Utilizing time-resolved IR to obtain photoassisted autoxidation profiles
irradiation of bicomponent resins induced a continuous decrease of the preoccupation band at 3010 cm−1, which is assigned to the mono/biallelic H of non-conjugated double bonds 8. The change in this ring reflects the curing rate. The conversion ( α ) was calculated using the equation : where A 0 is the optical density before radiotherapy and A deoxythymidine monophosphate is the optical density at vulnerability time t. The polymerization rate, R p, is derived from the conversion and calculated using the equation : The resultant role is shown in Fig. 2. Part a shows a plat of α versus t for four polymerizing systems. The reactivity of the monocomponent ITX ( marked as AlkydITX ) is obviously high ; the conversion of the formulation is drastically increased before 500 second, at corresponding to a conversion of approximately 30 %. After this, the conversion increases in a very smooth means as it approaches the final prize ( approximately 50 % ). The sum conversion crook shows a characteristic skew “ L ” determine. This indicates that ITX is most effective at the begin of the reaction. The monocomponent Co salt ( marked as AlkydCo ) shows a marked induction time period with a duration of 200 second in the early stage but peculiarly high activity in the final stage and the concluding conversion is over 80 %. The condition of the conversion curve is approximately half-arched. For the bicomponent system ( marked as AlkydCoITX ), ITX significantly increases the rate of polymerization without an induction period .Figure 2 Polymerization profile based on the bicomponent formulation, light volume : 40 mW·cm−2.
( a ) conversion ( α) vs. exposure time ( t ) ; ( b ). polymerization rate ( R p ) vs. conversion ( α ). Full size image In part b, the R p is plotted as a function of α. The plots show two energizing behaviors. The first is the plot of the AlkydITX resembles a diagonal-like occupation from the upper left to lower right ; i.e., the chemical reaction rate decreases dramatically with the conversion. The trace of the AlkydITX is unimodal and narrowed, which is similar to the demeanor of autoacceleration in the UV-curing of propenoate 9. The arch of the AlkydCo arrangement shows the quasi-symmetrical class of a shallow arch. For the AlkydCoITX system, the R p volt. α bend is similar to that of the AlkydITX system when α is less than 30 %, but the curve of AlkydCoITX is higher. The increase is caused by the high mobility of primary radicals, obviously induced by ITX. But when α is over 30 %, the behavior of AlkydCoITX is like to that of AlkydCo. According to a report on the autoxidation by E. Bouwman, pseudo -first-order kinetics is a common phenomenon in autoxidation initiated by Cobalt salt drier 10. indeed a conversion of 30 % can be considered a boundary line. The AlkydCoITX exhibits unexpected demeanor of non- pseudo -first-order kinetics before the boundary line because the two traces do not fit well together ; it follows the mechanism of photopolymerization initiated by photoinitiator at this prison term. then AlkydCoITX exhibits the common characteristics of pseudo -first-order after the boundary line as the two traces are coincident with each early .
Utilizing the linear free energy relationship to predict the ability of hydrogen abstraction
It is long-familiar that spectroscopy, such as UV/Vis, phosphorescence and fluorescence, is a means of studying excite states in molecules. Phosphorescence spectroscopy is more applicable to investigate the hydrogen abstraction reaction occurring as a result of the trio express of ITX. however, there are many difficulties hindering the broad application of phosphorescence probing techniques in industrial environments, since this technique is very sensitive to operating conditions. For case, in ordain to suppress non-radiative processes and increase quantum yields, one of the common procedures is to run luminescence measurements in frigid solutions and at cryogenic temperatures ( 77 K ). To avoid these difficulties, we resorted to UV/Vis spectroscopy, to evaluate the ability of hydrogen abstraction of Type II photoinitiator. Our overture is based on a linear detached energy kinship, which is intended to reflect the structure-property relationships of photoinitiators. The Kamlet-Taft expression has been found to be very successful for this function 11. It is shown below : v soap and v 0 are the solvent-dependent physicochemical properties in a given solvent or reference point solution, respectively. They may be the energy value ( e.g. E a ) or apparitional data ( e.g. λmax ), etc. that we were matter to in. π * is the dipolarity/polarizability, α describes the hydrogen-bond donate ( HBD ) ability, and β is the hydrogen-bond accept ( HBA ) ability. The parameters α, β, and π * were obtained from