Polymerization photoinduced

Figure 12. STM image of a monolayer of C9(DIA)C8ISA 63 (a) before and (b) after photoinduced polymerization of the diacetylene units. Pictures taken from ref. [41].

Figure 6.8 Photo-resist technology. A metal substrate (s) deposited on an insulator base (b) is protected by the resist (r). Irradiation of the resist through a mask (m) results in photoinduced polymerization or polymer destruction...

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 a similar way the complex (TPP)Al-0-C(C4H9) =CH-CH2-C2H5 was prepared [116] from A1(TPP)(C2H5) and rm-butyl vinyl ketene together with poly-ferf-butyl methacrylate. Every step of propagation in the photoinduced polymerization is conditioned by light absorption and includes the photoinsertion of an unsaturated molecule [115]. 

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. 

Caution This reaction should be carried out in a dark hood to prevent the photoinduced polymerization of the dione. 

Recendy, photopolymer systems have aroused increased interest because of their manifold applications in several high technologies [1-3]. Among such systems, those derived from photoinduced polymerization play an important role. The fundamental principles of these systems are based on the production of species X by photoreactions, which then initiates thermal reactions of low-molecular products leading to polymer or network formation see Eq. (1). In general, these thermal reactions are associated with low activation energies (about 60 kJ mol 1 for free radical chain polymerization). Therefore, such processes can also occur suffidentiy fast at room temperature. 

Some fundamental aspects of polythiophene preparation and chemistry have been investigated. Solution processing of a polythiophene having ester groups has been performed, which after ester cleavage and decarboxylation gave new access to the parent thiophene polymer <07MAC6012>. The mechanism of the photoinduced polymerization of thiophene... 

Photoinduced polymerization Polymerization of a monomer by a free radical or ionic chain reaction initiated by photoexcitation. 

For use in write/read/erase memories various organic photo-chromic materials have been studied, and these are described in detail in Section IV. A wide variety of organic materials have been considered for use in permanent memories, all apparently involving some form of photoinduced polymerization, and these are described in Section V. While high sensitivity is desirable in these applications, it is less critical than in holographic interferometry, and in many cases it is not necessary that the material be sensitive in the visible spectral region. 

The photo-induced polymerization of acrylonitrile, again in tetrahydrofuran solution, is greatly enhanced by the presence of benzophenone (46), which in this case exhibits a higher efficiency than TCMB. Both these compounds retarded the photoinduced polymerization of vinyl acetate in tetrahydrofuran, but did act as photoinitiators when toluene was used as solvent (46), possibly reflecting the differing abilities of the solvent-derived radicals to add to vinyl acetate. 

The possibility of applying similar intramolecular hydrogen abstraction reactions to the photoinduced polymerization of methyl methacrylate (MMA), has been tested [1] by using long-chain -alkyl N-substituted imides of 3,3, 4,4 -benzophenone tetracarboxylic dianhydride (BTDA) (Scheme 4). 

Benzoylated polystyrenes [poIy(VBP-co-St)] having different contents of VBP co-units have been checked [16] as initiators for the photoinduced polymerization of several unsaturated monomers using solvents with different hydrogen donating capability, such as dimethylformamide (DMF), tetrahydrofuran (THF), cyclohexane and aromatic hydrocarbons. 

Combinations of benzophenone and tertiary amine groups, one of which was anchored to a polymer matrix, have also been checked in the photoinduced polymerization and photografting of 2-ethylhexyl methacrylate (EHMA) [52]. Indeed, poly(VBP-co-St)s, in the presence of 7V,Af-diethylaniline (DEA), are found to display lower photoinitiation activity and photografting efficiency than the corresponding copolymers of styrene with 4-(Af,Af-diethylamino) styrene [poly(St-co-DEAS)] in the presence of benzophenone (BP), as revealed by conversion and fractionation data (Table 10). 

