Evaluate advantages and disadvantages of spin traps in comparison with other reversed inhibition methods of controlled/living radical polymerization. Give example.
Spin traps are inhibitors that can rapidly capture active radicals and convert them into new kinetically or thermodynamically stable radicals (spin adducts), preferably whilst retaining information about their nature. As opposed to active radicals, whose steady-state concentration in conventional liquid-phase reactions is low and, therefore, cannot be detected by ESR-spectroscopy, spin adducts can be accumulated in amounts sufficient for direct quantitative analysis. The choice of the spin trap and its application technique depend on the particular reaction being studied. Since the 1970s, the spin trap technique has been applied for investigation of a wide range of liquid-phase radical reactions, including radical polymerization reactions.
Radical reactions generally proceed as rapid chain processes and the direct determination of the individual reaction rate coefficients is usually difficult. By reacting competitively with the active species, spin traps break these chains and generate populations of stable detectable radicals. The stage at which the chain reaction is halted is determined by the rate constants and concentrations of reagents (primarily of the spin trap). At high spin trap concentrations (0.5 mol L-1 or above), the trap captures initiating radicals, but, as the concentrations decrease, the products of the deeper stages of the process are detected. This phenomenon underlies the use of spin traps for the study of the mechanism and kinetics of elementary stages of chain radical reactions. The introduction of RAFT agents into polymerization mixture results in appearance of numerous new reactions involving different active radicals. When a spin trap is used, these radicals are captured rapidly and adducts of many kinds emerge in such system. Thus, to acquire the values of kinetic constants of elementary stages of RAFT polymerization, one should further simplify the whole system so that the elementary reactions can be studied separately. To achieve this one can separate these reactions in time (e.g. using pulsed irradiation that separates initiation from the subsequent downstream reactions), and use simpler compounds, which would eliminate some of the competing reactions.
When C-phenyl-N-tert-butyl nitrone (PBN) is used as a spin trap, its adducts are usually more stable but less information is provided by their spectra.
18.Evaluate advantages and disadvantages of Ziegler-Natta catalysts.
The Ziegler-Natta catalysts have been used for ethylene polymerization since 1950. Ziegler-Natta catalysts have advantages of producing product with:
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1) high molecular weight;
2) high melting point;
3) controllable morphology.
However the disadvantages of Ziegler-Natta catalysts are:
1) less control of growing polymer branching due to multiple metal sites of transition metal;
2) encapsulation effect of polymer chains;
3) difficulty of catalyst removal from the final product.
Many modern Ziegler-Natta catalysts for stereospecific olefin polymerization employ the high surface area support matrices for active transition metal catalyst sites.
In the present study, Ziegler-Natta catalysts were prepared on supports that were synthesized via two different methods. The first method (method A) is called the recrystallization method, which employed MgCl2 as a precursor. The second method (method B) is called chemical reaction method, which employed Mg(OEt)2 as a precursor. The catalyst prepared by method A exhibited higher activity than that prepared by method B. However, the properties of polymer obtained from both catalysts were similar.
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