Rate of Photoinitiated Polymerization

Figure 8. Dependence of the rate of photoinitiated polymerization on the free energy for photo-induced electron transfer from borate to the excited state of cyanine dyes, listed in Table 2.

The same trend is observed for the rate of photoinitiated polymerization (Figure 22). This suggests that ... 

Gosh and Gosh [105] studied photoinitiated polymerization of methyl methacrylate initiated by the BP-TV,A-dimethylaniline couple, and Clarke and Shanks [106] tested the influence of a variety of amines on benzophenone-initiated polymerization. That amino radicals resulted during the initiation the polymerization by benzophenone-tertiary aromatic amines was shown by Li through the use of ESR and spin-trapping methods [107]. It was shown that the rate of photoinitiated polymerization depends on the structure of the amine. More recently [108] benzophenone-tertiary aromatic amines were studied as initiators of the free-radical polymerization of polyol acrylates. Illustrative kinetic curves recorded during photoinitiated polymerization of TMPTA are shown in Figure 23. 

A variety of amines (Table 8) and various xanthene dyes (Figure 25) were tested as visible-light photoinitiators of free-radical polymerization [108, 121]. The rate of photoinitiated polymerization depends on the type of amine used as the electron donor (Figure 26) [122, 123]. 

Photopolymerization experiments were carried out to compare the efficiency of the photoinitiated polymerization as a function of the SCCA structures. The rates of polymerization determined from the efficiencies of the photopolymerization after 6 min of irradiation are also presented in Table 11. A reference sample containing CB and other components, but without any co-initiator, did not show any polymerization. The results in Table 11 show that the rate of photoinitiated polymerization depends on the SCCA used. 

There is another interesting feature of Eqs. (54) and (55). They also show that the rate of photoinitiated polymerization is proportional to the square root of the quantum yield of free-radical formation (<1>r )- It is evident from Scheme 21 that there are two types of processes that could initiate polymerization. The quantum yield of a-CBR radical production can be taken to be equal to the quantum yield of CO2. The quantum yield of a-SR radical could be estimated by... 

As one can see in Figure 2 4, kinetic curves of polymerization in the presence of SGS have typical S-like shape. However, the rate of photoinitiated polymerization of diaciylate composition with increase of SGS content decreases as compared with initial polymerizing composition. Maximal rate of polymerization w at SGS content of 70% V. diminishes approximately by 2 times in comparion with w for initial composition, whereas, time of achievement of maximal rate increases by 2 times. Possible explanation of this fact may be that an inorganic constituent forms steric limitations for the process of polymerization of diaciylate monomer. An additional spatial network of nanoparticles of silica phase which appears as a result of sol-gel process leads to macroradicals decay and accordingly, to deceleration ofpolymerization process. 

When results are compared for polymerization experiments carried out at different frequencies of blinking, it is found that the rate depends on that frequency. To see how this comes about, we must examine the variation of radical concentration under non-stationary-state conditions. This consideration dictates the choice of photoinitiated polymerization, since in the latter it is almost possible to turn on or off—with the blink of a light—the source of free radicals. The qualifying almost in the previous sentence is actually the focus of our attention, since a short but finite amount of time is required for the radical concentration to reach [M-] and a short but finite amount of time is required for it to drop back to zero after the light goes out. 

Independent estimates of these quantities can be obtained from the temperature coefficients of photopolymerization. If the rate of photoinitiation is assumed to be independent of the temperature, the increase in rate must be due entirely to the change in kp/k] (see Eq. 13), hence the slope of the plot of log Rp against 1/T for the photochemical polymerization should yield Ep — Et/2. Burnett reported the value 5.5 kcal./mole for styrene, and Burnett and Melville found 4.4 kcal./mole for vinyl acetate, in satisfactory agreement with the values given above. 

For processes in which the rate of polymerization is not limited by the rate of electron transfer, the equation describing the rate of polymerization, one can obtain during the analysis of simple kinetics a scheme of photoinitiated polymerization. A mechanism describing a photoinitiated polymerization via PET (not considering the kinetics of free-radical formation, because this process does not affect the rates of polymerization), that contains all the major reaction steps, can be represented by Eqs. (34)-(39). 