the literature 12, with a, b-complex vitamin, and randomness, respectively, as their coefficients. The UV/Vis assimilation spectrum around the solvatochromic UV/Vis bands ( the longest-wavelength band of the π-π* transition ) of photoinitiators were measured in several common solvents at room temperature, as shown in board 1. The results from multiple squarely analyses of the ν soap values of the photoinitiator measured in the solvents, with the Kamlet-Taft solvent parameters, are summarized in table 2. THF is used as a character ( α = 0 ). For details, the proofreader is referred to Stefan Spange ’ s literature 13. The correlations statistically provide a solid base to understand the manifold solvent effects on the varying responsiveness of solute molecules. adjacent, we tried to predict the ability of hydrogen abstraction by applying this method. The rationale for this set about is as follows : in the alkyd resin, the active group initiating dry is the diallyl group from linoleic acerb and its esters, as mentioned before. The BDE of bis -allylic C-H in lipid has been reported to be lower. In a sense, alkyd monomers and Type II photoinitiators can be identified as the hydrogen donors and as the hydrogen acceptors, respectively. thus, the β can reflect the ability of hydrogen abstraction in a qualitative or quantitative room if the formation march of hydrogen-bonds is considered a part of the hydrogen transfer reaction. HBA and HBD are assumed to be the precursors of hydrogenated and dehydrogenated products. In general, the maximum vertex in the UV spectrum is induced by the excitation from the ground state to the arouse state. As a matter of fact, the across-the-board spectrum is composed of many separate electronic transitions. however, the maximal concentration point ( λ max ) was still selected for appliance. Since the reaction and property of organic compounds are frequently influenced by the solvent, in rate to simulate the H-donor condition, many alcohol solvents were selected for this experiment. We hope the influence of solution is about like the H-donor, i.e. alkyd monomers. The parent compound of ITX, thioxanthone ( TX ), was besides used to for comparison .Table 1 UV/Vis absorption maxima for photoinitiators in several solvents of different polarity and hydrogen bond ability. Full size tableTable 2 Values of the solvent-independent correlation coefficients according to the Kamlet-Taft equation. Full size mesa Comparing the two obtained equations, the bel value for ITX ( −0.590 ) is significantly lower than that for TX ( −0.403 ). As stated previously, β represents the HBA, despite the fact that there is no hydrogen bind between them. The results show that ITX is predicted to perform worse than TX based on its higher b-complex vitamin value. Two b values are negative numbers. This means both photoinitiators are characterized by hydrogen abstraction .
Utilizing transient laser technology to detect quenching rate constants
Quenching rate constants, k q, can be considered to reflect the interaction between an agitate molecule ( photoinitiator ) and a quencher ( alkyd monomer ). Laser flash photolysis is used to measure k q in order to investigate the photochemical summons of 3ITX/3TX with ethyl linoleate as a quencher. The typical approach is to carry out a study of the lifetimes of 3ITX/3TX in the presence and absence of monomers. The worldly behavior was recorded with the decay signal of the photoinitiator, F, versus clock time, t, after the end of the laser pulse. The life ( τ ) will decrease in the presence of a quencher if dynamic quench is the mechanism that involved. then, a energizing psychoanalysis yielded the quenching rate constants, based on the determine of the alkyd monomer concentration, Q, on the disintegrate sign of a photoinitiator. The Stern-Volmer analysis in a dynamic quench reaction was used to determine k q, as shown in Fig. 3a .Figure 3 Results of the time-resolved laser flash photolysis experiment.
( a ) Stern-Volmer plot for the snuff out of 3TX and 3TX in CH2Cl2. ( b ) Decay curve of 3TX quenched by ethyl linolenate as a descriptive model. Full size trope Q is the concentration of the quencher, τ 0 and τ are the lifetimes of the photoinitiator in the absence and presence, respectively, of the quencher. The spectrum after the laser pulse shows the decay in the signal of the photoinitiator at 700 nm. part b shows how the Stern-Volmer plot varies with the ethyl linolenate used as a quencher ; the bespeak follows clean biexponential kinetics on the microsecond scale. The values of k q from the Stern-Volmer calculation are 3.74 × 106 M−1.s−1 for TX and 3.65 × 106 M−1.s−1 for ITX respectively. The result shows that k q was alike or higher to those observed previously for benzoyl, phosphinoyl, or ketyl radicals ( 105–107 M−1.s−1 ) 14, 15, indicating effective interaction .
