Photobehaviour of phenyl-containing methacrylate polymers—II: Dependence upon the number of chromophores in the structural unit

Emission spectra and photobehaviour were investigated for polymers and copolymers derived from poly(methyl methacrylate) with one to three phenyl groups for each monomer molecule. The results show that excimer emission increases with the number of phenyl groups for the structural unit and with the temperature, and that it decreases when the thermodynamic character of the solvent improves. The results are interpreted in terms of excimer formation between non-nearest neighbours.     

Free radical polymerization in the presence of triethylborane

The polymerization of methyl methacrylate initiated by triethylborane-peroxides and by triethylborane-AIBN mixtures has been studied. From the dependence of rate of polymerization and molecular weight of the polymer on the triethylborane concentration. It is concluded that the initiation is complex. An initiation mechanism involving triethylborane-monomer and triethylborane-cocatalyst complexes is proposed.     

QUENCHING AND PHOTOBLEACHING OF EXCITED POLYCYCLIC AROMATIC HYDROCARBONS BY CARBON TETRACHLORIDE AND CHLOROFORM IN MICELLAR SYSTEMS

—Aromatic hydrocarbons incorporated into cetyltrimethylammonium chloride (CTACl) or sodium dodecylsulphate (SDS) micelles are efficiently quenched and photobleached by carbon tetrachloride. Similar results were obtained employing chloroform, but the efficiency of this compound as a quencher is nearly 10³ times smaller than that of carbon tetrachloride.
Bimolecular quenching constants, with the quencher concentration given in moles per liter of micellar pseudophase, have been evaluated and compared with those obtained in homogeneous solutions. For slow processes, the values obtained would indicate that the polarity sensed by the probe depends both upon the probe and the surfactant. For diffusionally controlled processes (i.e. quenching of biphenyl), bimolecular quenching constants are considerably smaller in the micellar pseudophase. For these processes the pseudo-first-order quenching rate constants are larger in SDS than in CTACl micelles. This effect is partly due to the smaller size of the SDS micelle but also is a consequence of a lower "microviscosity" of these micelles.     

Reactions of tert-butoxy radicals with vinyl monomers

Tert-butoxy radicals generated in the photolysis of di-tert-butyl peroxide in benzene at 25°C react with vinyl monomers by double-bond addition and, in most cases, also by competitive hydrogen abstraction. The rate constant for the double-bond addition changes by a factor of 17 between isoprene, which shows the highest reactivity, and the methacrylate derivatives, which are the least reactive of the monomers considered. The fraction of tert-butoxy radicals that react by hydrogen abstraction varies considerably with the monomer structure, ranging from 0.9 (cyclohexyl methacrylate) to less than 0.05 (styrene and conjugated diolefins). In the methacrylate derivatives, most of the hydrogen abstraction takes place, not in the α-methyl group, but in the alkyl chain.     

Autooxidation of diethyl hydroxylamine

The autooxidation of diethyl hydroxylamine (DEHA) has been studied both in solution and in the gas phase between 0° and 60°C. The results obtained are interpreted in terms of a nonchain free radical mechanism in which reaction (1) is the rate-determining step. This reaction is followed by the reaction of HO₂ radicals with DEHA to give hydrogen peroxide. The initial stoichiometry is given by which implies that the hydrogen peroxide reacts with DEHA to give water and diethyl nitroxide radicals. The activation energy of reaction (1) is 16.5 ± 2 kcal/mol, leading to a dissociation energy of 69.5 ± 2 kcal for the O?H bond in DEHA. The oxidation of DEHA in the gas phase is nearly ten times slower than that observed in chlorobenzene solution under similar experimental conditions. This result is related to the stabilization of the free radicals produced by the solvent.     

Polymerization photoinitiated by carbonyl compounds. VIII. Solvent and photoinitiator concentration effects

The interaction of benzil and thioxanthone triplets with triethylamine leads to the production of free radicals that are able to initiate the polymerization of methylmethacrylate and vinyl acetate. The process is mediated by a charge transfer complex. The initiation efficiency is higher in benzene than in acetonitrile, a result explained in terms of competition between back electron transfer, production of free ions and ketyl radical formation. The initiation efficiency decreases when the carbonyl concentration increases, because of the occurrence of a radical molecule reaction such as The α-hydroxy radicals produced in this reaction are unable to initiate the polymerization chain.