Photoinitiated Addition Polymerization

The photoinduced polymerization of 3-alkylated-2-pyrrolidones has been found to increase with substitution on the pyrrolidone ring system, while vinylidene fluoride (VDF) in the presence of hydrogen peroxide gives telechelic PVDF through the formation of hydroxyl radicals, which can react with the 

Photoinitiated Addition Polymerization.—Carbonyl-based photoinitiators continue to attract widespread interest. Carlini et have developed polymers 

Ledwith, Polym. Prepr., Am. Chem. Soc., Dio. Polym. Oiem., 1982, 23, 323. 

Benzophenone per-esters are apparently highly efficient photoinitiators for the photopolymerization of methyl methacrylate, and Neckers and coworkers have compared the efficiencies of several compounds (1) of this 

In the anthraquinone-sensitized photopolymerization of acrylate monomers, the percent conversion depends markedly on the nature of the solvent as well as on the type of monomer. Ethyl and n-butyl methacrylate, for example. 

Nishikubo and T. lizawa, Proc. lUPAC, Macromol. Symp.. 28th, 1982, 183. 

Photoinitiated Addition Polymerization.—The role of carbonyl compounds in initiating photopolymerization continues to be an area of prolific research. Laser flash photolysis has provided a valuable insight into both the photophysical and photochemical processes involved in carbonyl-initiated photopolymerizations. For example, Fouassier and co-workers have found that carbonyl-initiated vinyl polymerization proceeds faster in a micellar system and an example of this interesting effect is shown in Table 1. Laser flash photolysis has shown that 

Sierocka, J. Paczkowski, A. Wrzyszczynski, and A. Zakrzewski, Pr. Wydz. Nauk. Tech., Bydgoskie Tow, Nauk. Ser. A, 1980, 14, 67. 

Longnot, A. Merlin, P. Jacques, and J. P. Fouassier, Makromol. Chem., Rapid Common., 1980, 1, 687. 

Schnabel and co-workers have examined the behaviour of aromatic carbonyl compounds in laser flash photolysis. For l-phenyl-2-hydroxy-2-methy 1-propan-1-one (1) in the absence of a hydrogen-atom donating solvent, a-cleavage to give (2) and (3) was the dominant initiating step. 

1 Photoinitiated Addition Polymerization. - Many new photoinitiator systems continue to be developed for photopolymerization. Novel macroinitiators 

2-dimethoxy-2-phenylacetophenone as a photoinitiator continued in the dark for 90% of the total conversion processes. 

Polymer effects on the photoactivity of the thioxanthone chromophore have also been found to be negligible whereas in the anthraquinone induced photopolymerisation of methyl methacrylate the polarity of the solvent was found to markedly influence the rate of polymerisation . In this case the rate was directly proportional to the solvent polarity whereas the molecular weights were found to be inversely related. 

Where A = Benzophenone or 9- Fluorenone derivatives XM/ = Any growing polymer radical NR3 = Tertiary Amine 

2-dimethoxy-2-phenylacetophenone as photoinitiator, an increase in the laser repetition rate resulted in a decrease in both the polymer yield and molecular weight . This was associated with the formation of small molecular radicals quenching living polymer radicals which in turn were directly related to the initiator concentration. 

Mixtures of photoinitiators have been actively studied. Michler s Ketone and benzoyl peroxide have been shown to effectively induce the photopolymerisation of methyl methacrylate through the formation of an initial complex shown in scheme 3 7, Although the exact initiating radical does not appear to be ascertained it is almost certainly the arylalkylamino radical from the Michler s Ketone. In the interaction of benzil and thioxanthone with triethylamine in the photoinduced polymerisation of acrylic monomers their is a competition between reverse electron transfer and ketyl radical formation . As the carbonyl concentration increases the bimolecular termination rates due to radical recombination increases. The same workers also studied the same system but replaced the ketone initiators with pyrene . Their inability to identify pyrene end groups indicated that the active initiating species arise from a complex between the pyrene and the triethylamine. 

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Photoinitiation is not as important as thermal initiation in the overall picture of free-radical chain-growth polymerization. The foregoing discussion reveals, however, that the contrast between the two modes of initiation does provide insight into and confirmation of various aspects of addition polymerization. The most important application of photoinitiated polymerization is in providing a third experimental relationship among the kinetic parameters of the chain mechanism. We shall consider this in the next section. 

