Ivan Kmínek

Long wavelength photoinitiated cationic polymerization using diphenyliodonium salt and catena-poly (phenyl-4-phenylphenylsilicon)

Abstract The photolysis of catena-poly (phenyl-4-phenylphenylsilicon) at λinc = 365–400 nm leads to products (probably free radicals) which are oxidized by diphenyliodonium ions. The resulting ionic species is capable of readily initiating the cationic polymerization of tetrahydrofuran, n-butyl vinyl ether, cyclohexene oxide and N-vinylcarbazol.

Photochemical cationic polymerization of cyclohexene oxide in solution containing pyridinium salt and polysilane

Abstract The cationic polymerization of cyclohexene oxide (CHO) is initiated upon u.v. irradiation (λinc > 350 nm) of dichloromethane solutions containing N-ethoxy-2-methyl pyridinium hexafluorophosphate (EMP+PF6−) and poly(methyl phenyl silane) or poly(dimethyl diphenyl silane). A feasible initiation mechanism involves the photogeneration of silylene biradicals and silyl radicals by extrusion and main-chain scission of the polysilanes, respectively. Oxidation of these radicals by EMP+ ions yields reactive cations capable of initiating the polymerization of CHO.

Modification of poly(methyl phenylsilane) by the attachment of π-conjugated substituents. Synthesis and photochemical studies

Abstract Poly(methyl phenlylsilane), PMPSi, has been partly chloromethylated and the chloromethylated PMPSi has been formylated via the pyridinium salt and the nitrone (Kroehnke transformation). Subsequently, the aldehyde groups have been converted by condensation with aniline, 4-aminoazobenzene and 4-nitroaniline to the corresponding Schiff bases. Also, the aldehyde groups have been converted to 2,4-dinitrophenylhydrazone groups. The extension of the π-conjugated system of the phenyl groups by attachment of chromophoric groups does not cause a detectable shift of the u.v. absorption band of the δ-conjugated backbone. Apart from the formylated derivative, all substituted polysilanes are much less photolabile than the original PMPSi and the partly chloromethylated PMPSi. Emission studies have revealed that the fluorescence of PMPSi is strongly quenched by the attached chromophores.

Free charge carrier formation in polymers under illumination

In addition to many extrinsic processes, free charge carriers in polymers can be photogenerated intrinsically via an intermediate stage of bound ion-pairs formed in consequence of band-to-band transitions or exciton autoionization. The excitation can be realized within the framework of (i) a chemical bond in the polymer backbone or (ii) a side group skeleton. An electron—hole pair with a short separation distance created by light on the same polymer chain or on the same side group usually recombines geminately very fast and does not contribute to the photocurrent. An inter- or intrachain charge transfer or transfer of the electron from the main chain to a side group is necessary to form a more stable ion-pair. Dissociation of the ion-pairs by Brownian motion subjected to a combination of Coulomb and applied electrical field, into free charge carriers, can be described in terms of the Onsager theory of geminate recombination. The photogeneration yield can be improved by an electron acceptor doping or by an electron accepting groups attached to side chains.

Charge carrier photogeneration in poly(methylphenylsilylene) and its derivatives with π-conjugated side groups

Abstract The photogeneration of charge carriers in thin films of poly(methylphenylsilylene) (PMPSi) and its derivatives with attached pendant π-conjugated chromophores was investigated as a function of the electric field and temperature by the photoinduced discharge current method. Six new substituted polysilylenes were used in this study. The electric field dependence of photogeneration efficiency suggests that the Onsager theory of geminate recombination is a suitable model for the description of the electron-hole pair dissociation during the photogeneration process in these polymers. The most effective separation distances of electron-hole pairs participating in the photogeneration process were found to be 1.8–3.5 nm and 0.9–1.75 nm for low and high electric fields, respectively.

A water-soluble poly(methylphenylsilylene) derivative as a photoinitiator of radical polymerization of hydrophilic vinyl monomers

SummaryRadicals formed during photoscission of the pyridinium salt of partially chloromethylated poly(methylphenysilylene) (Q-PMPSi) in aqueous solution are capable of initiating the polymerization of hydrophilic vinyl monomers, e.g. acrylic and methacrylic acid, acrylamide, 2-hydroxyethyl methacrylate and 1-vinyl-2-pyrrolidone.

Charge carrier photogeneration on some substituted polyacetylenes

The photogeneration of charge carriers was studied with the following polymers: poly-[N-(2-propinyl)-phenothiazine] (PPPT) and copolymers of N-(2-propinyl)-carbazole with N-(2-propinyl)-phenothiazine (PCz+PPT) and N-(2-propinyl)-carbazole with phenylacetylene (PCz+PA). In the case of PCz+PA, the experimentally found dependence of the photogeneration efficiency on the strength of an externally applied electric field could be well fitted with the curve calculated on the basis of Onsagers model of geminate recombination. In the cases of PPPT and PCz+PPT, on the other hand, the experimental values deviated strongly from the theoretical curve at field strengths between 106 and 107 V/m.Equal values for the separation distancer0 and the primary charge carrier yieldη0 were found for all polymers:r0=2.0 nm andη0=0.20 atλinc=254 nm;r0=2.5 nm andη0=0.15 atλinc=355 nm.With PPT and PCz+PPT a strong dependence of the electric resistance on the humidity content of surrounding air was observed.

Poly(silylene)s: Effect of Polar Acceptor Side Groups on the Charge Carrier Photogeneration and Transport

Abstract The influence of polar electron-acceptor chromophores (both admixed and covalently bonded to the polymer backbone) on photoelectrical properties of poly[methyl(phenyl)silylene] was studied. Whereas the charge carrier mobility decreased for any concentration of the chromophore, the photogeneration efficiency increased for concentrations up to about 4 mole %. For higher concentrations, some decrease was observed. The decrease in mobility was ascribed to the electron-dipole interaction and to the broadening of the density-of-states distribution affecting the hopping transport.

New two-step synthesis of N-(2-ethylhexyl)-2,7-diiodocarbazole as a monomer for conjugated polymers

A reasonably efficient two-step synthesis of N-(2-ethylhexyl)-2,7-diiodocarbazole is presented. In the first step, the 4,4′-diiodobiphenyl was nitrated to a mixture of 4,4′-diiodo-2-nitrobiphenyl and 4-iodo-4′-nitrobiphenyl. In the second step, the mixture of these compounds was converted by simultaneous carbazole ring closure and N-alkylation to the N-(2-ethylhexyl)-2,7-diiodocarbazole by means of tris(2-ethylhexyl) phosphite. The synthesis represents a shorter alternative to the known more tedious procedures. The prepared compound is a suitable monomer for synthesis of conjugated polymers for optoelectronic applications.

4-Nitrophenylhydrazones of partially formylated polystyrene, their properties and crosslinking

SummaryThe title polymers were prepared as potential non-linear optical materials with a degree of substitution 3–13 mole %. During UV-irradiation and/or heating near the glass transition temperature these polymers crosslink through the hydrazone groups and their UV-vis absorption band diminishes. Degrees of crosslinking of swollen samples were estimated. A possibility of simultaneous crosslinking and poling of these polymers is discussed.