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Kargin readings
Polymer Science Vol. 33, No. 9, pp. 1903-1905, 1991 Printed in Great Britain.
0965-54b'X/91 $1.5.00+ . ~ ~) 1992 I~-i~laon Press l,td
KARGIN READINGS* ON 21 JANUARY 1991, the XXI Kargin Readings took place at the Chemical Faculty of the M. V. Lomonosov Moscow State University. They were opened by Academician V. A. Kabanov. He described the leading role of V. A. Kargin in the development of polymer science in the U.S.S.R., mentioning the enormous prognostic strength of his ideas, which is being increasingly confirmed in the course of time. The 20 preceding readings have played a substantial role in the consolidation of our scientists in the polymer field and in the education of young scientists. In the name of the Organizing Committee of Kargin Readings, Academician V. A. Kabanov cordially greeted two new Corresponding Members of the U.S.S.R. Academy of Sciences who were present, Professors Yu. B. Monakov and A. R. Khokhlov; they had been elected in December of 1990. The lecture of the Corresponding Member of the U.S.S.R. Academy of Sciences, Yu. B. Monakov (Institute of Chemistry, BNC of the Ural branch of the U.S.S.R. Academy of Sciences, Ufa) dealt with the polymerization of dienes in the presence of lanthanoid catalysts. Lanthanoid catalysts exhibit good activity and are suitable for the preparation of polybutadiene and polyisoprene with a high content of c/s-l,4-units (up to 98-99%). The rubbers prepared with these catalysts do not contain oligomers and branched structures. Studies aimed at developing the possibilities of lanthanoid catalysts have therefore been going on in many countries. On systems of the type LnHal3.3L-AIR3 (where L is an electron-donating ligand, for example tributylphosphate, sulphoxides, THF) the main factors were demonstrated that affect the activity and stereospecificity of the catalysts, as well as the molecular-mass characteristics of the polydienes formed. The dependence of the rate constant of the propagation reaction (and of the overall activity) on lanthanoid type is not simple--the maximum corresponds to neodymium. The stereospecificity of the studied catalysts in butadiene polymerization is practically independent of the composition of the lanthanoid component (types of lanthanoid, halogen or organic ligand), but with isoprene, and particularly with piperylene (i.e. with more complicated structures of the polymerizing diene), the microstructure of the corresponding polydienes becomes increasingly affected by the above named factors. The sum of the obtained data indicates that in such a complicated and dynamic conglomerate like the active centre of the lanthanoid catalyst, its stablility, reactivity and stereospecificity are determined by temperature, type of hydrocarbon solvent and composition of the initial components forming the catalytic system. The lecture on the kinetics and mechanism of radical polymerization in the presence of iniferterst was presented by Candidate of Chemical Science, A. V. Olenin; coauthors were V. B. Golubev, M. Yu. Zaremskii, Ye. S. Garina, M. B. Lachinov and S. I. Kuchanov (Chemical Faculty of M. V. Lomonosov Moscow State University). A general approach to studies of polymerization in the presence of iniferters was formulated, and all stages of polymerization in their presence were discussed; the general rules of polymerization in the presence of iniferters were used to show the ensuing important practical consequences. * Vysokomol. soyed..433: No. 9, 2027-2029, 1991. l'The term "iniferters" is derived from three English words: initiator, tranffer and terminator.
