Polymer Photochemistry最新文献

Photolysis of poly[1-(4-substituted phenyl]-2-propen-1-ones] in film of thickness 5–10 μm was followed by automatic viscometry, lowangle laser light scattering (LALLS) and gel permeation chromatography (GPC) under irradiation at 313 and 285·8 nm in air. The substituents in position 4 of the benzene ring were: H, fluoro, chloro, methoxy and ethyl. The monitoring of degradation by automatic viscometry and LALLS enabled determination of the Mark-Houwink parameters α and K for the above polymers in chlorobenzene at 25°C as follows. H: α = 0·76 ± 0·03, K = 3·8 × 10⁻³; 4-F: α = 0·69 ± 0·02, K = 6·1 × 10⁻³; 4-Cl: α = 0·79 ± 0·03, K = 1·4 × 10⁻³; 4-C₂H₅: α = 0·85 ± 0·03, K = 1·3 × 10⁻³; 4-OCH₃: α = 0·75 ± 0·06, K = 2·3 × 10⁻³. The plots of main chain scission versus time were curved, which indicates inhibition at the later stage of degradation. The initial quantum yields of main chain scissions are about 0·08 except for the 4-methoxy derivative, in which case strong oxygen quenching causes the quantum yield to lie one order of magnitude lower. The unsaturated ketone formed during photolysis quenches the main chain scissions with Stern-Volmer constants which lie within the range 50–150 mol⁻¹ for the polyketones under study.

Low molecular weight dioxane-lignin has been submitted to flash photolysis in deaerated solutions. The transient spectra show absorption maxima that can be identified with ketyl and phenoxy radicals, confirming that the initial mechanism is the abstraction of a phenolic hydrogen by a triplet excited carbonyl. The second-order decay rates, ca $\text{10}{}^9\text{M}{}^{\mathrm{-1}}\text{s}{}^{\mathrm{-1}}$, are also consistent with the nature of those radicals. Flash photolysis of model compounds vanillin and p-hydroxybenzaldehyde showed similar behaviour. The photolysis of a model compound (Ether I) with no carbonyl or phenol groups confirmed the occurrence of β-scission as the photolytic pathway.

The effect of light irradiation in the chiral complex of poly(l-lysine) (PLL) with Methyl Orange (MO) was investigated by means of absorption and circular dichroism (c.d.) measurements. Upon irradiation with ultraviolet or visible light, the magnitudes of the induced c.d. of the PLL-MO complex decreased together with a decrease in skew dimeric dye species. As more photons were irradiated, the induced c.d. decreased much faster. The lowermolecular-weight complex and higher temperature caused faster decrease of the induced c.d. The change was irreversible under dark or re-irradiation conditions.

In order to search for photoradical polymerization systems that are not subject to oxygen inhibition, photoradical polymerization of the N,N-dialkylaminoethyl methacrylate—aryl ketone system has been studied. It was found that photoradical polymerization of this system, where N,N-dialkylaminoethyl methacrylate functions both as a co-initiator and as a monomer, is accelerated in the presence of oxygen. The polymerization rate was faster with N,N-diethylaminoethyl methacrylate than with the N,N-dimethyl analogue. Crosslinking occurred rapidly in the presence of a multifunctional acrylate monomer, resulting in the formation of gels. As compared with photopolymerizations of acrylate monomers using non-polymerizable, low-molecular-weight alkylamine-aryl ketone combinations as photo-initiators, the present system has an advantage that less low-molecular-weight amine remains in the exposed area because it polymerizes.

Solution polymerization of methyl methacrylate (MMA) was carried out in the presence of visible light (440 nm) using β-picolinium p-chlorophenacylide as a photoinitiator at 30°C. The initiator and monomer exponent values were calculated to be 0·5 and 1·0 ± 0·01, respectively. An average value of $\text{k}{}_{\mathrm{p}}{}^2\text{k}{}_{\mathrm{t}}$ was 4·07 × 10⁻². The polymerization was retarded in the presence of hydroquinone; however, a nonpolar solvent (cyclohexane) enhanced the rate of polymerization. Kinetic data and ESR studies indicate that the overall polymerization takes place by a radical mechanism via triplet carbene formation which acts as a source of free radicals.