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678.675:66 09 and N. I. Lytkina***The point of view that PA-6 with a minimal content of nonlinear macromolecules can only be obtained by twostage (in melt and solid phase) synthesis was advanced. The liquid-phase stage should take place according to a periodic scheme so that the maximum time the polymer remains in the melt state would be much less than the time of accumulation of a practically important amount of defective macromolecules in it. It was shown that when lithium halides are used, the temperature boundary of liquid-phase synthesis of PA-6 by hydrolytic polymerization of caprolactam can in principle be decreased to 483-473 K.Among the causes that make it difficult to attain strength in industrial polyamide (PA) fibres of the order of 100 cN/tex is the existence of side reactions in synthesis of PA-6 which cause formation of nonlinear macromolecules [1]. According to the available data [2][3][4], these reactions begin to take place at a marked rate at temperatures above 523 K. However, the specific features of all industrial variants of liquid-phase synthesis of PA-6 consist of the fact that more or less of the melt remains in the continuous polymerization units for an indefinitely long time and for this reason independently decreasing the synthesis temperature to T < 523 K cannot prevent the appearance of nonlinear macromolecules in the polymerizate.Moreover, it is probably more correct to assume that the formation of such macromolecules can take place at any melt temperature, and the number of them in the polymer is proportional to the total time in this state. As a consequence, it is very important to not simply lower the synthesis temperature but also to do so so that the maximum time the PA-6 melt remains at this temperature is much less than the time required for accumulation of a practically important number of defective macromolecules.This essentially means that high-molecular-weight PA-6 with a minimum content of nonlinear macromolecules can only be obtained with two-stage (in melt and solid phase) synthesis. The stage of synthesis in a melt should take place according to a periodic scheme and in a way to guarantee a maximum two-fold (in comparison with the average duration of the reaction) increase in the time melt residues from the preceding batch spend in the reactor.There are no published data on how the temperature of liquid-phase synthesis of PA-6 by hydrolytic polymerization of caprolactam (CL) affects the ratio of the rates of the basic and side reactions, although it is probably possible to hypothesize that the lower it is, the more this ratio will change to the benefit of the basic reactions. For this reason, it is extremely important to know the lower temperature boundary of such synthesis in general. The experimental data concerning this problem are discussed in the present article.We know [5] that crystallization of flexible-chain polymers only takes place in sufficiently strong (by 10-20°C in comparison to the melting point T m of the polymer) supercooling of the melt and it is pr...