Taking into account [81,82] that photodegradation of poly(4,4-dimethyl-l-penten-3-one) [poly(BVK)] and poly(3-methyl-3-buten-2-one) [poly(MIK)] proceeds predominantly through a Norrish type I mechanism via the triplet state (Scheme 15), the above homopolymers have been studied as initiators in the photoinduced polymerization of vinyl monomers such as MMA, St, AN and VAc [83]. 

As reported in Table 15, the kinetic data clearly indicate that the photoinitiation activity of poly(BMOA-co-MtA) is not substantially affected by the content of BMOA co-units along the polymer chain and is of the same order of magnitude as that found for the model compound BMOAc. The absence of a polymer effect in the above photoinitiators has been interpreted [84] in terms of a photodegradation mechanism of the macromolecules involving the ftee radical species anchored to the main chain, even in the presence of acrylic monomers, analogous to what is reported in Scheme 18. Moreover, the induction period of the HDDA/BA photoinduced polymerization increases, on decreasing the content of... 

The main question arising during the analysis of the photoinduced polymerization is how one can know that the dye-coinitiator system, after excitation, forms a species that is able to start polymerization chain reaction. The main factors are ... 

Aliphatic sulfides can be efficient co-initiators for the photoinduced polymerization induced by benzophenone [185, 186]. An exceptionally strong effect was observed for 2,4,6-trimethyl-1,3,5-trithiane (TMT). A model reaction for free-radical formation during photoreduction of an initiator triplet state by a sulfide is the photoreduction of benzophenone by dimethyl sulfide [171, 187-189]. In this process it was established that electron transfer from the sulfur atom to the triplet state of the benzophenone is a primary photochemical step. In this step, radical ions are formed. The overall quantum yields of photoproducts (ketyl radicals and radical anions) are low (Ed) 0.26) in aqueous solution, in the range 0.16-0.20 in mixed water-acetonitrile solution and less then 0.01 in pure acetonitrile. These results suggest that, in organic solvents, back electron transfer within the radical-ion pair to regenerate the reactants is the dominant process. 

Coatings which harden or "cure" by photoinduced polymerization offer a way to reduce air pollution and to conserve energy. With such coatings, the change from liquid to solid is accomplished by an increase in molecular weight rather than by removal of solvent. 

In the same year, a patent was issued to Licari and Crepeau (14) for the photoinduced polymerization of epoxide resins by diazonium tetrafluoroborates for use in the encapsulation of electronic components and the preparation of circuit boards. 

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 fluorine sites to give carboxyl end groups. The photoinduced polymerization of... 

Part of grafting strategies can also be used in association with photoinduced polymerization. As previously stated, grafting from methodology consists of polymerization of monomers initiated from active sites distributed along a polymer backbone or a surface. Photoinitiated polymerizations can well be adapted to this... 

The polymerization rate depends on both the monomer reactivity and the nucleophilicity of the counteranion of the Initiator salt. The order of reactivity in photoinduced polymerization correlates well with the known relative nucleophilicities of the anions, that is, SbF > AsFg > PFg > BF. ... 

The spontaneous topochemical polymerization of (SN)2 to (SN), at 0°C , the most prominent example of a polymerization reaction in S—N chemistry, has been reviewed. Crystals of (SN)j belong to the space group PZ /c and it has been proposed that the a axis of the dimer converts into the b axis of the polymer with chain extension occurring parallel to this axis . On the basis of EHMO calculations, this process is both thermally and photochemically allowed , and the photoinduced polymerization occurs at — 65°C in THF . ESR studies indicate a radical process, but ab initio MO and configuration interaction studies suggest that the polymerization involves a cascade effect initiated by only a few radical centers . 

Cationic photoinduced polymerizations. The predominant approach to Imaging photolnltlated polymerization by free-radical proceBses almost completely restricted the scope of useful monomers to unsaturated compounds and Introduced a limiting factor for the sensitivity by oxygen Inhibition. 

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