Initiators of Photopolymerization

Polymerization is caused by an initiator that produces radicals by UV irradiation. Hydrogen abstraction and decomposition are the main reactions for radical initiation. The following characteristics are required for radical initiators. 

Benzophenones are shown in Table 1.12. Benzophenone abstracts hydrogen from hydrocarbon (RH) by UV irradiation and produces radicals  

Michelar s ketone is a typical reagent of the benzophenone type. Michelar s ketone reacts with RH by an equation similar to that of (1.65)  

4/4 -Tetra(t-butylperoxycarbonyl)benzophenone is an initiator for high sensitivity and hardening after baking  

4-Benzoyl-N-trimethylbenzene methane ammonium chloride Quatercure BTC  

---

This result reveals that exciplex formation plays a principal role in the initiation of polymerization. Since the absorption band is broadened toward longer wavelengths as the result of formation of CTC between AN and aniline, a certain concentration of aniline can be chosen so that 365-nm light is absorbed only by the CTC but not by the aniline molecule. Therefore, in this case the photopolymerization may be ascribed to the CTC excitation selected. For example, a 5 x 10 mol/L aniline solution in AN could absorb light of 365 nm, while solutions in DMF or cyclohexane with the same concentration will show no absorption. Obviously, in this case the polymerization of AN is caused by CTC excitation. The rates of polymerization for different amines were found to be in the following order (Table 12) ... 

We have prepared a copolymer-bearing amino side group and used it either alone or in combination with BP to initiate the photopolymerization of MM A [89]. The gel permeation chromatography (GPC) plot of PMMA initiated by the former system showed a bimodal distribution of molecular weight because both the radicals produced initiate polymerization as follows ... 

The polymers initiated by BP amines were found to contain about one amino end group per molecular chain. It is reasonable to consider that the combination of BP and such polymers will initiate further polymerization of vinyl monomers. We investigated the photopolymerization of MMA with BP-PMMA bearing an anilino end group as the initiation system and found an increase of the molecular weight from GPC and viscometrical measurement [91]. This system can also initiate the photopolymerization of AN to form a block copolymer, which was characterized by GPC, elemental analysis, and IR spectra. The mechanism proposed is as follows ... 

The second section focuses on emerging classes of photopolymerizations that are being developed as alternatives to acrylates. Three types of polymerization systems are included cationic photopolymerizations, initiator-free charge-transfer polymerizations, and a thiol-ene reaction system. The last section covers four interesting emerging applications of photopolymerization technology. 

Quantitative aspects of photopolymerization have been described in Sec. 3-4c. There are some differences between radical and cationic photopolymerizations. The dependence of Rp on light intensify is half-order for radical polymerization, but first-order for cationic polymerization. Radical photopolymerizations stop immediately on cessation of irradiation. Most cationic photopolymerizations, once initiated, continue in the absence of light because most of the reaction systems chosen are living polymerizations (Sec. 5-2g). 

Condensed monolayer films of pure 6 polymerized rapidly, as did mixed 6/DSPE films of up to 75% DSPE, provided the monolayers were in the condensed state [33], In the liquid-expanded state, polymerization did not occur. In the condensed state, lateral diffusion of individual lipids within the monolayer is severely restricted compared to the liquid-like state. This precludes initiation of polymerization by diffusive encounter between excited-state and ground-state diacetylene lipids. In order for polymerization to occur in the condensed state, the film must be separated into domains consisting of either pure 6 or pure DSPE. A demonstration that the rates of photopolymerization for pure 6 and mixed 6/DSPE monolayers are equal would be a more stringent test for separate domains of the lipids, but no kinetic data have been reported for this system. 

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... 

Photooxidation of Eosin with periodate ion has been used to initiate the polymerization of acrylonitrile in aqueous solution [187]. Addition of acrylonitrile to a periodate solution shifts the absorption maximum from 220 to 280 nm. This spectral change is interpreted as being due to complex formation between the monomer and oxidizing agent. The rate of photopolymerization increases linearly with the absorbed light intensity and monomer concentration. The observed intensity dependence indicates the main chain terminator is not produced photochemically. Polymer is not formed when the concentration of periodate ion is lower than 0.5 mM and the rate of polymerization is independent of its concentration for higher values. 