1903
1904
Kargin Readings
A typical such system is benzyidithiocarbamate (BTC)-styrene, in which an iniferter polymerization occurs under UV irradiation. It was found that in this system, polymerization proceeds by the mechanism of reinitiating chains: the decomposition of the low-molecular iniferter, BTC, and of the macroiniferter, PS formed in the presence of BTC and containing at one of the chain ends a dithiocarbamate group, proceeds by the same mechanism, with a rate constant that contrary to the classical FIory principle depends on the polymerization degree of the macroiniferter, while the concentration of the propagating chains is equal to the original concentration of the iniferter. The polystyrene chains grow in discrete steps; from the moment of initiation or reinitiation of the polymerization with active benzyl or polystyryl radicals to the moment of termination on "foreign" poorly active dithiocarbamyi radicals, some amount of monomer molecules is added, depending on the polymerization conditions. The contribution of classical bimolecular chain termination to this polymerization reaction is negligible. Iniferters present the unique possibility of easy practical realization of molecular polymer design in radical polymerization, to prepare functional oligomers and polymers with the needed polymerization degree and narrow molecular mass distribution, and to perform controlled synthesis of graft, block and gradient copolymers. In addition, iniferters enable us to carry out polymerization of vinyl monomers to high conversion without the occurrence of a gel-effect. The conditions were discussed under which radical polymerization of vinyl monomers can proceed by the iniferter mechanism. The lecture of Doctor of Chemical Science Ye. M. Antipov (A. V. Topchiev Institute of Petrochemical Synthesis of the U.S.S.R. Academy of Sciences) was devoted to the features of interphase structure in some linear polymers. General principles and rules of the formation of mesomorphous conformationally disordered structures in various classes of flexible, semiflexible and rigid chain polymers were discussed. In the lecture the main characteristics of such structures and of their phase transitions in a broad temperature range were explained on the example of polyorganophosphazenes, polyvinylorganosilanes and thermotropic liquid crystalline polyethers. A classification scheme of polymeric conformationally disordered mesophase structures was also discussed; this was found useful for defining two types of mesomorphous states, dynamic and static, each of which has been subdivided by the author according to its structural features into a series of subtypes. For fully aromatic rigid chain copolyesters, for the first time the whole spectrum of the possible physical states of macromolecular compounds could be observed in a single system, in terms of the scheme "crystal-conformationally disordered mesophase structure-liquid crystal--melt". For composite polymeric materials on the basis of polyethylene, a systematic analysis was carried out on the influence of the degree of dispersity, type of second component, level of intercomponent interactions, conditions of thermo-orientational drawing, theological features of composite formation, thermal prehistory, effects of ionizing radiation and high pressures on the ability of the polymer to form a conformationally disordered mesophase, on the character of the transition and on the specificity of the structure formed; as a result, promising technological schemes for the processing of composite polymeric materials were proposed. The last lecture of these Readings was presented by Corresponding Member of the U.S.S.R. Academy of Sciences A. R. Khokhlov (Physical Faculty of M. V. Lomonosov Moscow State University) on conformational transformations in polyelectrolytic systems. The development of the conformational theory of polyelectrolyte solutions is one of the most
Kargin Readings
1905
complex problems of macromolecular statistical physics. The increased complexity, as compared to the theory of the solutions of neutral macromolecules is caused by the following: 1. In polyelectrolytic systems, two types of volume interactions between monomeric units exist: long-distance Coulombic, and short distance van der Waals interactions. 2. For polyelectrolyte solutions, a number of additional parameters have to be considered, which are absent for solutions of neutral macromolecules: the concentration of the low-molecular salt, pH of the solution and other factors. 3. Coulombic interactions not only contribute to the volume effects, but also affect the flexibility of the polymer chain ("electrostatic persistence length" effect). 