Living radical polymerization. Give definition, classification and characteristics of iniferters.
This living/ controlled radical polymerization method relies on completely pure reactions so that no termination caused by impurities occurs. These polymerizations stop only when there is no more monomer and not when termination occurs. The polymerization can continue upon the addition of more monomer. Block copolymers can be made by this way. This type of polymerization can be stopped and restarted at anytime. This pioneering technology enables the synthesis of tailored polymers with unprecedented control over composition and architecture. Complete living radical polymerization allows for control of molecular weight and dispersity; however, this is very difficult to achieve and instead a pseudo-living radical polymerization occurs in which there is only partial control of molecular weight and dispersity.
The basics for living radical polymerization
· Fast initiation as compared to propagation since all chains should begin to grow essentially at the same time and retain functionality.
· Fast exchange between active and dormant species, so that majority of the growing chains are in the dormant state and only a small fraction is present as propagating free radicals. ƒ
· The propagation rate should not depend on the degree of polymerization. ƒ
· The polymerization reaction should be free of termination and transfer reactions. ƒ
· Concentration of the propagating radicals should be sufficiently low ([P*] < 10-7 M) to enable chain growth on one hand and reduce termination events on the other. ƒ
· A single type of propagating species must exist; if different propagating species are present, then their inter conversion should be very fast. ƒ
· The number average molecular weight of the polymer should have a linear relation with conversion.
10. Explain features of three-dimensional living radical copolymerization.
Copolymerization is co-polymerization of various monomers to produce polymers with improved properties.
It is known that the reaction of radical copolymerization of monomers M1 and M2 leads to the formation of linear macromolecules in which the composition and the method for the formation of monomeric units in the chain, with the exception of special cases of disturbing the homogeneous distribution of the reagents, are determined by the composition of the initial monomer mixture and the copolymerization constants of the monomers. The real mechanism of radical copolymerization is much more complicated than model representations. The microheterogeneous mechanism of three-dimensional radical copolymerization and structural-physical inversions play a decisive role in the formation of the structure and properties of the densified polymers.
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From the moment of the formation of the microgel particles, the reaction medium becomes structurally and kinetically micro-inhomogeneous. This is due to the fact that, due to the low mobility of the particles, the constant of termination of radicals localized in them decreases sharply and the reaction goes into the auto acceleration regime.
The micro-gel particles can be considered as micro-reactors with two reaction zones, internal and external, substantially differing in the effective reactivity and rate of polymerization of the double bonds of the semi-functional oligomer.
In the reaction mixture, four elementary chain reaction reactions are possible:
11. Explain features of organometallic radical polymerization
In a number of cases, metal-metal compounds can actually act on relative to the growth radicals as reversible spin traps. When This can be accepted by radicals as paramagnetic metal complexes, and diamagnetic compounds. This direction in the foreign literature is commonly referred to as the Organometallic Mediated Radical Polymerization (OMRP). By and large this method is similar in reversible inhibition involving stable radicals. OMRP mechanism in the presence of organometallic complexes:
where Men is the metal atom in the oxidation state "n", Y is the halogen atom (or another substitute); L - ligand; m is a monomer molecule.
An active radical, formed either as a result of the disintegration of ordinary radical initiators, or directly in the splitting of the bond metal-carbon, reacts with the monomer molecule to form macroradical of growth, capable of interacting with a metal complex form a labile adduct that decomposes under certain conditions with regeneration of the same macroradical growth. This reversible reaction leads to alternation of the periods of "sleep" and "life" of the polymer radical: the monomer molecules (m) are successively attached to the macroradical, thus a consistent growth of the polymer chain takes place. The flow of the undesirable reaction of the bimolecular chain termination in As a result of dimerization of growing macroradicals, leading to spontaneous growth of molecular weight, is difficult. To date, polymerization by the mechanism of reversible inhibition with the participation of organometallic compounds has been studied in detail for cobalt and molybdenum complexes. Comparative analysis of controlled synthesis processes macromolecules flowing through the mechanisms of ATRP and OMRP, testifies that both methods are characterized as indisputable merits, and shortcomings. If we compare the amounts of catalyst, In both cases, process by ATRP mechanism. In this case, only catalytic amounts of metal complexes that can be are reduced to parts per million in relation to the monomer. Furthermore, the catalyst can be separated from the polymerization system, regenerated and re-used for the synthesis of macromolecules. In case of OMRP, the necessary amount of metal complex for its is determined by the number of polymer chains and, consequently, is significantly higher than in the case of ATRP. Another indisputable advantage ATRP is a wide range of multifunctional initiators and monomers, as well as the ease of obtaining block copolymers, which allows it is easy enough to synthesize various polymer structures, including nanoscale. Undeniable advantage of OMRP can be considered the possibility of obtaining polymers based on methacrylic acid and vinyl ethers, characterized by a narrow molecular weight distribution, which is not possible under ATRP conditions, and polymerization in aqueous media.
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