We, as well as Chesneau and Fouassier, find that the photospeed increases linearly with light intensity. From this observation one can conclude that chain termination reaction is not the usual interaction between two macroradicals. We have measured the initial rate of photopolymerization using thin foil calorimetry and find a linear relationship between the rate of photopolymerization at low conversions (less than 15%) and the absorbed light intensity. Using the same monomer but with a different photoinitiator (to be discussed in detail later) we observe an order of one half with respect to light intensity both by thin foil calorimetry and by measuring the polymer spike. Therefore we conclude that the linear dependence observed for the Eosin-triethanolamine system is real and not an artifact of the technique employed to determine the photospeed. 

Recently Fouassier and Chesneau [219] studied the photochemistry of the system Eosin-PDO-MDEA in aqueous acetonitrile using steady-state irradiation and laser flash photolysis. The photopolymerization of methyl methacrylate (MMA) sensitized by the photoreduction of Eosin is investigated in acetronitrile to understand the mechanism of initiation and the enhancement in the rate of polymerization caused by the presence of PDO, 3. Rates, quantum yields of photopolymerization, and number average molecular weights of the polymer are determined with MMA (7 M), Eosin (3 x 10 5 M), and MDEA (0.1 M) in the presence and in the absence of 2 x 10-3 M PDO. 

The photopolymerization of 7 M methyl methacrylate in acetonitrile was studied. Measurements of the quantum yield of photopolymerization and the molecular weight of the formed polymer indicate PDO increases the quantum yield of initiation and decreases the rate of termination. The data are shown in Table 9 with the quantum yield of initiation, i5 and k, /kt reported in arbitrary units. 

Fouassier and Chesneau [219] is not consistent with the experimental observations. From the values of the rate constants of triplet decay presented in Table 8, and taking into account that k3/k2 = 0.23 (as determined by Kasche and Lindqvist), we calculate the quantum yield of D + under the polymerization conditions. For Eosin (3 x 10 5 M) and MDEA (0.1 M) the yield of semioxidized Eosin radical is 4 x 10 3 M in the presence or in the absence of 2 x 10 3 M PDO. From the values for the quantum yield of photopolymerization and the molecular weight in the absence of PDO we calculate a quantum yield of initiation between 0.086 and 0.17, the actual value depending on the mode of termination. Therefore, we conclude that formation of a-amino radicals according to Scheme 10 represents only a minor contribution to the quantum yield of initiation observed in the presence of PDO. 

A more efficient photoinitiator has been designed by combining the Eosin-MDEA system with diphenyliodonium salt. The quantum yield of photopolymerization of methyl methacrylate (7 M) in acetonitrile is 30 with MDEA (0.1 M) and diphenyliodonium (0.05 M). The molecular weight of the isolated polymer is 55,000. In the absence of quantum yield and molecular weight, respectively. Thus, the presence of diphenyliodonium decreases the quantum yield of initiation by approximately 40% and increases the value of k3/kt by a factor of 6. 

Heat evolution starts after an induction period, the reciprocal of which increases linearly with the light intensity and is independent of the borate concentration. The initial rate of photopolymerization is proportional to the square root of the light intensity, and at constant intensity the rate of photopolymerization is independent of borate concentration. Based on these results we write Eq. (37) for the initial rate of photopolymerization, where [M]0 is the initial concentration of double bonds (10.8 M, Rj the rate of... 

My colleagues and I developed a novel imaging system, which resulted in products for the prepress proofing industry that were successful for over 35 years. They were big earners because the research was done with an eye on establishing a strong patent position, and no effective competition came along. The photo-oxidants that were developed became enormously successful as photopolymerization initiators, which allowed DuPont to manufacture a series of novel products of importance to the printing and electronics industries. Now, some 40 years after this work was done, these biimidazole derivatives still appear to be initiators of choice and are found to play a role in current patents. 

Notable work in the area of photopolymerizations of donor monomers initiated by acceptor initiators was done by Shirota [5-7] in his study of polymerization of N-vinylcarbazole (VCZ) and by Hayashi and Irie [8] on the polymerization of a-methylstyrene (a-MSt). The initiation mechanism was proposed to proceed via the charge-transfer complex between VCZ (or a-MSt) and the acceptor, which then yields two kinds of ion-radicals D and A ... 

---