Jacques Lemaire

Photochemistry and environment

The transformation mechanisms of the three monochlorophenols in dilute aqueous solution are studied through the comparison of direct excitation and the use of photoexcited phenol. The primary photochemical process is a C‐Cl bond scission which is not influenced by oxygen. However, the position of the chlorine on the ring strongly influences the nature of this scission. The direct irradiation of 3‐chlorophenol yields to resorcinol whatever the pH, and the reaction can be photosensitized by phenol. A mechanism of direct photohydrolysis from the triplet state is proposed. Under conditions of direct excitation, 2‐chlorophenol can be deprotonated leading to a cyclopentadienic acid or hydrolysed to pyrocatechol. In the presence of excited phenol, radical mechanisms produce biphenyl derivatives. With 4‐chlorophenol, photohydrolysis appears to be a minor process, whereas the major pathway is a radical mechanism which can be induced by phenol excitation.

Photochemistry and environment. VII- The photohydrolysis of chlorobenzene: Studies of the excited state involved

Abstract Chlorobenzene excited in dilute aqueous solution is transformed quantitatively into phenol ( ф= 0.10±0.02 ). This reaction involves a triplet state, since it is quenched by biacetyl and sensitized by ketones. Based on quenching experiments, the lifetime of the lowest triplet of chlorobenzene was measured as 30 ns. From the sensitized phosphorescence of biactyl, the quantum yield of intersystem crossing (S1→T1) was found to be 0.6 ±0.1. The photophysical characteristics of chlorobenzene are identical in acetonitrile ( фisc S 1 → T 1 = 0.50±0.03 - τ T 1 = 35 ns ). Photosensitization of the photohydrolysis is observed with acetone and acetone D6 in aqueous solution. The quantum yield of the sensitized photohydrolysis is limited to 0.06±0.01. Hexadeuterated acetone appeared to be a more specific photosensitizer than acetone.

Degradations photothermiques de polyethylenes reticules. II.—Action photocatalytique d'oxydes metalliques

Resume Levolution des spectres infra-rouges dun echantillon de poly-ethylene reticule soumis a une oxydation photocatalysee permet de comparer les modifications chimiques du polyethylene a celles qui sont induites par une oxydation thermique ou photothermique. Des mesures en lumiere monochromatique permettent devaluer les rendements quantiques initiaux dapparition des cetones aliphatiques et leurs variations avec differents parametres macroscopiques (longueur donde dexcitation, intensite lumineuse, teneur en oxyde a action catalytique, nature des oxydes). En particulier, dans nos conditions experimentales, les oxydes de titane et de zinc manifestent des proprietes photocatalytiques voisines. Les energies dactivation des reactions photocatalysees ont pu etre mesurees en lumiere polychromatique. Le role conjugue dun oxyde absorbant a action photocatalytique et dun anti-oxydant quinoleique permet dassurer une bonne protection du polyethylene reticule contre la photodegradation oxydante.

Octene-1 linear low density polyethylene photooxidation

Abstract Low-temperature thermooxidation and photooxidation at long wavelengths ( λ > 300 nm) of octene-1 linear low density polyethylene have been examined. It is shown that both oxidations proceed according to mechanisms which are very similar to those thought to occur in free radical low density polyethylene presenting a low content of vinylidene unsaturation. Hydroperoxidation in the α-position to the vinylidene groups gives rise to isolated hydroperoxide groups that absorb at 3550 cm −1 and are stable at 85°C. A parallel hydroperoxidation occurs on the saturated chain affording hydrogen-bonded hydroperoxides which are fairly unstable at 85°C. In photooxidation at long wavelengths both hydroperoxides are produced and readily photolysed. Variations in the number of branch points which provoke large variations in density have no significant influence on the photooxidation rate.

Cadmium sulphide photocatalytic oxidation of crosslinked polyethylene

Abstract The photo-oxidation of crosslinked polyethylene (XLPE) modified by cadmium sulphide is investigated and compared to a photothermal oxidation and a zinc oxide-photocatalytic oxidation of XLPE. Cadmium sulphide is shown to afford a photochemical protection of hydroperoxides and ketones, but to a lesser extent than zinc oxide. Cadmium sulphide is also shown to act as a poor photocatalyst. The pigment absorbs wavelengths

Etude en phase polymere de la reticulation chimique du polyethylene basse densite

Resume Une etude analytique detaillee, par spectrophotometrie infra-rouge conventionnelle et par microsonde Raman, de la reticulation du polyethylene “basse densite” effectuee a laide de peroxyde de dicumyle ou de ditertiobutyle, permet datteindre la probabilite de dissociation dans la matrice polymere des radicaux primaires issus du peroxyde. Le devenir de ces radicaux peut etre modifie par des coagents de reticulation presentant, soit une fonction methacrylique, soit un groupement allylique. Une etude par spectrophotometrie infra-rouge associee a une etude cinetique realisee avec un rheometre, permet alors de preciser les interventions tres differentes de ces deux classes de coagents dans le mecanisme de reticulation du polyethylene.