Photoinitiation of polymerization

These photoinitiation processes which depend on the formation of free radicals in some photochemical reaction lead to chain reactions, since each molecule of initiator can promote the addition of many monomer units to a polymer chain. The quantum yield of monomer addition can therefore be much larger than unity, but it cannot be controlled since the growth of a polymer chain is then limited by termination reactions in which two free radicals react to produce closed-shell molecules. 

Photo-crosslinking and the reverse process of photodissociation of pre-existing crosslinks relies on a cycloaddition reaction (and on the reverse dissociation of the cyclic adduct). For example, derivatives of vinyl cinnamic acid can form crosslinks which are dissociated by irradiation with short wavelength light (e.g. 254 nm produced by low-pressure mercury arcs). In this process the polymer chains become separated, and the polymer itself is then soluble in organic solvents. 

Photoinduced polymerization can also be obtained through the dissociation of organic salts such as sulfonium, diazonium and similar salts. The photodissociation leads to several species, a radical cation, a neutral free radical and a closed-shell anion for example. The radical cation can then react further, e.g. through hydrogen abstraction from a substrate, ZH, to form another free radical Z.  

In all these cases the important reaction is the formation of reactive free radicals which induce the addition of further monomer molecules to the end 

The major impurities which are found in any polymer are the unreacted monomer itself, unreacted initiator (peroxides and all types of photoinitiators) and catalysts used in the polymerization process, as well as traces of the solvent and of water. Within the polymer chain itself there will be some defects or impurity sites which result essentially from oxidation reactions during the making of the polymer. The polymerization process on an industrial scale cannot be performed in the absence of atmospheric oxygen, and this will attack the growing polymer chain at random points to produce 

---

Photopolymerization, in general, can be defined as the process whereby light is used to induce the conversion of monomer molecules to a polymer chain. One can distinguish between true photopolymerization and photoinitiation of polymerization processes. In the former, each chain propagation step involves a photochemical process [1,2] (i.e., photochemical chain lengthening process in which the absorption of light is indispensable for... 

Photoinitiation of polymerization has played an important role in the early developments of polymer chemistry. The main features of this type of initiation are ... 

In more recent years, photoinitiation of polymerization proved to be of immense value in the understanding of the precise nature of polymerization. Several systems used for the initiation of radical polymerization were reviewed by Oster and Yang [3], Rabek [10], and Davidson [5,6]. 

Strohmeier and Hartmann [14] first reported in 1964 the photoinitiation of polymerization of ethyl acrylate by several transition metal carbonyls in the presence of CCI4. Vinyl chloride has also been polymerized in a similar manner [15,16] No detailed photoinitiation mechanisms were discussed, but it seems most likely that photoinitiation proceeds by the route shown in reaction Scheme (9). 

It has generally been concluded that the photoinitiation of polymerization by the transition metal carbonyls/ halide system may occur by three routes (1) electron transfer to an organic halide with rupture of C—Cl bond, (2) electron transfer to a strong-attracting monomer such as C2F4, probably with scission of-bond, and (3) halogen atom transfer from monomer molecule or solvent to a photoexcited metal carbonyl species. Of these, (1) is the most frequently encountered. 

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 ... 

Photoinitiation of polymerization of MMA and styrene by Mn(facac)3 was also investigated, and it was shown that the mechanism of photoinitiation is different [33] from that of Mn(acac)3 and is subject to the marked solvent effect, being less efficient in benzene than in ethyl acetate solutions. The mechanism shown in Schemes (15) and (16) illustrate the photodecomposition scheme of Mn(facac)3 in monomer-ethyl acetate and monomer-benzene solutions, respectively. (C = manganese chelate complex.)... 

Kaeriyama and Shimura [34] have reported the photoinitiation of polymerization of MMA and styrene by 12 metal acetylacetonate complex. These are Mn(acac)3, Mo02(acac)2, Al(acac)3, Cu(bzac)2, Mg(acac)2, Co(a-cac)2, Co(acac)3, Cr(acac)3, Zn(acac)2, Fe(acac)3, Ni(a-cac)2, and (Ti(acac)2) - TiCU. It was found that Mn(a-cac)3 and Co(acac)3 are the most efficient initiators. The intraredox reaction with production of acac radicals is proposed as a general route for the photodecomposition of these chelates. 

Aliwi and coworkers have investigated many vanadium (V) chelate complexes as photoinitiators for vinyl polymerization [36-43]. The mixed ligand complex of chloro-oxo-bis(2,4-pentanedione) vanadium (V). VO(a-cac)2 Cl is used as the photoinitiator of polymerization... 

