Photo-induced polymerization

Since the optical transitions near the HOMO-LUMO gap are symmetry-forbidden for electric dipole transitions, and their absorption strengths are consequently very low, study of the absorption edge in Ceo is difficult from both an experimental and theoretical standpoint. To add to this difficulty, Ceo is strongly photosensitive, so that unless measurements arc made under low light intensities, photo-induced chemical reactions take place, in some cases giving rise to irreversible structural changes and polymerization of the... 

Reaction Mechanism of Vinyl Polymerization with Amine in Redox and Photo-Induced Charge-Transfer Initiation Systems... 

A substantial number of photo-induced charge transfer polymerizations have been known to proceed through N-vinylcarbazole (VCZ) as an electron-donor monomer, but much less attention was paid to the polymerization of acrylic monomer as an electron receptor in the presence of amine as donor. The photo-induced charge-transfer polymerization of electron-attracting monomers, such as methyl acrylate(MA) and acrylonitrile (AN), have been recently studied [4]. In this paper, some results of our research on the reaction mechanism of vinyl polymerization with amine in redox and photo-induced charge transfer initiation systems are reviewed. 

III. PHOTO-INDUCED CHARGE TRANSFER POLYMERIZATION OF VINYL MONOMERS... 

Using the first-principles molecular-dynamics simulation, Munejiri, Shimojo and Hoshino studied the structure of liquid sulfur at 400 K, below the polymerization temperature [79]. They found that some of the Ss ring molecules homolytically open up on excitation of one electron from the HOMO to the LUMO. The chain-like diradicals S " thus generated partly recombine intramolecularly with formation of a branched Sy=S species rather than cyclo-Ss- Furthermore, the authors showed that photo-induced polymerization occurs in liquid sulfur when the Ss chains or Sy=S species are close to each other at their end. The mechanism of polymerization of sulfur remains a challenging problem for further theoretical work. 

Fig. 28 Raman spectra of polymeric sulfur (S ) prepared by various methods [109,173], of large disordered rings (S ) [182], and of photo-induced amorphous sulfur (a-S) [119], respectively. The spectrum of a-S has been smoothed for clarity. The position of the stretching vibration of a-S is pressure-shifted to higher wavenumbers. The very weak signals in the spectra of Sj, at ca. 150 and 220 cm are probably caused by the presence of Sg. In addition, the weak shoulder at ca. 470 cm observed in spectra of Sj, may originate from Sg, too...

At least five high-pressure allotropes of sulfur have been observed by Raman spectroscopy up to about 40 GPa the spectra of which differ significantly from those of a-Sg at high pressures photo-induced amorphous sulfur (a-S) [57, 58, 109, 119, 184-186], photo-induced sulfur (p-S) [57, 58, 109, 119, 184, 186-191], rhombohedral Se [58, 109, 137, 184, 186, 188, 191], high-pressure low-temperature sulfur (hplt-S) [137, 184, 192], and polymeric sulfur (S ) [58, 109, 119, 193]. The Raman spectra of two of these d-lotropes, a-S and S, were discussed in the preceding section. The Raman spectra of p-S and hplt-S have only been reported for samples at high-pressure conditions. The structure of both allotropes are imknown. The Raman spectrum of Se at STP conditions is discussed below. 

Fig. 31 Evolution of the Raman spectra of a high-pressure and photo-induced sample of Se while decreasing the pressure at ca. 300 K [109]. The spectrum at 3.9 GPa shows the onset of the transformation S6 p-S. The asterisks indicate the Raman signals typical for p-S whereas the peaks of two stretching vibrations of p-S coincide with those of Se at about 458 cm and 471 cm (not indicated by asterisks). The Raman spectrum of the sample recovered at ambient pressure (0 GPa) is evidently a superposition of the spectra of a-Sg and polymeric sulfur, Sj, arrows indicate plasma lines of the Ar ion laser at 515 nm, which have been used for calibration). For Raman spectra under increasing pressure, see Fig. 23 in [1] and references cited therein...

