Polymer photoactivators

The processes by which unsaturated monomers are converted to polymers of high molecular weight exhibit the characteristics of typical chain reactions. They are readily susceptible to catalysis, photoactivation, and inhibition. The quantum yield in a photoactivated polymerization in the liquid phase may be of the order of 10 or more, expressed as the number of monomer molecules polymerized per quantum absorbed. The efficiency of certain inhibitors is of a similar magnitude, thousands of monomer molecules being prevented from polymerizing by a single molecule of the inhibitor. ... 

The concept of dynamic silver clusters capable to transfer between molecules was also pointed out recently by Ras et al. for silver clusters prepared by photoactivation using PM A A as scaffold [20], Every specific initial ratio of silver ions to methacrylate unit, Ag+ MAA, results in distinct spectral bands (Fig. 12a, b). Thus, an initial ratio 0.5 1 gives an absorption band at 503 nm, whereas a ratio 3 1 gives a band at 530 nm. The shuttle effect was proven when for a given silver cluster solution with ratio 3 1 and absorption at 530 nm, a blue shift was achieved by the addition of pure PMAA. For instance if the added amount of polymer decreases the ratio Ag+ MAA from 3 1 to 0.5 1, the new optical band will match exactly with the band corresponding to a solution with initial ratio 0.5 1, that is 503 nm (Fig. 12c). The explanation given for this blue shift was the redistribution of the existent silver clusters in PMAA chains over the newly available PMAA chains, in other words that the clusters shuttle from partly clusters-filled chains to empty ones. 

Liu and Rauch (2003) of Motorola investigated oligonucleotide probe attachment onto polystyrene (PS), polycarbonate (PC), polymethyl methacrylate (PMMA), and polypropylene (PP) plastic surfaces. They utilized three different immobilization processes SurModics surface modification solution (that allows attachment of adsorbed reactive groups to a surface by photoactivation of polymers at 254 nm). Pierce Reactive-Bind coating solution, and CTAB (cetyltrimethylammonium bromide, a cationic detergent). Not surprisingly, the microarray performances on these plastics varied. 

We have shown how the band structure of photoexcited semiconductor particles makes them effective oxidation catalysts. Because of the heterogeneous nature of the photoactivation, selective chemistry can ensue from preferential adsorption, from directed reactivity between adsorbed reactive intermediates, and from the restriction of ECE processes to one electron routes. The extension of these experiments to catalyze chemical reductions and to address heterogeneous redox reactions of biologically important molecules should be straightforward. In fact, the use of surface-modified powders coated with chiral polymers has recently been reputed to cause asymmetric induction at prochiral redox centers. As more semiconductor powders become routinely available, the importance of these photocatalysts to organic chemistry is bound to increase. 

Examples of metallic organic compounds are listed in Table 1 along with their times to failure. Likewise examples of photoactivators are listed in Table 2. The times to failure obviously vary widely and since only selected samples were analyzed for the actual amounts of degrading agents present, the true amounts incorporated in the polymer film are not known in most cases. 

All of the work reported up to this point has left a big unknown and that is the actual amounts of compounds absorbed into the polymer were not known. Films were prepared using individual solutions of three metallic organic compounds and one photoactivator and the amount of compound determined for each film. The conditions for film preparation and the... 

Princeton Polymer Laboratories has discovered that mixtures of a metallic organic compound and a photoactivator produce a degradative effect in certain polymers that may be as much as ten fold greater than that produced by the individual compounds. The ratio of the components, type of plastic and total amount of additive are some of the important factors that affect the time to failure. Because of this strong synergistic effect, the amount of additive required is quite small, thus resulting in a very low cost, which has been estimated at less than 0.1 cent per pound of finished plastic. 

As expected, solutions containing mixtures of nanocrystal sizes exhibited spectral profiles corresponding to the sum of the spectra of the individual sizes (Fig. 5.3). The initial PL peak positions of 2.8 and 5.6-nm CdSe nanocrystals in toluene solution were at 541 and 636 nm, respectively. We incorporated the CdSe nanocrystal mixtures into polymer matrices rationally selected based on polymer-sensing properties (Table 5.1). CdSe nanocrystals exhibited spectral shifts upon their immobilization in polymer films and photoactivation with the 407-nm diode laser. Photoactivation is an important aspect of the performance of the CdSe and other semiconductor crystals as chemical sensor materials.16 Different spectral shifts of steady-state PL emission and different emission intensity were observed upon incorporation of the 2.8 and 5.6-nm CdSe nanocrystals in polymer films as illustrated in Fig. 5.4. Polymer 2 (PMMA) was selected for the detailed evaluation because it has been used previously as a matrix for incorporation of CdSe nanocrystals.1636... 

Fig. 5.4 Positions (a) and intensities (b) of PL peaks of CdSe nanocrystals of 2.8 and 5.6-nm in polymer films 1-9 upon photoactivation. Initial PL peak positions of 2.8 and 5.6-nm CdSe nanocrystals in toluene solution are at 541 and 636nm, respectively...

Figure 5.5 illustrates an example of PL spectra, where 2.8 and 5.6nm diameter nanocrystals were incorporated into a PMMA film. The 2.8 nm nanocrystals had an initial emission maximum in toluene at 541 nm, which was slightly red-shifted to 543 nm upon an addition of a polymer solution. A strong blue shift to 519nm was observed in a dry film containing 2.8 nm nanocrystals and PMMA, with an additional blue shift to 504nm upon an extended photoactivation. As shown in Fig. 5.6, the peak shift of 5.6-nm nanocrystals in PMMA was less dramatic upon photoactivation, but mirrored the behavior of the 2.8-nm nanocrystals. The peak positions for... 

R. D. Sanner, R. G. Austin, M. S. Wrighton, W. D. Honnick, C. U. Pittman, Jr., Photoactivation of Polymer-Anchored Catalysts. Iron Carbonyl Catalyzed Reactions of Alkenes, Chapter 2 in Interfacial Photoprocesses Energy Conversion and Synthesis, M. S. Wrighton, Ed., pp. 13-26, Advances in Chemistry Series 184, ACS Publishers, 1980. 

The intrinsic photoconductive property was found for a cobaltacyclopentadine polymer, 32.68 Photoresponse of current-voltage (I-V) characteristics for ITO/32/ITO indicated that the polymer had a low conductivity in the dark and the photocurrent was four times larger than the dark current. It was proposed that the metal -character orbitals localized at cobalt sites, plus their energy level lying between valence and conduction bands acted as the trapping sites of holes generated by photoactivation of electrons from the valence band to the conduction band. 

Udal tsov, A.V. (1997) Characterisitcs of donor-acceptor complexes formed in porphyrin-polymer systems and their photoactivation in electron transfer photoreaction, J. Photochem. Photobiol. B Biol., 37, 31-39. 

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