Evaluate advantages and disadvantages, common features and differences between radical and controlled/living radical polymerization.
Evaluation advantages and disadvantages of radical polymerization in the presence of alkoxyamines in comparison with other methods of controlled / living radical polymerization. Give example.
In the chain polymerization of macromolecular elements as a result of the disclosure of multiple bonds or cycles of monomers under the action of active centers located on the ends of growing chains on them. There are three types of active sites in chain polymerization - a radical, a cation and an anion, whereby radical, cationic and anionic polymerizations are distinguished.
Only compounds containing double carbon-carbon bonds are subjected to radical polymerization. In this case, the active center is carbradical, i.e. a carbon atom having one unpaired electron. Such an atom (radical) is very reactive due to the tendency of an unpaired electron to form a pair with a second electron. Compounds containing a π-bond are a suitable object for realizing this trend, since π-bond electrons are much more weakly bound than σ-bonds. Therefore, the radical easily selects one of the π-bond electrons in order to form a pair of electrons, that is, a new σ-bond:
The chemical act described above is shown in the diagram for the polymerization of ethylene; the chemical bonds of the atoms participating in the reaction are represented electron pairs for clarity. The radical at the end of the growing chain is called the growth radical, and its reaction with the monomer - the main polymerization reaction - is a growth reaction. It can be seen from the scheme that the addition of the monomer to the growth radical is accompanied by the regeneration of the active site at the end of the chain - the carbon atom with the unpaired electron.
Live polymerization. Block copolymers
Abkin and Medvedev observed live anionic polymerization for the first time in the 1930s, but Schwartz systematically investigated this process in the 1950s. XX century, and by now anionic polymerization has become widespread.
Carbanions are relatively more stable and therefore less reactive particles compared to carbocation, therefore, in the anionic polymerization of nonpolar monomers in nonpolar and low polarity solvents, for example styrene or butadiene-1,3 in benzene, tetrahydrofuran or 1,2-dimethyloxyethylene, there are no all the main reactions of the termination of the material and kinetic chain. The polymerization continues until the monomer is completely depleted, and at its end, the active centers (anions) of the macromolecules persist for 1-2 weeks. During this period, the polymerization can be resumed by adding a new portion of the monomer. It is believed that the slow deactivation of active sites is associated with reactions beginning with the transfer of the hydride ion to the counterion:
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The allyl anion formed as a result of the latter reaction is incapable of initiating anionic polymerization. Causes that impede the live polymerization of polar monomers, such as (meth) acrylates, are less clear, presumably associated with the reactions of the functional groups of the monomers with both the initiator organometallic initiator and carbanions. Live polymerization of polar monomers is carried out at a very low temperature in order to avoid these reactions.
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