Figure 5.2. Gel permeation chromatogram (GPC) of the liquid silicon precursor for Si film formation for (a) cyclopentasilane (CPS) and (b) UV-irradiated CPS, both of which were diluted with toluene (1 vol.%) before GPC measurements. The UV-irradiation conditions were 405 nm, 100mW/cm2, and a 10-min irradiation for 1cm3 of CPS. The broad peak around Mw = 2600 corresponds to polysilanes of various molecular weights, as a result of the photo-induced polymerization of CPS. [Reproduced with permission from Ref. 10. Copyright 2006 Nature Publishing Group.]...

Dual Photo/Thermal Initiation Studies. A series of studies were performed using reactive formulations containing both a photoinitiator and a thermal initiator dissolved in the Derakane resin. The objective of these studies was to investigate a dual cure strategy in which the heat liberated by the photo-induced polymerization leads to the production of additional active centers by the dissociation of a thermal initiator. In this way, the dual cure strategy could offer both the temporal control of the start of the reaction afforded by the photopolymerization, as well as enhanced reaction rate and completeness of cure provided by the thermal initiation. 

Atmospheric oxygen may initiate in situ polymerization [55] as well as hght as exemplified by the photo-induced isomerization/polymerization of (Z,Z)-muconate anions in the gallery space of LizAl LDH [61] and the photoisomerization of indolinespirobenzopyran [62]. 

In the laboratory, the source of radiation may be a high-intensity mercury lamp. However, for simple test purposes, sunlight is quite suitable. Even a lightly overcast sky furnishes sufficient UV radiation for photo-induced polymerizations. Naturally, bright sunlight is more effective. 

The information provided by the 31P-NMR spectra of DOPE/16 as a function of temperature and the extent of polymerization was critical to the characterization of the nature of the lipid structures responsible for the destabilization of the photolyzed DOPE/16 vesicles [73]. The progressive appearance of an isotropic NMR signal at the expense of the lamellar signal (Fig. 14) indicated that a lipid structure with isotropic symmetry was associated with the photo-induced leakiness of DOPE/16 vesicles. The enriched domains of PE facilitates the interaction and formation of intermediate lipid structures between bilayers, with the eventual development of an ILA(s) that connect the bilayers of an... 

Detailed studies on the decomposition of organic peroxides are of fundamental interest and of high importance in polymerization reactions. The time-scales of intermediate radical formation and of their subsequent decomposition determine process parameters such as the initiator efficiency in radical polymerizations. An improved understanding of the mechanism and dynamics of photo-induced fragmentation is achieved by systematic investigations in which quantum-chemical calculations are carried out in conjunction with highly time-resolved experiments. 

Organomineral peroxides also undergo thermal and photo-induced homolysis, yielding free radicals that are effective for initiating polymerization of vinyl monomers. 

This review has demonstrated that a wide variety of organic and organometallic electron donors and a smaller number of electron acceptors can function as coinitiators for the photo-induced polymerization of monomers using photoreducible or photooxidizable dyes as absorbers of incident light. These compositions can be used for a number of applications in the printing, graphics arts, and electronics industries where sensitivity to visible wavelengths is necessary. 

An MIP-QCM chemosensor has been devised for determination of thymine [141]. A new functional monomer, vis. methacryloylamidoadenine (MA-Ade), as well as the EDGMA cross-linker and AIBN initiator have been used for preparation of the thymine-imprinted polymer film by photo-induced polymerization. This film was deposited on the QCM resonator, featuring homogeneous binding sites for... 

Phospholipid-bilayer membrane 96, 102 Photo-induced charge carriers 97 Photocrosslinking 805 Photocurable polymeric 107 ac Photocurrent 89 Photocurrent-voltage curve 91 Photolithographic 828 Physical adsorption 808 Pipecolic acid 64, 67 Pirimiphos-methyl 531, 687, e299 Plasma e5 water 6... 

A charge-transfer complex (CT complex or CTQ is involved in all thermal charge-transfer polymerizations and in most of the photo-induced charge-transfer copolymerizations. Mixing of a donor (D), such as VCZ, with an acceptor, such as tetracyanoethylene (TCNE), immediately produces color [31] due to the formation of a CT complex ... 

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. 

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