The ion-pair complex formed by the interaction of hydroxobis(8-quinolyloxo) vanadium (V) [VOQ2OH] and /i-butyl amine is also effective in photoinitiation of polymerization of MMA in bulk and in solution [40]. The quantum yield of initiation and polymerization determined are equal to 0.166 and 35.0, respectively. Hydroxyl radical ( OH) is reported to be the initiating radical and the following photoreaction is suggested ... 

The best evidence for the photolytic decomposition of mercaptans and disulfides into free radicals involves photoinitiation of polymerization of olefins. Thus, photolysis of disulfides initiates the copolymerization of butadiene and styrene,154 as well as the polymerization of styrene207 and of acrylonitrile.19 Thiophenol and other thiols promote polymerization upon ultraviolet irradiation.19 Furthermore, the exchange of RS-groups between disulfides and thiols is greatly accelerated by light. Representative examples are benzothiazolyl disulfide and 2-mercapto-thiazole,90 tolyl disulfide and p-thiocresol, and benzyl disulfide and benzylmercaptan.91 The reaction probably has a free radical mechanism. Similar exchange reactions have been observed of RS-groups of pairs of disulfides have been observed.19... 

Photoinitiation of polymerization can be obtained through a variety of photochemical reactions which produce reactive free radicals. These radicals then lead to the formation of the polymer chains through the addition of further monomer units to the end of a chain in a sequence of radical addition reactions (Figure 6.10). A photoinitiator of polymerization is therefore a molecule which produces free radicals under the action of light. Benzo-phenone and other aromatic ketones can be used as photoinitiators, since a pair of free radicals is formed in the hydrogen abstraction reaction. Some quinones behave similarly, for example anthraquinone in the presence of hydrogen donor substrates such as tetrahydrofuran. 

The high rate constants for chemical quenching of triplet ketones by amines provide two sidelights of considerable importance. Photoinitiation of polymerization has received widespread and varied industrial applications. One problem is that many vinyl monomers quench triplet ketones very rapidly either by charge transfer or energy transfer mechanisms, without forming any radicals. Most solvents cannot compete with the olefins for the triplet ketone. However, tri-ethylamine quenches at rates close to diffusion-controlled, so that radical formation and polymerization initiation are quite efficient 158>. 

We conclude that the dominant process under photoinitiation of polymerization of (meth)acrylates by TPO (Scheme 12.1) is a tail addition of phosphinoyl radical to a double bond (Scheme 12.8). 

There are numerous references in the Patent Literature (72—76) to the use of anthraquinone and similar compounds as photoinitiators of polymerization or crosslinking required in the preparation of printing plates etc, although apparently little is known of the detailed reaction mechanisms. One literature report (77) described the polymerization of aqueous methyl acrylate by sodium anthraquinone-2-sulphonate in the presence of chloride ions, with a conclusion that the initiating species are chlorine atoms. 

In the present ehapter we consider the inter- or intramolecular photoinduced electron transfer phenomenon. We mainly focus on photoinduced electron transfer processes that lead to the photoinitiation of polymerization, and on processes initiated by photoredueed or photooxidized excited states. We concentrate especially on a description of the kinetic schemes, a description of the reactions that follow the primary proeess of eleetron transfer, and the characteristics of intermediates formed after electron transfer. Understanding the complexity of the processes of photo-initiated polymerization requires a thorough analysis of the examples illustrating the meehanistie aspects of the formation of free radicals with the ability to start polymerization. 

The absorption band (Amax 488 nm) in the transient spectrum corresponds to the pyrene radical ion (Py ) [155, 156], while the band at Amax 400 nm is assigned to the absorption of the 1-hydro-1-pyrenyl radical (Py ) [157, 158]. Steady-state photolysis of pyrene in the presence of TEA leads to its disappearance, and addition of vinyl monomers decreases the rate of pyrene photoreduction. The photobleaching process follows first-order kinetics. Encinas et al. [154] suggest that the photoinitiation of polymerization by pyrene-TEA is catalyzed by the pyrene radical ion. 

The iodonium and sulfonium salts have found the most application in both the photoinitiation of polymerization and polymer-based photoimaging. Amenability of these salts to spectral sensitization by either an electron-transfer [21] or a triplet energy transfer pathway [22] is an important factor. The reduction potential of the phosphonium salts, -2.1 to —1.55 V versus SCE [10], is too negative for efficient electron transfer sensitization from most sensitizers of practical interest (see below). As a result, most of the fundamental studies have focused on the iodonium and sulfonium salts, as will this review. 