4. In many cases, electrostatic interactions in polymeric systems cannot be regarded as weak, so that the linearized Debye-Hiickel approximation cannot be applied. The best known non-linear effect in polyelectrolyte solutions is the condensation of counter-ions. 5. Finally, even when the linearized equations can be used (for example for weakly charged polyelectrolytes), the electrostatic shielding is nevertheless realized in a much more complicated manner than in a system of point charges, as the charged units are connected in a chain. In the lecture it was described how the compatibility of polymeric blends can be improved when one of the components is weakly charged. This effect can be used for introducing a small fraction of a flexible polymeric component into a liquid-crystalline phase formed by rigid-chain molecules. Also extensive experimental evidence has been presented for considerable improvements in the compatibility of polymer blends by the addition of a small amount of charged units. A second reason for the interest in blends of weakly charged polyelectrolytes is that in such systems so called microdomain structures can be formed, in which domains enriched in one of the components alternate with domains enriched in the other. So far such structures have only been observed in melts and in concentrated solutions of biock-copolymers with mutually incompatible blocks. Theoretical calculations have shown that such periodical structures can also be formed in polyelectrolyte systems, where the structure period can be varied in a broad range depending on parameter values--from tens to thousands of/lngstr6ms. The experimental discovery of microdomain structures in polyelectrolyte systems could open the way to new types of materials with a regular and easily variable microstructure. One more example of conformational changes in polyelectrolyte systems are those occurring during the formation of interpolyelectrolyte complexes. One of the basic problems in the theory of such complexes at present is the following: is the structure of the complex nearer to a completely disordered mixture of two mutually entangled coils, or is it of ladder type, where two chains are arranged in parallel? Theoretical data, as well as the results of computer calculations favour the first of the above possibilities, but a final solution cannot yet be presented. P. V. KOZLOV
Translated by D. DOSKOt~ILOV,~
0965-54b'X/91 $1.5.00+ . ~ ~) 1992 I~-i~laon Press l,td
KARGIN READINGS* ON 21 JANUARY 1991, the XXI Kargin Readings took place at the Chemical Faculty of the M. V. Lomonosov Moscow State University. They were opened by Academician V. A. Kabanov. He described the leading role of V. A. Kargin in the development of polymer science in the U.S.S.R., mentioning the enormous prognostic strength of his ideas, which is being increasingly confirmed in the course of time. The 20 preceding readings have played a substantial role in the consolidation of our scientists in the polymer field and in the education of young scientists. In the name of the Organizing Committee of Kargin Readings, Academician V. A. Kabanov cordially greeted two new Corresponding Members of the U.S.S.R. Academy of Sciences who were present, Professors Yu. B. Monakov and A. R. Khokhlov; they had been elected in December of 1990. The lecture of the Corresponding Member of the U.S.S.R. Academy of Sciences, Yu. B. Monakov (Institute of Chemistry, BNC of the Ural branch of the U.S.S.R. Academy of Sciences, Ufa) dealt with the polymerization of dienes in the presence of lanthanoid catalysts. Lanthanoid catalysts exhibit good activity and are suitable for the preparation of polybutadiene and polyisoprene with a high content of c/s-l,4-units (up to 98-99%). The rubbers prepared with these catalysts do not contain oligomers and branched structures. Studies aimed at developing the possibilities of lanthanoid catalysts have therefore been going on in many countries. On systems of the type LnHal3.3L-AIR3 (where L is an electron-donating ligand, for example tributylphosphate, sulphoxides, THF) the main factors were demonstrated that affect the activity and stereospecificity of the catalysts, as well as the molecular-mass characteristics of the polydienes formed. The dependence of the rate constant of the propagation reaction (and of the overall activity) on lanthanoid type is not simple--the maximum corresponds to neodymium. The stereospecificity of the studied catalysts in butadiene polymerization is practically independent of the composition of the lanthanoid component (types of lanthanoid, halogen or organic ligand), but with isoprene, and particularly with piperylene (i.e. with more complicated structures of the polymerizing diene), the microstructure of the corresponding polydienes becomes increasingly affected by the above named factors. The sum of the obtained data indicates that in such a complicated and dynamic conglomerate like the active centre of the lanthanoid catalyst, its stablility, reactivity and stereospecificity are determined by temperature, type of hydrocarbon solvent and composition of the initial components forming the catalytic system. The lecture on the kinetics and mechanism of radical polymerization in the presence of iniferterst was presented by Candidate of Chemical Science, A. V. Olenin; coauthors were V. B. Golubev, M. Yu. Zaremskii, Ye. S. Garina, M. B. Lachinov and S. I. Kuchanov (Chemical Faculty of M. V. Lomonosov Moscow State University). A general approach to studies of polymerization in the presence of iniferters was formulated, and all stages of polymerization in their presence were discussed; the general rules of polymerization in the presence of iniferters were used to show the ensuing important practical consequences. * Vysokomol. soyed..433: No. 9, 2027-2029, 1991. l'The term "iniferters" is derived from three English words: initiator, tranffer and terminator.
