Cage Effect under Photolysis in Polymer Matrices

Abstract

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.

Preparation of environment‐friendly hydrophilic rubber from natural rubber grafted with sodium acrylate by reactive melt mixing

“…At a high viscosity of the molten NR medium, the formation of free radicals on NR molecules has very low efficiency due to the cage effect that promotes the selftermination of free radicals. [47,48] Moreover, at too high a NaAA concentration, there is more chance of NaAA to act as a trap for free radicals, and so higher NaAA concentrations result in less polymer radicals and more homopolymerization of NaAA. [40] However, it could be noticed from this observation that an increased grafted NR percentage tended to increase the Mooney viscosity of the NR-g-NaAA, such that the viscosity of NR-g-NaAA at 38%, 58%, and 69% were 45, 75, and 84 MU, respectively.…”

Catalytic conversion of heavy naphtha to reformate over the phosphorus-ZSM-5 catalyst at a lower reforming temperature

“…Unlike dodecane cracking, the main products obtained from heavy naphtha reforming were formed through bimolecular reactions occurring inside the zeolite pores allowing the paraffin and olefins to further react and produce aromatics, iso-paraffins, and naphthenes. Whereas the free radicals from β-scission were affected by the surrounding molecules and did not overcome the cage effect of the feedstock, 63,64 and cracked molecules went via bimolecular reactions through zeolite pores. 64 …”

“…However, the heavy naphtha reforming over the modied 40% ZSM-5 in 60% binder by the hydrothermal treatment and phosphorus impregnation showed more selectivity towards the bimolecular reaction, which may be assisted by the increase in the Lewis acid sites aer hydrothermal treatment with steam. Furthermore, the products from monomolecular cracking from b-scission were affected by the surrounding molecules and did not overcome the cage effect of feedstock, 63,64 which were elevated to further reactions, including cyclization and isomerization via zeolite pores to produce more aromatics and isoparaffins. The zeolite pore shape selectivity slightly improved and maintained the cyclization reaction when co-feeding 10% steam with heavy naphtha feedstock.…”

Fast Recombination of Free Radicals in Solution and Microviscosity

“…The cage effect (φ) plays a vital role in the photoinitiation of chemical reactions and free-radical chemistry [18][19][20][21]. φ is the fraction of radicals that decayed near each other after photolysis or thermolysis of molecules in solution; 0 ≤ φ ≤ 1.…”

“…φ is the fraction of radicals that decayed near each other after photolysis or thermolysis of molecules in solution; 0 ≤ φ ≤ 1. It is known that φ increases with the solvent viscosity η increase [18][19][20][21]. Tyler and coworkers [18,19] studied the dependence of experimentally measured φ vs. η under photolysis of an organometallic dimer.…”