Sensitized photo-oxidation of en-containing EPDM rubber

Terpolymers of ethylene, propylene and ethylidene norbornene (EN) rapidly react with singlet oxygen generated by dye sensitization. The singlet oxygen attacks the ethylidene double bond forming at least two different allylhydroperoxides. The instability of these hydroperoxides causes crosslinking of the photo-oxidized terpolymers at relatively low temperatures. Reduction of the hydroperoxide groups with triphenylphosphine yields a thermally stable hydroxylated polymer. Terpolymers of ethylene, propylene and dicyclopentadiene (DCPD) are inert towards singlet oxygen.     

Accelerated aging and stabilization of radiation-vulcanized EPDM rubber

The effect of different antioxidants and their mixtures on the thermal aging and accelerated weathering of γ-radiation vulcanized EPDM rubber in presence of crosslinking coagent, was investigated. The compounds used were either a synergistic blend of phenolic and phosphite antioxidants, i.e. 1:4 Irganox 1076: Irgafos 168 or a blend of arylamine and quinoline type antioxidants, i.e. 1:1 IPPD:TMQ, at fixed concentration. Tinuvin 622 LD hindered amine light stabilized (HALS) was also used. The response was evaluated by the tensile strength and elongation at break for irradiated samples after thermal aging at 100°C for 28 days and accelerated weathering (Xenon test) up to 200 h.     

Metastable ion study of organosilicon compounds. Part III—trimethoxyphenylsilane

The unimolecular decomposition of trimethoxyphenylsilane (1) was investigated by mass-analysed ion kinetic energy (MIKE) spectrometry, deuterium-labelling studies and from high resolution data. The characteristic fragmentations of metastable molecular ion of 1 were losses of C₆H₆ and C₇H₇^· with rearrangements. Almost complete H/D scrambling occurred in the molecular ion prior to these decompositions. The other important fragmentation routes corresponded to expulsions of CH₃O^· and C₆H₅^·. These fragmentations were followed by consecutive elimination of an H₂CO molecule, as commonly observed in the mass spectra of alkoxysilanes. In these fragmentation processes, H/D scrambling increased as the internal energy of the molecular ion was lowered. The fragmentations of 1 were compared with those of its carbon analogue, α,α,α-trimethoxytoluene.     

Radikale aus Polymethin-Farbstoffen: III—Polymethinoxonole

The e.s.r. spectra of neutral radicals in solution obtained by electrolytic reduction of a series of dinuclear polymethine oxonols have been interpreted. The values of the e.s.r. coupling constants, ¹H n.m.r. results from the parent compounds and HMO calculations are used to discuss conformation, π-electron density distribution and C? C bond lengths in these radicals.     

Phosphoranes par fermeture trans-annulaire: III—Conformation—configuration—labilité

The analysis of the n.m.r. spectra of the two isomers of 1,6,6-trimethyl-3-phenyl-2,8-dioxa-5-aza-1-phospha ^vbicyclo[3.3.0]octane allows us to propose a conformation for each 5-membered ring and for the configuration of the phosphorus atom. The lability of the H? P proton, revealed by high temperature experiments and by P? H ? P? D exchange, is significantly different in the two isomers.     

The chemical effects of photo-oxidation on butadiene rubber

The photo-oxidation of butadiene rubber (BR) was studied in detail using films of a commercially available material, containing 33% cis-1,4-, 56% trans-1,4- and 12% 1,2-vinylic units. The spectral data of the products showed that during photo-oxidation the following functional organic groups are formed: hydroperoxides, alcohols, ketones, carboxylic acids and esters. ¹³C-NMR spectroscopic evidence for the formation of epoxides is presented. By comparison with spectra of model compounds, signals were assigned to ester and carboxylic acids in the allylic position of the oxidized polymer. The presence of ketones and carboxylic acids was also proved by chemical methods. It could be demonstrated that the formation of ketones, carboxylic acids and esters occurs from the photolysis of the OO bond of the hydroperoxides. Kinetic measurements with films containing benzophenone showed a strong increase in the quantum yield for the formation of products in comparison with the pure polymer sample. The opposite effect is observed with films containing the commercially used anti-oxidant, 2,6-di-t-butyl-p-hydroxytoluene. Also, the quantum yield for formation of the oxidation products decreases with increase in the light intensity. This result shows that cross-linking plays an important role in the photo-degradation of BR.     

