Photoinitiation anionic polymerization

In subsequent papers Kutal et al reported that they found that ferrocene and ruthenocene (FeCp2 and RuCp2, where Cp is rj -CsHs) complexes will also photoinitiate anionic polymerizations of a-cyanoacrylate. They suggest that the mechanism of initiation by ferrocene is an attack on the monomer and formation of a radical anion through electron transfer... 

YAM 07], For example, they have reported the implementation of the Pt(acac)2 complex toward the photoinitiated anionic polymerization of ethyl-a-cyanoacrylate (ECA) [PAL 95], It is understood that the initiating species were free acetylacetonate anions freed by UV excitation of the complex (Diagram 3.14). 

Also, it has been shown that the polymerization of alkyl cyanoacrylates can be driven forward by tertiary amines such as pyridine whereby the active mechanism is thought to move through a zwitterionic pathway [JOH 81]. Through work emanating from the authors laboratory, it has also been shown that weak photobase generator systems can be used to initiate a zwitterionic pathway toward a photoinitiated anionic polymerization of cyano acrylates. A-Alkoxypyridinium and N-phenacylpyridinium type salts were described as the active initiators for these systems (Diagram 3.15) [ARS 96, ONE 99, KAS 03a]. 

Diagram 3.15. N-alkoxypyridinium salt used for the photoinitiated anionic polymerization of ECA... 

Along with the photochemical generation of bases, Crivello and Dietliker have reviewed the field of photoinitiated anionic polymerizations. Until this point in time, there has been no report of the photochemical generation of carbanionic species equivalent to those obtained from organometallic compounds... 

There are, however, several highly reactive vinyl monomers such as 2-(trifluoromethyl) acrylates and 2-cyanoacrylates that undergo anionic polymerizations in the presence of even weak bases. The photoinitiated anionic polymerizations of these monomers have been achieved using a number of photosensitive metal complexes. For example, the irradiation of alkali salts containing the trans-[Cr(NH3)2(NCS)4] anion at wavelengths in the range of 350-532 nm releases the thiocyanate anion (SCN"). As depicted in Scheme 34, the thiocyanate anion is capable of initiating the anionic chain polymerization of ethyl 2-cyanoacrylate. ... 

As described by the Kutal research group, another approach to the photoinitiated anionic polymerization of ethyl 2-cyanoacrylate involves the photolysis of the platinum bis(acetylacetonate) complex, 117. The irradiation of 117 at wavelengths above 300 nm results in the liberation of the acetylacetonate anion (118, Scheme 35) that is capable of initiating the polymerization of the monomer in a manner similar to the thiocycanate anion as shown in Scheme 34. 

Kutal, C., Grutsch, P.A., and Yang, D.B. (1991) A novel strategy for photoinitiated anionic polymerization. Macromolecules,... 

Photoinitiators provide a convenient route for synthesizing vinyl polymers with a variety of different reactive end groups. Under suitable conditions, and in the presence of a vinyl monomer, a block AB or ABA copolymer can be produced which would otherwise be difficult or impossible to produce by another polymerization method. Moreover, synthesis of block copolymers by this route is much more versatile than those based on anionic polymerization, since a wider range of a monomers can be incorporated into the blocks. 

The block copolymer produced by Bamford s metal carbonyl/halide-terminated polymers photoinitiating systems are, therefore, more versatile than those based on anionic polymerization, since a wide range of monomers may be incorporated into the block. Although the mean block length is controllable through the parameters that normally determine the mean kinetic chain length in a free radical polymerization, the molecular weight distributions are, of course, much broader than with ionic polymerization and the polymers are, therefore, less well defined,... 

An interesting characteristic of mthenocene is that it is a photoinitiator for the anionic polymerization of ethyl 2-cyanoacrylate. Solutions with millimolar concentrations of the complex in neat cyanoacrylate polymerize to a hard, plastic solid within seconds upon exposure to light. The low concentration of mthenocene required for the polymerization makes it desirable in applications such as coating and adhesives that require solvent-free solutions of monomer. ... 

