Propose mechanism of ATRP in the presence of titanium complexes. Draw appropriate formulas.
Along with nickel complexes, the compounds of titanium and its analogs, which are known to be effective components of the Ziegler-Natta catalytic systems, are of undoubted interest in influencing the regularities in the synthesis of macromolecules under conditions of radical initiation. In this connection, we analyzed the features of the polymerization of vinyl monomers in the presence of cyclopentadienyl complexes of titanium, tungsten and niobium.
On the example of MMA and CT, it has been found that vinyl monochloride dichlorides, as well as the molecular weight characteristics of the corresponding polymers. Thus, the processes of homo- and copolymerization of vinyl monomers in the presence of these metal complexes proceed without a gel effect and are characterized by a linear increase in the molecular weight with conversion.
Unlike nickel complexes, titanium derivatives do not have a noticeable effect on the composition of the copolymer. At the same time, their introduction into the polymerization system leads to a slight increase in the syndiotacticity of polyMMA. This effect is especially noticeable in the case of silicon-containing titanium derivatives with a rigidly fixed structure.
The results of the study of these systems using the EPR method and quantum-chemical modeling indicate that during polymerization in the presence of titanium compounds reversible oxidation-reduction takes place according to the scheme:
Thus, in this case, the polymerization process passes through the ATRP scheme.
35. Propose mechanism of initiation in the presence of Ni complexes. Draw appropriate formulas.
Specify the factors influenced on the mechanism of controlled radical polymerization in the presence of organometallic compounds. Draw appropriate formulas.
Radical polymerization is often the preferred mechanism for forming polymers and most commercial polymer materials involve radical chemistry at some stage of their production cycle. The radical reactions can be initiated by various methods. They can be divided into the broad areas of thermolyses, photolyses, and electron transfer reactions .
Photoinitiators are compounds that decay on irradiation with UV or visible light. They are used not only in industrial processes involving coatings and surface polymerizations but also find applications involving accurate kinetic measurements.
The main advantage of photoinitiator used in polymerizing systems is the possibility to define exact start- and endpoints of the polymerization process via the duration of the irradiation period. In addition, the rate of (most) photoinitiator decomposition is almost independent of the reaction temperature, but depends strongly on the light intensity. An ideal photoinitiator for a specific polymerization may be defined via the following criteria:
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The photoinitiator should decompose on irradiation with the (UV) light source. For instance, an absorption should coincide with the radiation wavelength. The monomer(s) used in the specific polymerization process should not absorb light at the selected wavelength.
The efficiency of the initiator should be high, preferably close to 1, which says that all radicals generated start a growing chain.
At best, there should be only one type of free-radical species that is formed on laser irradiation .
According to the mechanism by which initiating radicals are formed, photoinitiators are generally divided into two classes:
type I photoinitiators, which undergo a unimolecular bond cleavage upon irradiation to yield free-radicals; and
type II photoinitiators, which undergo a bimolecular reaction where the excited state of the photoinitiator interacts with a second molecule (a co-initiator) to generate free radicals .
Within the type I initiator class, there are several structural variations.
The mechanism of the controlled radical polymerization of styrene and methyl methacrylate in the presence of dicyclopentadienyltitanium dichloride (Cp2TiCl2) was studied using quantum chemical calculations and electron spin resonance spectroscopy. It was established that the reduction of Cp2TiCl2 to Cp2TiCl during the macromolecule synthesis occurs through the living polymerization mechanism, which adjusts the growth of a polymeric chain. The geometrical structures of the molecular complexes between a growing macroradical and Cp2TiCl2 and transition states of radical inhibition steps were optimized and the thermodynamic and kinetic parameters of the elementary reactions were estimated for several feasible directions of the proces
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