Transient free radicals in viscous solvents

Cage Effect under Photolysis in Polymer Matrices

Photoinduced elementary reactions of low-MW compounds in polymers is an area of active research. Cured organic polymer coatings often undergo photodegradation by free-radical paths. Besides practical importance, such studies teach how the polymer environment controls elementary free-radical reactions. Presented here is a review of recent literature which reports such studies by product analysis and by a time-resolve technique of photochemical reaction inside the cage of a polymer and in the bulk of a polymer. It was established that application of moderate external magnetic field allows the control of the kinetics of free radicals in elastomers. Preheating and stretching of elastomers affect reactivity of photoproduced radicals.

Photoinduced Reactions of Benzophenone in Biaxially Oriented Polypropylene

The photoinduced reactions of benzophenone (B) in biaxially oriented polypropylene (BOPP) were studied with nanosecond laser photolysis (N₂ laser, λ337.1 nm). The first observed transient was a triplet state ³B*. Decay of ³B* led to formation of a radical pair (RP) of BH^• and R^•, where R^• is a radical formed by hydrogen abstraction from BOPP (RH) by ³B*. We studied BOPP after the preheating for a short time in a temperature range 298-423 K, which is essentially lower than its melting point of 453 K. All measurements with not-heated and with preheated (annealed) BOPP were made at 298 K. A radical pair (RP) apparently decays as a contact pair ³[BH^•, R^•] in nonheated BOPP. A critical phenomenon takes place: dissociation of RP with a formation of free radicals in the polymer bulk is observed at preheating temperature T_crit ≈ 403 K and at a higher T. The physical process of heating and cooling of BOPP apparently resulted in the restructuring of crystallites, their agglomeration, shrinking of the distribution of crystallites according to their sizes in BOPP. Overall BOPP becomes softer which manifests itself in the radical kinetics. The decay kinetics of ³B* and RP in the cage fits well the first-order law. Rate constants were obtained. Radicals BH^•, which exit into the polymer bulk at temperatures of preheating T ≥ 403 K, decay by cross-termination according to the second-order law. A relatively high rate constant ∼10⁸ M^-1·s^-1 for this reaction was obtained due to diffusion of BH^• enclosed in the soft amorphous phase of BOPP. Properties of BOPP containing B were studied with ESR, DSC, IR, and WAXD.

Thermodynamic and reactivity studies of a tin corrole-cobalt porphyrin heterobimetallic complex

A heterobimetallic complex, (TPFC)Sn-Co(TAP) (TPFC = 5,10,15-tris(pentafluorophenyl)corrole, TAP = 5,10,15,20-tetrakis(p-methoxyphenyl)porphyrin), was synthesized. The complex featured a Sn-Co bond with a bond dissociation enthalpy (BDE) of 30.2 ± 0.9 kcal mol^-1 and a bond dissociation Gibbs free energy (BDFE) of 21.0 ± 0.2 kcal mol^-1, which underwent homolysis to produce the (TPFC)Sn radical and (TAP)Co^II under either heat or visible light irradiation. The novel tin radical (TPFC)Sn, being the first four-coordinate tin radical observed at room temperature, was studied spectroscopically and computationally. (TPFC)Sn-Co(TAP) promoted the oligomerization of aryl alkynes to give the insertion products (TPFC)Sn-(CH 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 C(Ar)) _n -Co(TAP) (n = 1, 2, or 3) as well as 1,3,5-triarylbenzenes. Mechanistic studies revealed a radical chain mechanism involving the (TPFC)Sn radical as the key intermediate.

Radical Cage Effects: The Prediction of Radical Cage Pair Recombination Efficiencies Using Microviscosity Across a Range of Solvent Types

This study reports a method for correlating the radical recombination efficiencies (F_cP) of geminate radical cage pairs to the properties of the solvent. Although bulk viscosity (macroviscosity) is typically used to predict or interpret radical recombination efficiencies, the work reported here shows that microviscosity is a much better parameter. The use of microviscosity is valid over a range of different solvent system types, including nonpolar, aromatic, polar, and hydrogen bonding solvents. In addition, the relationship of F_cP to microviscosity holds for solvent systems containing mixtures of these solvent types. The microviscosities of the solvent systems were straightforwardly determined by measuring the diffusion coefficient of an appropriate probe by NMR DOSY spectroscopy. By using solvent mixtures, selective solvation was shown to not affect the correlation between F_cP and microviscosity. In addition, neither solvent polarity nor radical rotation affects the correlation between F_cP and the microviscosity.

Reactivity of Benzophenone Ketyl Free Radicals in an Elongated Elastomer Film

The effect of polymer film elongation leading to the thinning of the film up to three times on the decay of transients was studied. Kinetics of benzophenone (B) triplet state (3)B* and ketyl free radical BH• in soft rubber poly(ethylene-co-butylene) (abbreviated as E) was investigated by nanosecond laser flash photolysis. We monitored decay kinetics of the triplet state of (3)B* and of BH• decay in the polymer cage and decay of BH• in the polymer bulk. The fast exponential decay of (3)B*(lifetime τT ≈ 200 ns) is accompanied by hydrogen atom abstraction from E with the formation of BH• and a polymer free radical R•. The decay of BH• in the polymer cage occurs during τc ≈ 1 μs. Cage recombination, in turn, was followed by a cross-termination of BH• in the polymer bulk (τb ≈ 100 μs under our conditions) and is characterized by a rate constant kb ≈ 10(8) M(-1) s(-1). We studied changes of rates of transients decay upon elongation (thinning) of E. Decay of (3)B* is practically independent of elongation of the film. Recombination of BH• in the solvent bulk occurs with a two times lower kb than in a nonelongated E. The decrease in kb is ascribed mainly to a lower fractional polymer free volume Vf in elongated E compared with that in nonelongated E. Dependencies of log10 kb versus [Formula: see text], where [Formula: see text] is the thickness of the film, turned out to be linear with a negative slope. At the same [Formula: see text] recombination proceeds slower in the elongated elastomer compared with the nonelongated elastomer. Cage effect increases twice as well due to a lower rate of radicals escape from the polymer cage in the elongated film. We observed relatively large effects of external magnetic field (B = 0.2T) on the kinetics of cage recombination and recombination in the polymer bulk. Magnetic field effect on recombination rates in the cage and in the solvent bulk does not depend on elongation.

Kinetics of benzophenone ketyl free radicals recombination in a polymer: reactivity in the polymer cage vs. reactivity in the polymer bulk

The decay kinetics of intermediates produced under photolysis of benzophenone (B) dissolved in soft rubber poly(ethylene-co-butylene) films (abbreviated asE) was studied by ns laser flash photolysis in the temperature range of 263–313 K.