Reactive centers, living polymerization

Equation 15.16 describes cracking of the aliphatic part of the feedstock, i.e., detachment of the aliphatic fragments from the heavy residue. The reactive centers of polymerization/adsorption formed as a result of cracking cause the decrease in the concentration of the aliphatic component C that can be potentially converted to the cracking product. This reduction on the potential product yield is described by Equation 15.17. It is assumed that the polymerized (or adsorbed) products are radiation resistant and do not further participate in the cracking reactions. Equation 15.18 describes accumulation and disappearance of the long-living radicals R. ... 

The role of reactive centers is performed here by free radicals or ions whose reaction with double bonds in monomer molecules leads to the growth of a polymer chain. The time of its formation may be either essentially less than that of monomer consumption or comparable with it. The first case takes place in the processes of free-radical polymerization whereas the second one is peculiar to the processes of living anionic polymerization. The distinction between these two cases is the most greatly pronounced under copolymerization of two and more monomers when the change in their concentrations over the course of the synthesis induces chemical inhomogeneity of the products formed not only for size but for composition as well. 

The polymerization proceeds by a consecutive addition of monomeric units to reactive centers and after each addition cycle a new reactive center is regenerated at the end of the growing macromolecule. The propagation stops when all the monomer has been consumed. The polymerization proceeds again if a new amount of monomer is added. If one adds a second monomer B different from the first one A generally a block copolymer AB is obtained. The initiation of the polymerization of B by the living ends of A is only possible if the electroaffinity of B is higher or at least equal to that of A. When the two monomers have similar electroaffinities, the order of addition is indifferent. 

True block copolymers containing long blocks of each homopolymer in a diblock, triblock, or multiblock sequence are formed by simultaneous polymerization of the two monomers when n > 1 and r2 8> 1. However, block copolymers are prepared more effectively by either sequential monomer addition in living polymerizations, or by coupling two or more telechelic homopolymers subsequent to their homopolymerization. Alternatively, if the two monomers do not polymerize by the same mechanism, a block copolymer can still be formed by sequential monomer addition if the active site of the first block is transformed to a reactive center capable of initiating polymerization of the second monomer. 

The key feature distinguishing anionic (and cationic) from free-radical polymerization is the absence of spontaneous binary termination and has already been mentioned. Unless chain transfer occurs, polymer chains keep growing until all monomer is used up. At that stage, the polymer still carries reactive centers [65] —it is said to be a "living polymer" [66,67]—, and polymerization can be started anew by addition of further monomer. Block copolymers can be synthesized from a living polymer by addition of a different monomer [68,69]. 

Living ionic methods, however, have limitations as to the types of monomers that can be polymerized resulting from the incompatibility between the reactive centers and monomers. Radical polymerizations, on the other hand, do not really suffer from these drawbacks because a free radical is less discriminating re-... 

This method requires not only strictly living conditions but also the quantitative conversion of the living growing centers into reactive end-groups. At present, living polymerization of THF and 1-t-butylaziridine have been used for this purpose, e.g. ... 

Andrzej Duda is head of the Department of Polymer Chemistry at the Center of Molecular and Macromolecular Studies of the Polish Academy of Sciences in Lodz, Poland and currently chairman of the Polymer Section of the Polish Chemical Society as well as a member of the Polish National Science Center. He received his MSc degree from Lodz Univereity of Technology (1975), his PhD (1984, under the supervision of Stanislaw Penczek), and his DSc (1997) from the Polish Academy of Sciences. Since 2004, he has been a full professor in chemistry with the title conferred by the President of Republic of Poland. His research interests focus on thermodynamics, kinetics, and mechanisms of the ring-opening and ionic polymerizations, reactivity-selectivity relationships in polymerization, methods of controlled/living polymerization, macromolecular engineering, and polymers and monomers available from renewable resources. He is the author and coauthor of more than 100 scientific papers (including 5 book chapters). 

This trend was reversed after it was realized that lowering the intrinsic reactivity of propagating species could bring about many benefits. Inspired by the works in anionic polymerization by Szwarc, who discovered the features of living polymerizations, several teams have uncovered in the early 1980 the ways to control cationic polymerization. It was not yet a question of fiuly living polymerizations because transfer and termination reactions could not be completely avoided but, under certain experimental conditions, it was possible to separate the initiation step from that of propagation and secure a satisfactory persistence of the propagating active centers. 

The retrogradation of the dissociation equilibrium is brought about by addition to the reaction medium of common ion salts whose cation is inert toward the polymerization system. As for the nature of the counterions, those that bring about a certain covalency of the active centers are preferred. Addition of a weak nucleophile is also an efficient means to curb the reactivity of carbocations. The following systems (monomer, initiating system, solvent) qualify, more or less perfectly, for the category of living polymerizations ... 

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