The chemical effects of photo-oxidation on isoprene rubber

There has been a detailed study of the non-volatile products of the photo-oxidation of two kinds of cis-1,4-polyisoprene, viz. natural and synthetic (98% cis), by irradiating films in the solid state under ambient conditions. I. R., ¹H- and ¹³C-NMR and chemical methods indicated the formation of the following functional groups: hydroperoxides, alcohols, ketones, aldehydes, epoxides and carboxylic acids. There is also evidence for the formation of esters and ethers. Viscometric measurements showed that chain-scission predominates over cross-linking for irradiation of toluene solutions of the rubbers. Photo-oxidation kinetic measurements for films containing singlet oxygen quenchers, such as 1,2-diazabicyclo[2,2,2]octane and diphenyl-p-isobenzofuran, showed that singlet oxygen is not responsible for the initiation of the environmental photo-oxidation of cis-1,4-polyisoprene. Increasing the number of successive coagulation steps, in the purification of natural rubber, decreased its stability, confirming the presence of an intrinsic natural stabilizer. Synthetic isoprene rubber showed less stability than natural rubber.     

Doubly charged ion mass spectra: III—N-alkanes

Doubly charged ion mass spectra have been obtained for 15 n-alkane hydrocarbons. Spectra were measured using a Nier-Johnson geometry Hitachi RMU-7L mass spectrometer operated at 1.6kV accelerating voltage. Fragment ions, which resulted from C? C bond rupture and extensive H loss, dominated the spectra. Molecular ions have not been observed. The most intense ions in the doubly charged ion mass spectra of n-alkanes were [C₂H₄]²⁺, [C₃H₂]²⁺, [C₄H₃]²⁺, [C₅H₂]²⁺, [C₆H₆]²⁺, [C₆H₈]²⁺, [C₇H₆]²⁺, [C₇H₈]²⁺, [C₈H₆]²⁺ and [C₈H₈]²⁺. Appearance energies for forming the prominent doubly charged fragment ions have been measured and range from 27.5 eV to energies greater than 60eV. A geometry optimized SCF approach has been used to compute the energies and structures of prominent ions in the doubly charged mass spectra.     

Radiolytic unsaturation decay in polyethylene. Part III—the effect of certain chain transfer agents

Small amounts of certain halogenated compounds are found to have, at most, only a slight enhancing effect on the radiolytic decay rates of added poly-unsaturated compounds in polyethylene, but significantly increase the elastic modulus at 433 K (melt modulus) obtained thereby. Experiments with model chlorine-containing additives suggest that this increase is due to a more random distribution of polymer and monomer mediated crosslinks in the polymer, that it does not result from a significant increase in crosslinking and that it is mediated by chlorine atoms, in a similar manner to radiolytic hydrogen atoms, through facilitation of long range free radical migration. Although low molecular weight chloro-paraffins inhibit radiolytically induced growth of melt modulus in monomer containing polyethylenes, even very small additions of chlorinated polyethylenes, which form a separate phase, increase the melt modulus. This again indicates that the active species is the chlorine radical.     

Carbon-13 nuclear magnetic resonance spectra: III—2-heteroadamantanes

The ¹³C n.m.r. spectra of some 2-heteroadamantanes and 1-substituted 2-heteroadamantanes are reported. The influences of the heteroatoms in the adamantane framework, and those of the substituents attached to it, on the ¹³C chemical shifts of the adamantane carbons are investigated and compared with related compounds such as the corresponding heterocyclohexane derivatives and 2-mono- and 2,2-disubstituted adamantanes. The nonadditivity of the substituent effects for 1-substituted 2-heteroadamantanes, especially for the geminally substituted carbons, is substantially confirmed. In addition, the influences of a missing CH₂ group and of NCH₃ carbons upon the ¹³C chemical shifts of the carbons in the adamantane system are described.     

13C NMR spectra of thiophenes. III—Bromothiophenes

The ¹³C NMR spectra of all bromo substituted thiophenes have been obtained at 15·085 MHz. ¹³C signal assignments for monobromothiophenes have been confirmed by comparison with the spectra of their partially deuteriated derivatives; a revision is made for the assignment in 3-bromothiophene. The substituent effect of bromine is generally additive for the ¹³C chemical shifts. The substituent effect on various types of ¹³C, ¹H coupling constants is also obtained and discussed.     