The photoinitiation of polymerization of pentaerythritol tetraacrylate using phenyl-(p-anisyl)-iodonium triflate or triphenylsulfonium hexafluorophosphate, sensitized with either l,6-diphenyl-l,3,5-hexatriene or 1,3-diphenyl-2-pyrazoline, was illustrated by Smith [111b] in 1981. Under his conditions, direct photolysis of the onium initiators failed to initiate polymerization. Baumann and co-workers, however, found conditions for initiation of radical polymerization on direct irradiation of onium salts [18,122], consistent with the hypothesized generation of radicals capable of cage escape in direct photolysis. 

D. Ruhlmann and J.P. Fouassier, Structure-property relationships in photoinitiators of polymerization. 8. Sulfonyl ketone derivatives. Eur. Polym. J. 1993, 29(8), 1079-1088. 

Time-Resolved Laser Spectroscopy of Synergistic Processes in Photoinitiators of Polymerization... 

The photoinitiation of polymerization by the use of intermolecular hydrogen abstraction is well known.( ) Diaryl ketones, such as benzophenone, are used as the photoinitiator and a... 

R 466 J. G. Kempf and J. P. Loria, Theory and Applications of Protein Dynamics from Solution NMR , Cell Biochem. Biophys., 2002,37,187 R467 E. Kennett and P. Kuchel, Redox Reactions and Electron Transfer Across the Red Cell Membrane , lUBMB Life, 2003,55, 375 R 468 R. G. Khalifah, Reflections on Edsall s Carbonic Anhydrase Paradoxes of an Ultra Fast Enzyme , Biophys. Chem., 2003,100,159 R 469 A. A. Khrapitchev and P. T. Callaghan, Spatial Dependence of Dispersion , Magn. Reson. Imaging, 2003, 21, 373 R 470 I. V. Khudyakov, N. Arsu, S. Jockusch and N. J. Turro, Magnetic and Spin Effects in the Photoinitiation of Polymerization , Des. Monomers Polym., 2003, 6, 91... 

The light-induced synthesis of polymers is the topic of Part III. While the various modes of photoinitiation of polymerization processes are discussed in Chapter 10, related technical applications are treated in Chapter 11. The latter include curing of coatings and dental systems, printing plates (used to print newspapers), holography (important for data storage), and the synthesis of block-and-graft copolymers. 

Apart from the photoreactions of dithiocarbamate groups (last entry in Table 11.4), no details on the radical-generating photoreactions referred to in Table 11.4 are given here. These can be found in [84-86]. Dithiocarbamate groups play a special role with regard to the photoinitiation of polymerizations. This is... 

When photoinitiators consisting of mixtures of benzophenone, or 4-benzoylbiphenyl, or isopropylthioxanthone vdth a tertiary amine are combined with an electron deficient anhydride, rapid photoinitiations of polymerizations of acrylate esters result. Thus, additions of less than 0.1 wt. percent 2,3-dimethylmaleic anhydride to 1,6-hexanediol diacrylate containing any of the above diaryUcetones and N-methyldiethanolamine result in an increase in the polymerization rate maximum by a factor of as much as three that is attained for the same reaction without the anhydride. Laser flash photolysis results show that benzophenone, 4-benzoylbiphenyl, and isopropylthioxanthone triplets are readily quenched by dimethylmaleic anhydride. 

Also, free radieal photoinitiation of polymerization of multifunctional acrylate monomers (trimethylolpropane triacrylate and phthalic diglycol diacrylate) was reported to take place by a cationic cyanine dye-borate eomplex 1,3,3, r,3, 3 —hexamethyl-11 -chloro-10,12-propylene tricarbocyanine friphenyl-butyl borate. The dye-borate complex was illustrated as follows ... 

FOU 12c] Fouassier J.P., Lalevee J., Design of chromophores for photoinitiators of polymerization brief survey and recent achievements , in Moliere a., Vigneron E. (eds). New Development in Chromophore Research, pp. 212-254, Nova Sciences, New York, 2012. 

J. P. Fouassier and J. Lalevee, Design of Chromophores for Photoinitiators of Polymerization Brief Survey and Recent Achievements, in New Developments in Chromophore Research, Nova Science Publishers, Hauppauge, NY, USA, 2012. 

---