The compoimds listed in Table 10.11 have been found to photoinitiate the polymerization of neat ethyl or methyl 2-cyanoacrylate, CA, that readily polymerize by an anionic mechanism. Therefore, this has been taken as evidence for the occurrence of an anionic mechanism [57-59]. 

In the context of the anionic polymerization of CA derivatives, as considered in Section 10.3.2.2.1, it is notable that the polymerization of cyanoacrylates is also photoinitiated by substituted pyridine pentacarbonyl complexes of tungsten or chromium, i.e. M(CO)5L with M = Cr or W, and L = 2- or 4-vinylpyridine [60]. Photo-released pyridine adds to CA, and the resulting zwitterion initiates the anionic chain polymerization (see Scheme 10.16). 

Effect of Temperature and Photoinitiator Anion. The polymerization temperature has a significant effect on both the rate of polymerization and the final limiting conversion. For example. Figure 9 contains a series of plots of reaction rate as a function of time for photopolymerization of an epoxide monomer. This figure illustrates that as the temperature is increased, the peak reaction rate increases and the reaction time decreases. The increase in reaction rate with increasing temperature ultimately arises from the effect of temperature on the propagation rate constant (which increases with increasing temperature as described by the Arrhenius equation for the rate constants). The rate of photoinitiation is... 

There are some examples of the use of so-called photobase generators in the preparation of polymers by anionic means. Photobase generators are compounds that produce bases upon irradiation with light, examples include cobalt complexes 0-acyloximines and benzyloxycarbonyl derivatives. PBGs are generally important in polymer science due to their application in photoinitiated transformation of polymeric materials and epoxy resins. Kutal et al. have reported the anionic polymerization of alkyl 2-cyanoacrylates initiated by anionic species upon the irradiation of group 8 metallocenes [YAM 98, YAM 99, YAM 00, SAN 02, BRI 02, DIN 03, SAN 05,... 

FUK 87] Fukuchi Y., Takahashi T., Noguchi H. et al, Photoinitiated anionic coordination polymerization of epoxides, a novel polymerization process . Macromolecules, o. 20, pp. 2316-2317, 1987. 

As in the case of the diaryliodonium and triarylsulfonium salts, photoinitiated cationic polymerizations employing dialkylphenacylsulfonium and dialkyl-4-hydroxy-phenylsulfonium salts exhibit a marked dependency on the structures of both the cation and anion portions of the photoinitiator. Here also, the most efficient photoinitiators are those bearing anions having the poorest nucleophilic character possible i.e. BF4, PF, AsFg, and SbF. ... 

Meanwhile, it was found by Asai and colleagues [48] that tetraphenylphosphonium salts having such anions as Cl, Br , and Bp4 work as photoinitiators for radical polymerization. Based on the initiation effects of changing counteranions, they proposed that a one-electron transfer mechanism is reasonable in these initiation reactions. However, in the case of tetraphenylphosphonium tetrafluoroborate, it cannot be ruled out that direct homolysis of the p-phenyl bond gives the phenyl radical as the initiating species since BF4 is not an easily pho-tooxidizable anion [49]. Therefore, it was assumed that a similar photoexcitable moiety exists in both tetraphenyl phosphonium salts and triphenylphosphonium ylide, which can be written as the following resonance hybrid [17] (Scheme 21) ... 

The radiolysis of olefinic monomers results in the formation of cations, anions, and free radicals as described above. It is then possible for these species to initiate chain polymerizations. Whether a polymerization is initiated by the radicals, cations, or anions depends on the monomer and reaction conditions. Most radiation polymerizations are radical polymerizations, especially at higher temperatures where ionic species are not stable and dissociate to yield radicals. Radiolytic initiation can also be achieved using initiators, like those used in thermally initiated and photoinitiated polymerizations, which undergo decomposition on irradiation. 

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