Semiconductor Photocatalysis—Mechanistic and Synthetic Aspects

Preceding work on photoelectrochemistry at semiconductor single‐crystal electrodes has formed the basis for the tremendous growth in the three last decades in the field of photocatalysis at semiconductor powders. The reason for this is the unique ability of inorganic semiconductor surfaces to photocatalyze concerted reduction and oxidation reactions of a large variety of electron‐donor and ‐acceptor substrates. Whereas great attention was paid to water splitting and the exhaustive aerobic degradation of pollutants, only a small amount of research also explored synthetic aspects. After introducing the basic mechanistic principles, standard experiments for the preparation and characterization of visible light active photocatalysts as well as the investigation of reaction mechanisms are discussed. Novel atom‐economic C? C and C? N coupling reactions illustrate the relevance of semiconductor photocatalysis for organic synthesis, and demonstrate that the multidisciplinary field combines classical photochemistry with electrochemistry, solid‐state chemistry, and heterogeneous catalysis.     

Mass spectra of liquid crystals. III—phenylpyrimidine derivatives

The 70 eV electron impact mass spectra of 34 1-phenyl-2,5-pyrimidine derivatives are presented. Based on the observed mass shifts by the various substituents, the nature of the main fragment ions is rationalized.     

Mechanistic aspects of cyclization-elimination reactions: 2′-azabenzanilides

Prominent peaks in the mass spectra of 2′-azabenzanilides appear to involve molecular ion cyclization followed by the loss of CO and then H. Effects of structure on the abundance of these and [M? CHO]⁺ ions indicate that the ring nitrogen provides a radical-like reactive site for cyclization, and that stabilization in the product ions of the unpaired electron as well as of the ionic site is important for the reaction.     

Mesomere Kationen: III—13C-Fourier-Transform-NMR-Spektren von Harnstoffen,Thioharnstoffen und Guanidinen

The ¹³C-chemical shifts of methyl substituted ureas, thioureas and related compounds are reported and discussed. π-Electron densities, as obtained by simple HMO calculations, do not reproduce the ¹³C-chemical shift trend. A correlation has been found for the carbonyl and thiocarbonyl resonances which has been discussed as being mainly influenced by the mean excitation energy in the paramagnetic screening term.     

New aspects of the photo-oxidation and photo-stabilization of polymers

The mechanism of the photo-oxidation of an ethylene-propylene random copolymer (EPM) has been compared with that of a styrene-butadiene-styrene block copolymer (SBS). The former is considered as a model for polyolefins that undergo chain scission on irradiation and the latter as a model for unsaturated rubbers that become cross-linked. A hindered amine light stabilizer (HALS) that is a better inhibitor of the absorption of oxygen in SBS than in EPM is, however, less efficient to prevent formation of carbonyl groups and change of the mechanical properties. These results are discussed and an explanation is proposed.     

Some surface aspects of the photo-oxidation of polystyrene films as revealed by ESCA: Part III—Natural weathering

The changes in the surface chemistry of polystyrene films exposed to natural weathering in Saudi Arabia have been followed with ESCA. Extensive incorporation of oxygen is observed, involving reaction of the main backbone chain and the pendant phenyl groups. The rate of surface photo-oxidation is strongly dependent on the photon flux during the time of exposure.     

Stereochemical aspects of proton chemical shifts. III—Configurational assignments in pentacyclic systems without recourse to a karplus equation

It is shown that in contrast to the use of coupling constants, chemical shift criteria may lead to unambiguous structural elucidations in pseudorotational frameworks (pentacycles, heptacycles, etc.).     

Carbon-13 nuclear magnetic resonance studies of anthraquinones. Part III—correlation of substituent effects for 2-substituted anthraquinones

The ¹³C chemical shifts of 2-substituted and 2,6-disubstituted anthraquinones have been determined and assigned. The C-1, 2, 3, 4, 13 and C-14 chemical shifts of 2-substituted anthraquinones are correlated with the chemical shifts of monosubstituted benzenes. A three-parameter correlation with Swain and Lupton's ? and ? parameters and Schaefer's Q parameter provides relationships for the prediction of all chemical shifts of 2-substituted anthraquinones from the substituent parameters. Q values for the SCH₃, OCOCH₃, C₂H₅ and C(CH₃)₃ groups are proposed. The two types of correlations are compared for predicting chemical shifts.     

1H NMR utilization of through-space effects: III—configuration of oximes and analogous compounds

The configurational assignment of Z and E nitrogen derivatives () of 3,5,5-trimethyl-2-cyclohexen-1-one was made taking into consideration the through-space effects on oxime, O-methyloxime, dimethylhydrazone, tert-butylimine, N,N,N -trimethylhydrazonium iodide and oxime hydrochloride derivatives. The relationship between the magnitude of the chemical shifts of the α-protons and the dihedral angle formed by the α-C? H bond and the C?N? OH plane was interpreted in terms of the geometrical dependence of the electric field effect. For the different Y substituents, the change in chemical shift between the Z and the E configuration of the proton near the functional group was mainly dependent on the electric field effect.