1903
1904
Kargin Readings
A typical such system is benzyidithiocarbamate (BTC)-styrene, in which an iniferter polymerization occurs under UV irradiation. It was found that in this system, polymerization proceeds by the mechanism of reinitiating chains: the decomposition of the low-molecular iniferter, BTC, and of the macroiniferter, PS formed in the presence of BTC and containing at one of the chain ends a dithiocarbamate group, proceeds by the same mechanism, with a rate constant that contrary to the classical FIory principle depends on the polymerization degree of the macroiniferter, while the concentration of the propagating chains is equal to the original concentration of the iniferter. The polystyrene chains grow in discrete steps; from the moment of initiation or reinitiation of the polymerization with active benzyl or polystyryl radicals to the moment of termination on "foreign" poorly active dithiocarbamyi radicals, some amount of monomer molecules is added, depending on the polymerization conditions. The contribution of classical bimolecular chain termination to this polymerization reaction is negligible. Iniferters present the unique possibility of easy practical realization of molecular polymer design in radical polymerization, to prepare functional oligomers and polymers with the needed polymerization degree and narrow molecular mass distribution, and to perform controlled synthesis of graft, block and gradient copolymers. In addition, iniferters enable us to carry out polymerization of vinyl monomers to high conversion without the occurrence of a gel-effect. The conditions were discussed under which radical polymerization of vinyl monomers can proceed by the iniferter mechanism. The lecture of Doctor of Chemical Science Ye. M. Antipov (A. V. Topchiev Institute of Petrochemical Synthesis of the U.S.S.R. Academy of Sciences) was devoted to the features of interphase structure in some linear polymers. General principles and rules of the formation of mesomorphous conformationally disordered structures in various classes of flexible, semiflexible and rigid chain polymers were discussed. In the lecture the main characteristics of such structures and of their phase transitions in a broad temperature range were explained on the example of polyorganophosphazenes, polyvinylorganosilanes and thermotropic liquid crystalline polyethers. A classification scheme of polymeric conformationally disordered mesophase structures was also discussed; this was found useful for defining two types of mesomorphous states, dynamic and static, each of which has been subdivided by the author according to its structural features into a series of subtypes. For fully aromatic rigid chain copolyesters, for the first time the whole spectrum of the possible physical states of macromolecular compounds could be observed in a single system, in terms of the scheme "crystal-conformationally disordered mesophase structure-liquid crystal--melt". For composite polymeric materials on the basis of polyethylene, a systematic analysis was carried out on the influence of the degree of dispersity, type of second component, level of intercomponent interactions, conditions of thermo-orientational drawing, theological features of composite formation, thermal prehistory, effects of ionizing radiation and high pressures on the ability of the polymer to form a conformationally disordered mesophase, on the character of the transition and on the specificity of the structure formed; as a result, promising technological schemes for the processing of composite polymeric materials were proposed. The last lecture of these Readings was presented by Corresponding Member of the U.S.S.R. Academy of Sciences A. R. Khokhlov (Physical Faculty of M. V. Lomonosov Moscow State University) on conformational transformations in polyelectrolytic systems. The development of the conformational theory of polyelectrolyte solutions is one of the most
Kargin Readings
1905
complex problems of macromolecular statistical physics. The increased complexity, as compared to the theory of the solutions of neutral macromolecules is caused by the following: 1. In polyelectrolytic systems, two types of volume interactions between monomeric units exist: long-distance Coulombic, and short distance van der Waals interactions. 2. For polyelectrolyte solutions, a number of additional parameters have to be considered, which are absent for solutions of neutral macromolecules: the concentration of the low-molecular salt, pH of the solution and other factors. 3. Coulombic interactions not only contribute to the volume effects, but also affect the flexibility of the polymer chain ("electrostatic persistence length" effect). 4. In many cases, electrostatic interactions in polymeric systems cannot be regarded as weak, so that the linearized Debye-Hiickel approximation cannot be applied. The best known non-linear effect in polyelectrolyte solutions is the condensation of counter-ions. 5. Finally, even when the linearized equations can be used (for example for weakly charged polyelectrolytes), the electrostatic shielding is nevertheless realized in a much more complicated manner than in a system of point charges, as the charged units are connected in a chain. In the lecture it was described how the compatibility of polymeric blends can be improved when one of the components is weakly charged. This effect can be used for introducing a small fraction of a flexible polymeric component into a liquid-crystalline phase formed by rigid-chain molecules. Also extensive experimental evidence has been presented for considerable improvements in the compatibility of polymer blends by the addition of a small amount of charged units. A second reason for the interest in blends of weakly charged polyelectrolytes is that in such systems so called microdomain structures can be formed, in which domains enriched in one of the components alternate with domains enriched in the other. So far such structures have only been observed in melts and in concentrated solutions of biock-copolymers with mutually incompatible blocks. Theoretical calculations have shown that such periodical structures can also be formed in polyelectrolyte systems, where the structure period can be varied in a broad range depending on parameter values--from tens to thousands of/lngstr6ms. The experimental discovery of microdomain structures in polyelectrolyte systems could open the way to new types of materials with a regular and easily variable microstructure. One more example of conformational changes in polyelectrolyte systems are those occurring during the formation of interpolyelectrolyte complexes. One of the basic problems in the theory of such complexes at present is the following: is the structure of the complex nearer to a completely disordered mixture of two mutually entangled coils, or is it of ladder type, where two chains are arranged in parallel? Theoretical data, as well as the results of computer calculations favour the first of the above possibilities, but a final solution cannot yet be presented. P. V. KOZLOV
Translated by D. DOSKOt~ILOV,~