Photopolymerization, radical chains

The presence of one carbonyl group per oligomer molecule was also ascertained. The orange colour of the resin suggested that some minor event during the photopolymerization produced chromophores in small concentrations. The presence of furoin among the products corroborated the proposed mechanism, which was shown not to involve free radical chain reactions. 

Table 3-10 shows the values of the various concentrations, rates, and rate constants involved in the photopolymerization of methacrylamide as well as the range of values that are generally encountered in radical chain polymerizations. For the methacrylamide case, the experimentally determined quantities werei ,-, (Rp)s, [M], [I], kp/ltj1, ts, and kp/kf. All of the other... 

Polymerization of monomer by vinyl addition polymerization is a typical chain process. Three main reaction stages can be identified initiation, propagation and termination. During the initiation event, free radicals are created. In a photopolymerization, the initiating free radicals are formed in a photoprocess. Propagation is the process of addition of monomer to the growing free radical chain. The destruction of the free radical center occurs during termination. 

An advantage of this type of photopolymerizations is that as they are non-radical chain polymerizations, they are insensitive to oxygen. In addition, as the cation is relatively stable, the reaction is able to continue in the dark. Applications of this chemistry may be found in the fields of coatings, adhesives, printing inks, and also for photocurable composites and microelectronic photoresists. 

It has been generally assumed that the polymer is formed by a free-radical chain which is initiated by H atoms or alkyl radicals. While this mechanism may be true for the mercury-photosensitized polymerization, recent results on the direct photopolymerization suggest that the two systems may be very similar and that a free-radical chain mechanism is not tenable for the latter case. It is probably most logical to examine this system by first studying the polymer that is formed, and then establishing the kinetics and mechanism of its formation. 

The photopolymerization process taking place within a representative mixture of sensitizer, initiator, chain-transfer agent, and monomer, typical of positive Cromalin, has been studied in detail (41,42). The exact mechanism is still controversial, but a generalized reaction scheme can be postulated as follows, where L2 = biimidazole dimer, S = sensitizer, RH = chain-transfer agent, L2 = excited biimidazole dimer, L = biimidazole radical,... 

The PLP-SEC method, like the rotating sector method, involves a non-steady-state photopolymerization [Beuermann, 2002 Beuermann and Buback, 2002 Komherr et al., 2003 Nikitin et al., 2002], Under pulsed laser irradiation, primary radicals are formed in very short times ( 10 ns pulse width) compared to the cycle time ( 1 s). The laser pulse width is also very short compared to both the lifetimes of propagating radicals and the times for conversion of primary radicals to propagating radicals. The PLP-SEC method for measuring kp requires that reaction conditions be chosen so that no significant chain transfer is present. The first laser pulse generates an almost instantaneous burst of primary radicals at high... 

Moreover, it should be noticed that polymerization rates were determined from the maximum slope of the kinetic curves, namely at degrees of conversion between 20 and 40%. At that time, the large increase in viscosity of the photoresist may already have reduced the chain mobility, thus favoring radical isolation and first-order termination. It is therefore very likely that the intensity exponent of the photopolymerization rate equation will be less than 0.85 in the early stages and that it increases with conversion to reach almost unity in the solid network. Such a kinetic behavior was indeed observed for the photopolymerization of neat hexanedioldiacrylate (31). 

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... 

The quantum yield of photopolymerization, as well as the kinetic chain length ( p/ i), decrease with the square root of the absorbed light intensity. For an absorbed light intensity of 1.06 xlO-4 einsteins/1 x s and borate concentration of 10 mM the photopolymerization quantum yield is 1420. The quantum yield for radical generation is 0.067 [274], giving a kinetic chain length of 2.1 x 104, which compares favorably with the value of 2.9 x 104 reported for the UV photopolymerization of epoxy diacrylate/TMPTA in the presence of air [282]. 

The radicals shown in Table 14 are expected to be good initiators. We would expect them to add to unreacted monomer as soon as they are produced and, as a consequence, not to be available for termination when the photopolymerization is carried out under steady-state irradiation. However, our results indicate that these radicals are the main chain terminators in our systems. We postulate that this effect is partially due to RBAX being a long-lived species in the monomers we have employed in this work. From our bleaching results we estimate a lifetime shorter than 3 ns for RBAX" in ethyl acetate. It ispagate to a considerable extent before the terminator radical is generated. 

A very different process of photopolymerization relies on the reaction of photocondensation which is an addition of two molecules to form a longer adduct. There is no radical intermediate in this case and one photon (at least) is required for each step in the polymerization process. Some male-imide derivatives can be polymerized in this way, to form an insoluble crystalline polymer. It should be noted that the monomer molecules must have two reactive groups, one at each end, so that the polymer chains can extend in principle indefinitely (Figure 6.12). 

In the particular case of a sensitized photopolymerization process, the expression for Rp must be modified to reflect the photophysics of the sensitizer S and the efficiencies of the processes which intervene on the way to creation of the free radical R- which will eventually initiate chain formation. For the purposes of the present discussion, consider that R is formed by the following sequence of reactions ... 

A polymerization process requiring a photon for the propagation step is called photopolymerization or photoinduced polymerization, when polymerization of a monomer by a free radical or ionic chain reaction is initiated by photoexcitation. 

A PET in intramolecular CPs between pyridinium ions and bromide, chloride or thiocyanate ions for polymerization initiation is described, too [137-139]. As expected, an equilibrium exists among free ions, ion pairs, and CT, which is shifted to the free ions in polar solvents and to the complex in a less polar solvent That complex serves as the photosensitive species for the polymerization (see Scheme 10). The photodecomposition of the CT yields radicals of the former anion and N-alkylpyridinyl radicals. Probably, the photopolymerization is initiated only by X- radicals, whereas latter radicals terminate the chain reaction. By addition of tetrachloromethane, the polymerization rate is increased owing to an electron transfer between the nucleophilic pyridinyl radical and CC14 (indirect PET). As a result, the terminating radicals are scavenged and electrophilic -CQ3 radicals are produced. 

Application of a moderate MF accelerates photopolymerization initiated by PI leading to triplet RPs. " The main effect is an increase in/and an increase in the rate of initiation. One should expect a second weak effect leading to deceleration of a chain termination by bimolecular radical reaction. A MFE on an F parr was observed for the first time in Refs 23,24. 

In summary, the results on sensitized photopolymerization of diacetylenes are of relevance for potential application and, in addition, augment understanding of the polymerization process in general. In conjunction with what is known about chain propagation and self-sensitization, they indicate that the basic requirements for both chain initiation and propagation are presence of an unpaired electron — in form of either a radical or a charge carrier — on the reactive center and excitation of a librational motion of the reactant(s) in course of which the reaction distance is temporarily reduced to a critical value. 

All distribution curves are bimodal with maxima at P = 60 and 400. At lower temperatures longer chains are formed. Since there is no gradual shift of the maximum with temperature it must be assumed that the chain grows by at least two different active chain ends, the population of which is strongly temperature dependent. The chemical nature of these chain ends cannot be deduced by the kinetic data. However, it seems reasonable to infer that we are dealing with the same carbene and radical intermediates which have been identified in the photopolymerization of diacetylenes at low temperatures by Sixl and coworkers... 

Photoinitiated polymerization uses the energy of light for the rapid conversion of monomeric liquids to solid polymeric products. The term photopolymerization implies that the initiation step of a radical, cationic, or anionic chain reaction producing a macromolecule requires the absorption of a photon. Since the absorption of one photon may start the reaction of up to 10 monomeric units, photoinitiated polymerization is, in practice, one of the most powerful chemical amplification techniques. 

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