The results of these photochemical studies form guidelines for the choice of sensitizers, onium salts and other additives potentially useful in the cationic curing of coatings. The sensitized photochemistry of diphenyliodonium hexafluoroarsenate and triphenylsulfonium hexaflurorarsenate was investigated at 366 nm. Product quantum yields are compared to relative rates of photoinitiated cationic polymerization of an epoxy resin. 

In photoinitiated polymerization it is possible to commence the generation of radicals abruptly by exposure of the polymerization cell to the light source, and the time required for temperature equilibration in an otherwise initiated polymerization can be avoided. The rate of photoinitiation is given by Eq. (6.77) and Eq. (6.100) then becomes... 

As discussed in a previous section, a number of studies have been conducted to increase the rate of cationic polymerization of epoxides. In curing applications, polymerization should be rapid enough for high output of production. In a recent work, the effect of addition of tetraethylene glycol (TEG) or polyEPB on the rate of photoinitiated cationic polymerization of CY179, limonene dioxide (LDO), and 1,2,7,8-diepoxyoctane (DEO) has been investigated [150]. These hydroxyl containing additives were shown to obviously accelerate the polymerization, increase the total epoxide conversion and decrease the induction period. 

The advantage of photoinitiation lies in that the generation of radicals can be commenced (or stopped) instantly by turning on (or off) light to the polymerization cell and, moreover, time needed for temperature equilibration (when thermal initiators are used) can be avoided. Using Eq. (6.61) for the rate of photoinitiation, Eq. (6.79) becomes... 

Polyamic acids are useful resists especially when containing 2,2 -dinitrodi-phenylmethane segments, while a Ti sapphire laser has been found to be effective for 3D curing and microfabrication. On a theoretical note, a direct correlation has been found between the calculated Boltzmann-averaged dipole moment and the measured maximum rate of photoinitiated radical polymerization of acrylic monomers. ... 

Effect of Temperature and Photoinitiator Anion. The polymerization temperature has a significant effect on both the rate of polymerization and the final limiting conversion. For example. Figure 9 contains a series of plots of reaction rate as a function of time for photopolymerization of an epoxide monomer. This figure illustrates that as the temperature is increased, the peak reaction rate increases and the reaction time decreases. The increase in reaction rate with increasing temperature ultimately arises from the effect of temperature on the propagation rate constant (which increases with increasing temperature as described by the Arrhenius equation for the rate constants). The rate of photoinitiation is... 

In addition to the control which can be exercised over photoinitiated cationic polymerizations by manipulation of the structures of the diaryliodonium and triarylsulfonium salts, there are a number of additional factors which also influence these polymerizations. First, the emission spectrum of the irradiation source must be matched as closely as possible to the absorption characteristics of the specific photoinitiator. Fortunately, today there are available commercial light sources which provide intense bands in specific areas of any portion of the ultraviolet spectrum. Since the rate of photolysis of the photoinitiators varies as the first power of the light intensity, a simple doubling of the light intensity doubles the rate of photolysis of the photoinitiator. In practice it has also been observed that the rate of the polymerization of epoxy containing monomers is also doubled by a two fold increase in the light intensity... 

The dependence of the viscoelastic properties of IPNs based on PU and PUA on the segregation degree was shown in [88,256,257]. Taking into account the interconnection between the chemical kinetics and the kinetics of phase separation, various methods of IPN synthesis have been used, i.e., simultaneous and sequential methods. It is known that oHgourethane acrylate (OUA) in the presence of photoinitiators polymerizes at a high rate at room temperature, the reaction rate being much higher than that... 

Note that the initiator decomposition makes the largest contribution to E therefore photoinitiated processes display a considerably lower temperature dependence for the rate of polymerization. 

Local excitation was also studied for primary and secondary amines under irradiation at 313 nm. The results are summarized in Table 11. In order to estimate the photoinitiating efficiency of the amines, the measurement was performed at a chosen constant absorbance (0.40) of the reaction mixture. The rates of polymerization were found to be in the following order ... 

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