Polymeric photocatalysts

An emerging area of study in the area of photocatalysis is the development and investigation of new polymerization photocatalysts. Early work showed that ferricenium, generated by irradiation into the CTTS band of ferrocene in CCI4, was a catalyst for the radical polymerization of vinyl monomers (Equation (29)). ... 

Wang X, Maeda K, Thomas A et al (2009) A metal-free polymeric photocatalyst for hydrogen production from water imder visible light. Nat Mater 8 76-80... 

Another very important visible light-initiated reaction of alkyl aluminum porphyrins is their 1,4-addition to alkyl methacrylates to produce ester enolate species [Eq. (4)]. This enolate then acts as the active species in the subsequent polymerization of the acrylate monomer. For example, Al(TPP)Me acts as a photocatalyst to produce polymethylmethacrylate with a narrow molecular weight distribution in a living polymerization process [Eq. (4)]. Visible light is essential for both the initiation step (addition of methylmethacrylate to Al(TPP)Me) and the propagation... 

The coupling of photocatalysis and polymeric membranes has been carried out using Ti02 as photocatalyst compartmentalized in the reactor by a membrane [39]. Various types of commercial membranes (ranging from UF to NF) and reactor configurations have been investigated [39]. 

Photocatalysts have been also successfully heterogenized in polymeric (catalyti-cally active) membranes. 

We have already seen that photoactive clusters, e.g. CdS, can be introduced into vesicles and BLMs (Sect. 5.2 and 5.3). Similar support interactions are possible with both inorganic and organic polymeric supports. Photoactive colloidal semiconductor clusters can be introduced, for example, into cellulose [164], porous Vycor [165], zeolites [166], or ion exchange resins [167]. The polymer matrix can thus influence the efficiencies of photoinduced electron transfer by controlling access to the included photocatalyst or by limiting the size of the catalytic particle in parallel to the effects observed in polymerized vesicles. As in bilayer systems,... 

Metal Ion and N Co-doped TiCh as a Visible-Light Photocatalyst Co-doped samples prepared by polymerized complex or sol-gel method. Doped N species found to reside at interstitial lattice positions in the host. 316... 

The photochemistry of Cp(arene)Fe + complexes has been extensively examined since the discovery of photochemical arene loss by Nesmeyanov, and this class of compounds has received considerable attention as a cationic photocatalyst for the polymerization of epoxides, dicyanate esters, and styrene among others. As in the case of the bis(arene)metal complexes described above, photolysis is believed to initiate an r] - transformation whereupon solvent enters the open site and thermally displaces the arene ligand. Consistent with this the quantum yields indicate a significant associative component in the ligand displacement. Time-resolved studies indicate that the initial photochemical event occurs on the fs or ps scale. There has been substantial confusion concerning the role of singlet and triplet excited states in the observed photochemistry. Sensitization studies established a... 

These two complexes were also found to be active photocatalysts for the polymerization of ethylene (77, 47, 47a). Irradiation of solutions of either complex under 1 atm ethylene resulted in the rapid formation of high-molecular-weight polyethylene. The rate of ethylene polymerization was increased by the addition of various metal halides prior to photolysis. The mechanism of this reaction was not investigated but the authors postulated that the active species were photogenerated Ziegler-Natta-type catalysts (77). 

A fully aromatic poly(cyclodiborazane) was prepared by hydroboration polymerization between DCA and mesitylborane. Since DCA is a highly fluorescent compound and widely used as a photocatalyst, the obtained polymer is expected to act as a novel type of electron transport material or polymer photocatalyst. The polymerization was carried out as Scheme 5, by adding a slight excess amount of mesitylborane (in THF) to a THF suspension of DCA at room temperature, and stirring the reaction mixture for 12h. The Mn of the polymer was found to be 4400 from the GPC measurement (CHCI3, PSt standards). 

A membrane-induced structure-reactivity trend that may be exploited to achieve selective processes has been recently observed in polymeric catalytic membranes prepared embedding polyoxotungstates, W(VI)-oxygen anionic clusters having interesting properties as photocatalysts, in polymeric membranes [17]. These catalytic membranes have been successfully apphed in the photooxidation of organic substrates in water providing stable and recyclable photocatalytic systems. 

After light absorption by the photocatalyst the next step is photoproduction of free radicals that initiate polymerization of the acrylate unsaturated monomers, oligomers, and polymers contained in the UV-curable coating system. 

The reaction can be used to introduce chromophores onto the surface of polymeric films containing hydroxy groups. When a poly(2-hydroxyethylmethacrylate) (PHEMA) film 14 was immersed in an acetonitrile solution of dimethyl terephthalate (DMTP) and l-methyl-2-arylcy-clopropane 13 (where the aryl group is 1-pyrenyl or 9-phenanthryl) and then irradiated at 313nm, a chemically modified film 15 was obtained that gives absorption and fluorescence spectra similar to 13, and therefore may be useful as a photocatalyst, photochromic device etc. - ... 

Nowakowska et al. also used nitroxide-capped pSSt macroinitiators (in this case, HTEMPO) as the macroinitiator for the preparation of a block copolymer [151]. Their goal was to incorporate vinyl naphthalene (VN) and use the resulting block copolymers as photocatalysts. Although there was little data presented to support the conclusion that the polymerization was living , there was significant spectroscopic evidence to confirm the presence of some pVN in the block copolymer and that the behavior of the block copolymer was different from the statistical copolymer [151]. 

Crivello has published extensively on onium compounds and their function as catalysts (3,4). Generally, such compounds are photocatalysts, but are stable in the absence of light. Irradiation at the proper wavelength leads to a complex photodecomposition of the onium ion in which reactive cation radicals and neutral radicals are formed as short lived intermediates. Also formed are acids, corresponding to the anion of the onium compound, which catalyze the epoxy polymerization. Equations 2 and 3 show the decomposition of iodonium cations. 

Crivello (5) also found that these photocatalysts could be activated thermally when used in combination with copper compounds. For those conditions, Crivello has proposed a cationic polymerization mechanism which does not... 

Figure 10.12 Examples of photocatalysts anchored to polymeric matrices.

Maeda, K. Sekizawa, K. Ishitani, O. A polymeric-semiconductor-metal-complex hybrid photocatalyst for visible-light C02 reduction. Chem. Commun., 2013, 49, 10127-10129. 

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