. Block copolymerization reactions can thus also easily be addressed in flow reactions, again underpinning the versatility of the process, as well as the high livingness of the polymerization reactions. The versatility of UV-irradiation over thermal initiation makes this method particularly suitable for green industrial synthesis processes via thiol–ene chemistry using limonene and multifunctional thiols. An increase in functionality of the reacting system leads to a higher number of chemical crosslinks per unit volume of material and, therefore, the chances to extract any fraction of unreacted material occluded are lower. This is in addition to the cross linking effect caused by the polymerization of multifunctional monomers and oligomers.
But the inks for this purpose should be specially designed to match with the high light flux of the flash. In MRT flow reactors, channel widths and thus optical path-lengths are very small, allowing for very reproducible and easily upscalable reaction conditions.
For the tubular reactor, a UV-light source with 400 W was chosen to achieve fast polymerization reactions.
Tests show that directly after deoxygenation of the solution (and thus right before injection into the reactor) no reaction has yet taken place as no polymerization occurs in the dark.
The mass-spectrometric analysis thus nicely confirms the above made observation of excellent livingness of the process. The thiol–ene system evaluated herein serves as a model example for the sustainable incorporation of natural diolefinic monomers into semi-synthetic thiol–ene networks exhibiting a range of thermo-mechanical properties.
The course of the reactions was monitored via1H-NMR and FT-Raman spectroscopy by following the disappearance of individual double bonds and thiol functional groups, respectively. Indeed, it was observed that networks having the highest functionality will reach full crosslinking upon UV-cure regardless of the amount of thiol used and practically no leaching of the unbound fraction was detected. However, the degrading effect of radiation on the other constituents of the ink, especially the color-imparting pigments, should be considered. Employing UV-flow reactors does not only allow for continuous production of materials, but also for an increased efficiency of the reactions. The peak wavelength in both cases was ? = 365 nm, whereby the irradiation spectrum was relatively broad. More recently, Meier and co-workers (2012) demonstrated the efficient thiol–ene coupling of cysteine hydrochloride to R-(+)- and S-(?)-limonene in the preparation of new diamine functionalized renewable monomers which served as platform precursors for polyamide and polyurethane synthesis.10 In an early report, the authors developed optimal experimental conditions for the selective functionalization of the terminal vinylidene group with methyl thioglycolate as a model compound further enabling the synthesis of a diversity of monomer precursors and ensuing polymers. After solvent evaporation, the two resins were obtained as clear, faintly yellow, highly viscous liquids exhibiting a fresh citreous odor note. In contrast, materials having the lowest average number functionality showed much higher tendency to leach out upon swelling in a very good solvent, such as THF, even if full thiol–ene cure is achieved with respect to the limiting enes. The electrons that are generally obtained by heating a metallic cathode in a vacuum chamber may be accelerated by two techniques.
Since residence times are usually short in micro- and milliflow reactors, interfering reactions stemming from degradation of materials from extended UV-illumination are generally avoided, thus leading to less side products and overall increased product quality.
It should be noted that in the case of the microreactor, a conventional glass chip was used, thus optical transparency is limited at wavelengths below approximately 350 nm. UV-irradiation may result in several side products, ranging from self-initiated chains over crosslinking to polymer degradation. Direct polymerization of limonene with dithiols was also reported in this study.57 In another paper the authors also successfully described the attachment of alcohol and ester functionalized thiols to the two enantiomers of limonene via thiol–ene coupling affording monofunctional, homo-difunctional and hetero-difunctional terpene-modified monomers. It is remarkable the differences in extraction capacity observed between systems with and without the presence of one extra propionate ester arm (on average)! In the first one, the accelerating force, which is an electric field is derived from an electrostatic potential.
For the present case study, we used methyl acrylate as a monomer and dimethyl sulfoxide as a solvent as good results were reported for this combination in batch processing. The practical absence of all such product species confirms that the flow conditions only favor the desired product, but do not cause other processes as are often observed for UV-initiated batch reactions at similar high light intensities. The difunctional addition products were subsequently homo- and co-polymerized with short-chain diols using cyclo guanidine 1,5,7-triazabicyclododecene (TBD) as the polycondensation organocatalyst because of its high transesterification activity.
Dispersities of the first three individual bands corresponding to the oligomers took unit values although moving in the direction of low elution volumes an increasing loss of resolution (separation capacity) is verified. Statistically, the probability for di-coupling reactions between macromonomers and multifunctional thiols having the same average number functionality in reactive groups is higher for the combination M1 + T3 than for the combination M2 + T4 which dramatically reduces the possibility for higher order coupling reactions necessary for the creation of a large macromolecular cluster.
In the second one, the required electric field is provided by the electric field component of electromagnetic radiation3,4 that usually lies in the radio frequency or microwave region.
It should thereby be noted that also some samples taken at even higher conversions were analyzed with virtually the same result. The same strategy was employed to synthesize high molecular weight polyesters (from 9 up to 25 kDa) based on limonene and fatty-acid methyl ester derivatives.9 These three recent reports illustrate the extreme versatility of the thiol–ene reaction in the design of new linear polymers issued from limonene in combination with distinct classes of renewable monomers (natural or modified) and chemistries. Therefore, the chances for having a higher fraction of free or partially attached material within the network are higher for the first case as illustrated in Scheme 5.
The Van de Graff generator and Linear Accelerator (linac) respectively work on these principles. While the exact mechanism has not yet been elucidated, photo-activation of the Me6TREN ligand followed by carbon–halogen bond breakage appears to be the dominant mechanism for chain initiation. This indicates that approximately 68.6% of the overall synthesis mixtures were constituted by primary (mono-addition) and secondary (di-addition) coupling products (complete and partial oligomers).
These results confirm the impact of the system co-functionality on the crosslinking density achieved after cure.
In 25% of individual macromonomers 4 or 5, about 21% results from coupling at the exo-unsaturation and only ?4.6% from coupling at the endo-unsaturation. One should note, however, that even in the worst case situation the fraction of unbound material is always lower than 25%.
But in the actual ink scenario, much lower electron energies are used, typically in the range of 125–150 keV (kilo electron volt).
This indicates, at one instance, the importance of having good control over the composition mole ratio in the formulation of pure stoichiometric thiol–ene networks; and, in another instance, the properties of the final thermosets can be further modulated by adding small incremental amounts of thiol acting as a plasticizer. Such an energy range may be attained by a single gap accelerator technology,5 where the beam current can be several milliamperes.
The fact that resin 2 yielded a slightly higher dispersity index over resin 1 associated with a much broader molecular weight distribution curve is very likely to be attributed to the influence of an extra mercapto propionate ester arm which increases the probability for high order coupling combinations in the first synthesis step.
This effect is obviously translated into development of much higher viscosity for resin 2 when compared to resin 1 observed during and after synthesis.
All these results demonstrate the effectiveness of the free-radical thiol–ene reaction in the preparation of multifunctional polydisperse macromonomer resins based on limonene suitable for the synthesis of polymeric thermoset materials.
In a rare gas-discharge lamp, a rare gas like xenon is used, which gives a useful spectral range very similar to that of the sun.



Does glass ionomer bond to enamel naturally
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Comments Uv polymerization of methyl methacrylate msds

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