Conditions of synthesis of polymers with known in advance molecular weight and with desired end-groups were discussed. This problem was connected with the principle saying that the more reactive are the substrates, the less selective they are (the reactivity-selectivity principle, RSP). A quantitative comparison of the course of anionic and covalent (pseudo-anionic) polymerization of ε-caprolactone (KL) from the point of view of RSP was presented. It was shown that the propagation with the participation of ionic pairs ··· -O-Na+ is in this case much faster than the propagation with the participation of covalent species ··· -O AIR2; the respective rate constants are: kp± ≥1.7 l/mole·s and kpcov=3.0·10-2 l/ mole·s (THF,20°C). Simultaneously with propagation an intermolecular transesterification (back-biting) takes place leading to undesirable cyclic oligomers. The rate of formation of cyclic dimer with a rate constant of kb (2) was used as a measure of the side-reactions rate. The following values of the selectivity coefficients kp/kb(2 ) (in l/mole) in dependence on the type of active centres were determined: 1.55·103 (for ··· -O-Na+), 4.6·104 [for ··· -OAl(C2H5)2] and 7.7·104 [for ··· -OAl(i-C4H9)2]. The measured kinetic parameters show that RSP can be used to explaining the phenomenon observed in the polymerization of KL: a linear high-molecular-weight polymer is formed with a much high er yield in the pseudo-anionic process with the participation of covalent centres which are less reactive than the ionic active centres. A series of examples of cationic and radical polymerizations of vinyl monomers was cited in which the reduction in reactivity of active centres made it possible to elaborate more convenient conditions of polymer synthesis.
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