The chemistry involved in LfV-curable resin systems has been extensively investigated and thoroughly surveyed [88-94]. LfV-radiation polymerization, is in principle, completely analogous to the conventional addition polymerization. A photoinitiator is used in UV polymerization. Its function is the same as the free-radical initiator. A conventional initiator possesses a thermally labile bond which is cleaved to form free-radical species, but the photoinitiator has a bond which breaks upon absorption of radiant energy. Benzoin ethers, benzyldialkyl ketals, benzophenone, and acetophenone derivatives are the important LfV-photoinitiators [95-99]. 

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. 

Photocurable coatings are widely used for metal, plastics wood and paper. Photoinitiated free-radical polymerization, however, can only be applied to vinyl monomers. The studies of Crivello have broadened the scope of monomers. In addition, photoinitiated cationic polymerization is not sensitive toward oxygen (air). Photoinitiated free-radical polymerization sometimes requires working in inert atmosphere in order to avoid the inhibition through oxygen1). 

A typical cationic uv adhesive formulation contains an epoxy resin, a cure-accelerating resin, a diluent (which may or may not be reactive), and a photoinitiator. The initiation step results in the formation of a positively charged center through which an addition polymerization reaction occurs. There is no inherent termination, which may allow a significant postcure. Once the reaction is started, it continues until all the epoxy chemistry is consumed and complete cure of the resin has been achieved. Thus, these systems have been termed living polymers. 

Not enough is known for one to predict whether ionic or radical cleavage will occur. Many a-chloro and cc-bromo phenyl ketones are used as photoinitiators for polymerizations 52>, so they clearly produce radicals readily. Irradiation of chloroacetone in solution initiates the addition of CCI4 and thiols to olefins 197). Careful analysis of product structures suggests that only radical cleavage occurs. For example, in anisole the main product is orf o-methoxyphenylacetone. Radicals but not carbonium ions add preferentially ortho to monosubstituted benzenes. 

Free radical promoted, cationic polymerization also occurs upon irradiation of pyridinium salts in the presence of acylphosphine oxides. But phosphonyl radicals formed are not oxidized even by much stronger oxidants such as iodonium ions as was demonstrated by laser flash photolysis studies [51, 52]. The electron donor radical generating process involves either hydrogen abstraction or the addition of phosphorus centered or benzoyl radicals to vinyl ether monomers [53]. Typical reactions for the photoinitiated cationic polymerization of butyl vinyl ether by using acylphosphine oxide-pyridinium salt combination are shown in Scheme 10. 

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). 

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. 

The low activation energy of the thermal addition polymerization reaction confirms the necessity of a (librational) motion of the molecules in the initiation process. The first addition process differs from all the following addition proccesses by the metastable monomer diradical structure, which — in contrast to the DR , AC , and DC structures with n > 2 — has a limited life-time given by the phosphorescence decay of the monomer triplet state. Therefore, the librational excitation must be performed during the life-time of the monomer reaction centre. In the case of the low temperature photopolymerization reaction the librational excitation has to be prepared optically via the decay of the electronic excitation. This is in contrast to the photopolymerization reaction at high temperatures, where numerous molecular motions are thermally and stationary present in the crystals. Due to this difference two photons (2hv) are required in every dimer initiation process at low temperatures and only one photon (hv -i- kT) is required at high temperatures. The two paths of the photoinitiation reaction are illustrated below by the arrows in Fig. 26. The respective pair states are characterized by M M and M M as discussed below. 

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 results of these photochemical studies form guidelines for the choice of sensitizers, onium salts and other additives potentially useful in the cationic curing of coatings. The sensitized photochemistry of diphenyliodonium hexafluoroarsenate and triphenylsulfonium hexaflurorarsenate was investigated at 366 nm. Product quantum yields are compared to relative rates of photoinitiated cationic polymerization of an epoxy resin. 

Acceleration of photoinitiated cationic polymerization by using additives... 

Photoinitiated cationic polymerization of benzoxazines by onium salts was investigated mechanistically. It was postulated that the first step involves the addition of photocemically generated proton (or carbocation) either to oxygen or to nitrogen atom. Then, the polymerization proceeds via two different routes leading to the formation of different structures (Scheme 11.34) [2,118]. 

Acceleration of Photoinitiated Cationic Polymerization by Using Additives... 

As mentioned before, addition of plasticizing agents, which decrease the viscosity, into polymerization medium increase the rate of polymerization. This effect was observed in photoinitiated cationic polymerization of epoxides in the presence of epoxidized soybean oil [133],... 

Crivello s group followed either or both of two strategies that described for the additives in acceleration of photoinitiated cationic polymerization of epoxide monomers. These are stabilization of free radicals and cations by resonance and inductive effect, and the activated monomer mechanism. Comparative studies of novel monomers with conventional monomers show that newly designed monomers given in... 

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