Photopolymerization triplet

An alternative approach envisages the stimulating idea to produce an all-carbon fullerene polymer in which adjacent fullerenes are linked by covalent bonds and align in well characterized one-, two- and tliree-dimensional arrays. Polymerization of [60]fullerene, with the selective fonnation of covalent bonds, occurs upon treatment under pressure and relatively high temperatures, or upon photopolymerization in the absence of a triplet quencher,... 

Photopolymerization of MMA was also carried out in the presence of visible light (440 nm) using /3-PCPY as the photoinitiator at 30°C [20]. The initiator and monomer exponent values were calculated as 0.5 and 1.0, respectively, showing ideal kinetics. An average value of kp /kt was 4.07 x 10 L-mol -s . Kinetic data and ESR studies indicated that the overall polymerization takes place by a radical mechanism via triplet carbene formation, which acts as the sources of the initiating radical. 

A variety of routes for incorporation of the succinimide group into a polymer backbone are provided by the use of bis(maleimide) monomers (8 Scheme 5). Examples include Michael addition of diamines or dithiols (or H2S) to produce poly(imides) (9) (B-80MI11101), and photopolymerization (via the triplet state) to prepare cyclopolymers (10) (73PAC213) having molecular weights of about 30 000. 

It has been suggested in the literature that the a-amino radical is the species that initiates polymerization [210], This view is supported by our observation that, in spite of the relatively high quenching rate constant of Eosin triplet by triphenylamine (Table 5), the system Eosin-triphenylamine does not sensitize the photopolymerization of multifunctional acrylates. Thus, it is necessary that the amine contains a hydrogen at the a-carbon to be released as a proton after oxidation of the amine by the dye triplet. This deprotonation prevents the back electron transfer and forms a carbon radical that is sufficiently long-lived to be captured by the monomer. 

Fouassier and Chesneau [219] is not consistent with the experimental observations. From the values of the rate constants of triplet decay presented in Table 8, and taking into account that k3/k2 = 0.23 (as determined by Kasche and Lindqvist), we calculate the quantum yield of D + under the polymerization conditions. For Eosin (3 x 10 5 M) and MDEA (0.1 M) the yield of semioxidized Eosin radical is 4 x 10 3 M in the presence or in the absence of 2 x 10 3 M PDO. From the values for the quantum yield of photopolymerization and the molecular weight in the absence of PDO we calculate a quantum yield of initiation between 0.086 and 0.17, the actual value depending on the mode of termination. Therefore, we conclude that formation of a-amino radicals according to Scheme 10 represents only a minor contribution to the quantum yield of initiation observed in the presence of PDO. 

Other carboxylate-dye interactions have been reported. Ethylenediamine tetracarboxylic acid (EDTA) and its salts are well known reductants for a variety of dyes (54,55). The amino-acid N-phenylglycine can be photooxidized and induce polymer formation (26,56,57). Studies of the efficiency of photopolymerization of acrylate monomers by MB/N-phenylglycine combinations as a function of the pH of the medium suggest that either the amino group or the free carboxylate can act as an electron donor for the dye excited state, but that the amine functional-lity is the more efficient coinitiator (10). Davidson and coworkers (58) have shown that ketocarboxylic acids are photode-carboxylated by electron transfer quenching of dye triplet states under anaerobic conditions. Superoxide formation can occur when oxygen is present. 

Table Z Data of the photopolymerization of methyl methacrylate with ketone-amine initiator systems (IE = ionisation potential, 3k H = quenching constant of ketone triplet state with amine in benzene, AV/At = dilatometric contraction) [1]...

The first reaction describes the excitation of uranyl ions. The excited sensitizer can lose the energy A by a non-radiative process (12b), by emission (12c) or by energy transfer in monomer excitation to the triplet state (12d). Radicals are formed by reaction (12e). The detailed mechanism of step (12e) is so far unknown. Electron transfer probably occurs, with radical cation and radical anion formation these can recombine by their oppositely charged ends. The products retain their radical character. Step (12g) corresponds to propagation and step (12f) to inactivation of the excited monomer by collision with another molecule. The photosensitized initiation and polymerization of methacrylamide [69] probably proceeds according to scheme (12). Ascorbic acid and /7-carotene act as sensitizers of isoprene photoinitiation in aqueous media [70], and diacetyl (2, 3-butenedione) as sensitizer of viny-lidene chloride photopolymerization in a homogeneous medium (N--methylpyrrolidone was used as solvent) [71]. 

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. 

The photopolymerization of the polymers studied is influenced by the sensitizer as well as the intensity of light used. Aromatic hydrocarbons, like anthracene and tetracene, are used as singlet sensitizers whereas 1-chlorothioxanthone (CTX) is a triplet sensitizer. From the comparison of the polymerization rates of the systems studied as a function of sensitizer it follows that there is a dependence on the type of sensitizer. As seen from Fig. 2, CTX as triplet sensitizer has been found to be the best sensitizer for vinyl ether systems. 

Feast. Dibenzoylbenzene derivative (114) was irradiated with tetramethylallene for 3.5 minutes in benzene to produce a polymeric structure in which there were 90% oxetane structures incorporated along the backbone. This process constitutes an example of step-growth photopolymerization via the triplet state of an aromatic carbonyl moiety. The products are thermally unstable and readily degraded by acids, but are quite soluble in organic solvents and may possess useful properties as new materials. 

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 type a photopolymerization via the triplet excited state is exemplified by the photopolymerization of N,N -nonamethylene-blsdimethylmaleimlde in dichloromethane solution (190). 

Photochemical generation of the radical cations derived from A -vinylcar-bazole/acceptor charge-transfer complexes and subsequent polymerization is well known. Perhaps somewhat more interesting are the cationic photopolymerizations of styrene and a-methylstyrene. With these monomers of relatively weak electron donor character photolysis of the charge-transfer complexes formed with tetracyanobenzene and pyromellitic dianhydride produces monomer radical cation species from both singlet and triplet states, and the photophysics of the primary processes have been elucidated in some detail. ... 

Stannett and coworkers (17, 18) have shown that anthraquinone-sensitized photopolymerizations on celluloses and nylon involve the triplet state of the dye and semiquinone intermediates in photoinitiation. In addition, anthraquinones (17, 18) were also known to photosensitize degradation of polymers. 

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