Rates photoinitiated polymerization

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. 

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 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. 

Difunctional vinvl ether/difunctional N-maleimide. Up until this point, our results have centered on the reactivity of monofunctional maleimide divinyl ether mixtures. From Kloosterboer s26 work for acrylate polymerization, it is known that the rate of polymerization of a free-radical process is increased dramatically as the functionality of the acrylate is increased. In order to enhance the polymerization rates of maleimide divinyl ether systems, it was decided to synthesize difimctional maleimides for copolymerization with difunctional vinyl ethers. The results in Table V indicate that the photoinitiated TTDBM [bismaleimide made from maleic anhydride and 4,7,10-... 

Furthermore, the wide range of polymerization rates is controlled by the photoinitiation conditions. Specifically, the initiator concentration and incident light intensity control the rate of polymerization and, therefore, the rate of heat released upon curing. These conditions can be conveniently altered for in vivo applications to minimize local tissue necrosis from the... 

The above expression indicates that the rate of polymerization is proportional to the square root of both the incident light intensity and the photoinitiator concentration. This expression is accurate to within 2.5% for thin films where e A]b < 0.2. As the thickness of the sample is increased, the light intensity can decrease appreciably across the sample, and Eq. (4) must be used. 

Photoinitiated free radical polymerization is a typical chain reaction. Oster and Nang (8) and Ledwith (9) have described the kinetics and the mechanisms for such photopolymerization reactions. The rate of polymerization depends on the intensity of incident light (/ ), the quantum yield for production of radicals ( ), the molar extinction coefficient of the initiator at the wavelength employed ( ), the initiator concentration [5], and the path length (/) of the light through the sample. Assuming the usual radical termination processes at steady state, the rate of photopolymerization is often approximated by... 

In stereolithography the positions of polymerization x and y are controlled by a mirrored scanner which reflects the laser onto the surface of the to-be-polymerized monomer at a point x,y and the z dimension is determined by the position of the elevator, as shown in Figure 5. During the polymerization z, the depth to which reaction occurs, is held constant through the use of a UV photoinitiator that bleaches either not at all or very slowly, relative to the rate of polymerization. 

If a swelling agent is added to the reaction mixture, a maximum rate is usually noted for some fairly low agent-to-monomer ratio. DMF, which is merely a swelling agent when mixed with monomer, gives a maximum rate at 10 mol-% in photoinitiated polymerization, at about 25 mol-% in polymerization catalyzed by benzoyl peroxide (9), and at about 30 mol-% with gamma rays (114), all near room temperature. On the other hand, as little as 10 mol-% of DMF reduces the rate at 60° by a factor of about 15. It decreases also the ratio of the fast reaction at 60° to the 25° rate. 

A monolithic hydrophobic polymer formed by photoinitiated polymerization for on-chip solid-phase extraction is shown in Figure 5.6. The polymer mixture includes butyl methacrylate (BMA) and ethylene dimethacrylate (EDMA), with the pore size controlled by the composition of the hexane/methanol porogenic mixture. The degree of pre-concentration depends on the flow rate, as shown in the pre-concentration of GFP at three flow rates (see Figure 5.7). The factors of pre-concentration were 355, 756, and 1002 for the flow rates of 3, 1.03, and 0.53 rE/min, respectively [342]. 

When using photoinitiators such as phenyl acetophenone derivatives, addition of an hydrogen donating solvent (Table IV) to the system improved ultimately the percent grafting and, in addition, the concentrations of initiator required were lower. Moreover, the presence of THF is necessary with benzophenone derivatives. These results are in agreement with similar observations with respect to MMA polymerization in solution in fact, the rate of polymerization was enhanced in the presence of THF. 

Tt is well known that the presence of precipitated polymer can influence the course of polymerization. In bulk acrylonitrile polymerization the effects are most dramatic and have been the subject of many studies. The literature on this subject has been reviewed by Bamford et al. (4) by Thomas (29), and by Peebles (23). Under conditions where the system becomes heterogeneous owing to precipitation of small particles of polymer, a protracted acceleration period is observed at the start of polymerization, and the final rate is found to depend on the 0.8 power of the concentration of free radical initiator. Unusual post-polymerization effects are observed in photoinitiated polymerization of acrylonitrile, owing to the presence of trapped radicals which can be detected by electron spin resonance. None of the detailed mechanisms proposed to... 

The effect of pulsed discharge on plasma polymerization may be viewed as the analogue of the rotating sector in photoinitiated polymerization. The ratio r of off time 2 to on time ti, r = l2lti, is expected to influence the polymerization rate depending on the relative time scale of t2 to the lifetime of free radicals in free radical addition polymerization of a monomer. This technique was used to estimate the average lifetime of free radicals in the polymerization. 

