Rates photoinitiation

These results point to two processes, premature radical chain termination and film shrinkage, which compete in determining the ultimate polymerization conversion efficient of multifunctional acrylates. It is obvious that critical attention must be paid to the pulse repetition rate, photoinitiator concentration, and acrylate functionality in developing any photopolymerizable system for laser-initiated polymerization. Future publications on laser-initiated polymerization of multifunctional acrylates will deal with monomer extraction of partially polymerized films, mechanical properties of laser polymerized films, and the Idnetics of single-pulsed systems. 

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. 

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. 

The second type of photoinitiators, ie, those that undergo electron transfer followed by proton transfer to give free-radical species, proceed as follows, where is the rate constant for intersystem crossing. 

When free-radical initiation is used, cocatalysts, eg, phosphites (112), and uv photoinitiators such as acetophenone derivatives (113) can be used to increase the rate and conversion of the olefins to the desired mercaptans. 

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 ... 

The rate of formation of radical from the photoinitiator molecule (the only light-absorbing molecule in the system) Vr- is generally given by Eq. (4) ... 

Photoinitiation is an excellent method for studying the pre- and posteffects of free radical polymerization, and from the ratio of the specific rate constant (kx) in non-steady-state conditions, together with steady-state kinetics, the absolute values of propagation (kp) and termination (k,) rate constants for radical polymerization can be obtained. 

Studies in the photoinitiation of polymerization by transition metal chelates probably stem from the original observations of Bamford and Ferrar [33]. These workers have shown that Mn(III) tris-(acety]acetonate) (Mn(a-cac)3) and Mn (III) tris-(l,l,l-trifluoroacetyl acetonate) (Mn(facac)3) can photosensitize the free radical polymerization of MMA and styrene (in bulk and in solution) when irradiated with light of A = 365 at 25°C and also abstract hydrogen atom from hydrocarbon solvents in the absence of monomer. The initiation of polymerization is not dependant on the nature of the monomer and the rate of photodecomposition of Mn(acac)3 exceeds the rate of initiation and the initiation species is the acac radical. The mechanism shown in Scheme (14) is proposed according to the kinetics and spectral observations ... 

Clearly, unless monomer is the intended photoinitiator, it is important to choose an initiator that absorbs in a region of the UV-visible spectrum clear from the absorptions of monomer and other components of the polymerization medium. Ideally, one should choose a monochromatic light source that, is specific for the chromophorc of the photoinitiator or photosensitizer. It is also important in many experiments that the total amount of light absorbed by the sample is small. Otherwise the rate of initiation will vary with the depth of light penetration into the sample. 

If the rate of addition to monomer is low, primary radical termination may achieve greater importance. For example, in photoinitiation by the benzoin ether 12 both a fast initiating species (13, high k) and a slow initiating species (14, low... 

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. 

In order to formulate an answer to the obviously important question of the length of this interval of acceleration and to ascertain under what conditions it may be long enough to observe experimentally, we shall examine the non-steady-state interval from the point of view of reaction kinetics. Let us suppose, however, that the polymerization is photoinitiated, with or without the aid of a sensitizer. It is then possible to commence the generation of radicals abruptly by exposure of the polymerization cell to the active radiation (usually in the near ultraviolet), and the considerable period required for temperature equilibration in an otherwise initiated polymerization can be avoided. Then the rate of generation of radicals (see p. 114) will be 2//a s, and the rate of their destruction 2kt [M ]. Hence... 

The net result is that the rate of polymerization is lower by about a factor of 10, when polysilanes are compared with conventional photoinitiators such as benzoin methyl ether. 

Consequently conventional antioxidant mechanisms must be expected to protect against photo-oxidation. Thus hydroperoxide decomposition to inert molecular products will reduce the rate of photoinitiation and scavenging of any of the free radical species will be beneficial, although the effectiveness of conventional antioxidants in photo-oxidations is limited by their own stability and the photo-sensitizing propensity of their products (3,). 

The crosslinking of such types of silicones can be described by means of a polymerization reaction. The reaction rate (rP) of this process is a function of the light intensity, the exposure time, the acrylate content, the molecular weight of the uncrosslinked silicone, the photoinitiator and also of the oxygen content of the system. A typical reaction rate/time profile is shown in the Fig. 1. 

Calorimetric results demonstrate that the chain process is inhibited and terminated by oxygen. The inhibition period depends on oxygen, the light intensity and the type of photoinitiator. The measured values vary from 40 to 11 sec (variation of the light intensity (I0 = 4.15. .. 1.0 mW/cm2), p(air) = 1000 mbar), from 40 to 7 sec (variation of the air pressure (p(air) = 1000. .. 6 mbar, Ie = 1.0 mW/cm2)), and from 3 to 30 sec (variation of the initiator). Using values of the inhibition time and reaction rate one can estimate the relative efficiency of several radicals in the chain process. 

Complexes (181)-(183) may also be used to polymerize acrylates449 and methacrylonitrile450 in a living manner, although (181) again requires photoinitiation. Acrylates such as BuA polymerize faster than methacrylates. The rate of propagation of methacrylonitrile is much slower than methacrylates, although in the presence of (185), 100 equivalents are consumed within 3 hours. 

Various industrial pilot plants and full-scale operations, using radiation-chemical processing have been reported, with production rates -50 to -1000 tons per year (Spinks and Woods, 1990 Chutny and Kucera, 1974). Production rates less than -50 tons per year are not considered viable. These operations are or have been conducted in countries such as the United States, the former U.S.S.R., Japan, and France. However, some operations have also been reported in the former Czechoslovakia and Romania, especially in connection with petroleum industry. In the United States, chlorination of benzene to gammexane (hexachlorocyclohexane) was hotly pursued at one time by radiation or photoinitiation. Since the early seventies the activity has dwindled, presumably due to lack of demand and environmental considerations. 

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