Utilizing quantum chemistry to analyze the thermodynamic relation and kinetic process
In an attempt to probe the mechanism of the reaction, we began to explore the mechanistic details using quantum chemistry. Because of the miss of electron correlation, the energies from Hartree-Fock ( HF ) calculations are always greater than the exact energy. The disadvantage of semiempirical calculations is that the results can be argument dependant and erratic. thus fewer properties can be predicted faithfully although they are much faster than ab initio calculations. For Density Functional Theory ( DFT ), the energy of a atom can be determined from the electron density. This results in faster calculations than HF and gives more accurate results at the lapp time. The B3LYP hybrid running with 6–31 G ( d ) basis set has been successfully used on a similar system contained a photoinitiator 16, 17. consequently, we calculated the transition submit ( TS ) of ITX compared to TX in the reactions with two compounds, methyl linoleate ( MLO ) and methyl oleate ( MO ) as model compounds of alkyd resins ( MLOR and MOR are their match radicals after dehydrogenation ). figure 4 shows the procedure, and the results of the calculations and their correspond symbols are summarized in postpone 3. The activation energies ( E a ) and reaction enthalpies ( ΔH ) of the four reactions and two thermoneutral reactions ( MLO + MLOR and MO + MOR ) were obtained from the results of optimization. Two thermoneutral reactions were used to calculate the Polanyi factor ( α ), which is obtained by the Evans-Polanyi kinship, and the Wigner tunnel factors is expressed by the follow equations 18. The kinetic results are shown in the same table .Table 3 The result of quantum chemistry of the four reactions (MLO + TX/ITX and MO + TX/ITX) and two thermoneutral reactions (MLO + MLOR and MO + MOR). Full size tableFigure 4 The proposed mechanism for photoassisted autoxidation. Full size persona where α is Polanyi factor, C is the activation energy of the thermoneutral reaction ( MLO + MLOR and MO + MOR ) respectively. κ( T ) is Wigner tunneling factor. ω ≠ is the fanciful frequency of the conversion state of matter. T is the temperature. k boron and h are the Boltzmann constant and Planck constant, respectively. We found that the structure of unsaturated monomer has a dramatic effect on the activation barrier in hydrogen abstractedness reaction 19. MO is known as a mono-ene. MLO is a especial isolate diene because it has a methylene group group between the two doubly bonds, therefore MLO is besides classified as a jump diene. This is the shortest interval between two double bonds. The result shows that the hydrogen abstractedness reaction had a lower likely energy when MLO was the reactant, and the equate rate constants were closely two orders of magnitude bigger than that of MO. This suggests that dehydrogenation from a hop diene was a lot easier. Comparing the data of two thermoneutral reactions between monomers, we found that the chemical reaction of MLO had a lower energizing barrier than MO besides. however, the determine of photoinitiators was relatively little as their rate constants are of the lapp magnitude ( e.g. ~10−15 for ITX ). The electron-donating group in ITX can not decrease the E a. Both E a values of TX are rebuff smaller than that of ITX. The solution of calculation is close to the previous prediction from linear free energy relationship. The mean of α is ambiguous, but it is represented as a transfer coefficient. however, according to the empiric results, it ranges from 0.2 to 0.6 when Δ H lies in the range of −40 to 0 kcal/mol for an exothermic atom transfer reaction 20. On this footing, the results of MLO as H-donor fall within the setting of the α value while the results of MO do not accord with this rule.
The Wigner tunneling factor can be easily obtained and only requires the complex number frequency of the transition department of state as the input signal parameter. This agent reflects the human body of the TS wind : polish or steep. We found that the effect of the monomer on the tunnel factor is very obvious. In other words, the bigger the tunnel factor is, the exorbitant the TS wind and the higher the activation barrier. however, this was not relevant for the thermoneutral reaction. Compared with other hydrogen abstraction reactions ( for example, the value of a benzoyl radical reacting with a cyclohexadiene or a 2,5-heptadiene is 3.55 or 3.32 ) 18, tunneling factors in trio express systems are much smaller. The analysis is shown later in the discussion. From the above discussion, it is besides suggested that the alkyd that contained a cut diene group was a better monomer than the alkyd that had one double over shackle. Based on the above results, the proposed mechanism for photooxidation is summarized in Fig. 4 ( the effect of Co is ignored ). From studies of polymerizations initiated by the combination of a hydrogen abstractor with a cobalt complex, conclusions about the reaction mechanism of these autoxidations can be drawn, specially the character of the photoinitiator : ITX is excited to 1ITX, then 3ITX after irradiation, which abstracts a bis -allylic hydrogen atom from the polyunsaturated lipids, and pentadienyl radicals are yielded. These pentadienyl radicals react with oxygen, and then alkyl hydroperoxides are generated quickly. This access is represented as free-radical-promoted autoxidation. This appears to reduce the doorsill for the Co salts so that it reacts more easily with the alkyd monomer. The curing reaction quickly passes around the hydrogen abstraction stage, which is considered a potent obstacle in autoxidation .