Solvent effectiveness in the benzophenone-photoinitiated polymerization of methyl methacrylate was in the order tetrahydrofuran > isopropanol > toluene > benzene (46). In the case of TCMB, isopropanol gaw slightly higher polymerization rates than tetrahydrofuran, but both wore again considerably more effective than toluene and benzene. As might be expected, monomers which thonselves contain ether grouj , e.g. diethylene glycol diacrylate (7), do not require addition of a separate hydrogen-donor solvent for efficient photoinitiation by benzophenone. 

As a photoinitiator for the polymerization of bulk methyl methacrylate, benzil was found to be considerably less efficient than benzoin or benzoin methyl ether (20), e,g. at photoinitiator concentrations of 10 M, the polymerization rate observed using benzoin was eight times that observed using benzil, despite the fact that the latter was found to absorb three times as much of the incident radiation (Fig. 1). The photo-initiating efficiency of benzil was improved by a factor of three on addition to the methyl methacrylate of 10% v/v tetrahydrofuran, whereas the same additive had no appreciable effect on rates of benzoin- and benzoin methyl ether-photoinitiated polymerizations direct evidence that photoinitiation by benzil proceeds by a hydrogen abstraction mechanism rather than by fragmentation. 

In the photopolymerization of methacrylamide by benzoin methyl ether, chain-transfer to monomer has been found to be important, and benzalde-hyde is reported to be an inefficient photoinitiator of methyl methacrylate polymerization unless benzophenone and triethylamine are present. Acetophenone has been found to sensitize the cycloaddition of maleic anhydride to 7-oxabicyclo[2.2.1]heptan-5-one-2,3-dicarboxylic anhydride, , a-hydroxy-acetophenone derivatives have been found to be non-yellowing initiators, and h.p.l.c. has been used to determine residual carbonyl photoinitiators in u.v.-cured resins. In the emulsion-polymerization of methyl methacrylate using an aromatic ketone and a continuous or intermittent laser, the former conditions were found to be similar to those under continuous u.v. irradiation. The dependence of the polymerization rate and average chain-length on the absorbance of the initiator used in the photoinitiated polymerization of vinyl monomers has been studied. Interestingly, irrespective of all conditions, maximum conversion is observed when initiator absorbance is 2.51. "... 

The photoreduction of aromatic ketones by tertiary amines is reported [38] to proceed at rates which are substantially faster than those observed for the corresponding photoinduced hydrogen abstraction from, e.g. alcohols. A limit case is given by fluorenone, the photoreduction of which does not occur in alcohol, ether or alkane solution, but readily takes place in the presence of amines, tertiary amines being the most effective [39,40]. Xanthone has also been reported to be easily photoreduced by iV,A-dimethylaniline [41], but not by 2-propanol [42]. However, the oxidation of tertiary amines photosensitized by fluorenone and xanthone is much less efficient than when sensitized by benzophenone, apparently because of lower rates of hydrogen abstraction [43]. Fluorenone/tertiary amine systems have been used successfully to photoinitiate the polymerization of MMA, St, MA and AN [30,38,44] and rather similar results have been obtained in the photoinitiated polymerization of MA by the benzophenone/EtsN system [45]. Thus, the great variety of substrates participating in exciplex formation has been readily extended to polymer-based systems. 

Table 9. Photoinitiated polymerization rate (Rp) of MMA in benzoie solution by low- and high-molecular-weight systems based on fluorenone moiety in combination with different amines [30] ...

It is moreover worth mentioning that the polymerization rate Rp is [83] appreciably higher for poly(MIK) and poly(BVK) than for the model compound 3,3-dimethyl-2-butanone (MBK) in the photoinitiated polymerization of AN and St, which are known to act as singlet and triplet quenchers, respectively (Table 13). 

Figure 6. Rate of heat evolution during argon-ion laser-photoinitiated polymerization of 1-methyl-2-pyrrolidinone (MP), 9 ML of 2-ethyl-2-(hydroxymethyl)-l,3-propanediol triacrylate (TMPTA), and the dyes at a concentration of 10 m. Initiators 1) 19, 2) 18, 3) 10, 4) 16, 5) RBAX (rose bengal derivative prepared from rose bengal, which is first decarboxylated and then acetylated [36]),...

Section 4.4) on the photoinitiation process, one can anticipate that under certain conditions (identical free radicals formed), the rules regulating the primary processes can also be applied for the secondary processes. The results presented in Figure 8 confirm this expectation. It is clear from the data (Figure 8) that the rate of polymerization as initiated by the series of cyanine borates in Table 2 increases as the driving force of the electron transfer increases. This behavior is predicted by the classical theory of photoinduced electron transfer. 

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.

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