Solid-phase synthesis of polyamide-6

Abstract

678.675:66 09Solid-phase synthesis of PA-6, simultaneously discovered with the reaction of conversion of CL into a polymer and of practical interest primarily as a method for obtaining a polymer constructed of strictly linear macromolecules and containing a minimum amount of CL and especially cyclic oligomers, is not solid-phase in the generally understood meaning of this term. This essentially concerns a set of liquid-phase reactions which differ from ordinary reactions due to the fact that some of the participants and primarily the solvent molecules, which the kinetic segments of transfer chains act as, are free of translational mobility, and the entire reaction volume is in a stressed state due to differences in the coefficients of thermal expansion of amorphous and crystalline microregions of the polymer and the developed process of additional crystallization. The presence of an important reaction volume which persists in a relatively wide temperature range and the very large amplitudes of vibrations of translationally immobile reactive groups makes the process kinetically more advantageous than solid-phase polycondensation of monomeric ε-aminocaproic acid.Low-temperature hydrolytic polymerization of caprolactam (CL) accompanied by crystallization of the polycaproamide formed (PA-6) and the subsequent evolution of the reaction as formally solid-phase was described for the first time in the patent [1] obtained by Hubert and Ludewig in Germany in 1939. The possibility of this process also follows from the sense of the patents [2, 3] obtained by Schlack in Germany in 1938 and in the US in 1941.In 1942, this version of synthesis of PA-6 was used by Staudinger and Warder .4, p. 19/] for obtaining a polymer of rigorously linear structure. However, it has not yet been implemented on the industrial scale due to the lack of an acceptable method of giving the solid polymer a shape convenient for processing it into yarn [5, p. 90].The studies conducted in [1959][1960][1961][1962][1963][1964][1965][1966][1967][1968][1969][1970][1971][1972] showed that Klares ideas [5, p. 90] concerning solid-phase synthesis of PA-6 from ε-aminocaproic acid were also practically unrealizable.The low rate of the process and mainly the very narrow temperature range in which the first stage can take place conversion of the monomer into an oligomer are a stumbling block in this case.Moreover, the research conducted in this period on solid-phase additional polycondensation of demonomerized PA-6 obtained by purely liquid-phase hydrolytic polymerization of CL to significantly increase its molecular weight and correspondingly the physicomechanical indexes of the spun yarns allowed Kudryavtsev, Nosov, and Volokhina to conclude in the second half of the 70s that the technical and economic advantages of solid-phase complete polyamidation of such PA-6 are obvious, but the process and implementation of the process require further development [6].Today solid-phase additional polycondensation is widely used in synthesis of high-molecular-weight polyethy...

The role of post-crystallization of polyamide-6 in its solid-phase polycondensation

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The character of the effect of additional crystallization of the polymer on the kinetic parameters of its solid-phase polycondensation is discussed based on data on the kinetics of the change in the molecular weight an density of polyamide-6 on heating in a vacuum at 453-483 K. This reaction is polychronic and consists of pseudomonomolecular dehydration of contact pairs formed by the amino and carboxyl groups in the macromolecules in crystallization of the polymer melt.Solid-phase polycondensation of polyamide-6 (PA-6) purified of unreacted caprolactam and cyclic oligomers (demonomerized) by aqueous extraction at 433-483 K in a vacuum or inert gas stream in conditions of an irreversible reaction has become [1] an important process stage in production of the polymer used in manufacture of industrial fibres and different injection-molded items.However, in this temperature region, additional crystallization (post-crystallization) of the polymer should and actually does take place [2, 3], which reduces the volume and perhaps also alters the structural organization of the amorphous regions of PA-6 in which the polycondensation reaction strictly takes place.As we noted previously in [3][4][5][6], the main difference between the solid-phase polycondensation that takes place in amorphous regions of the partially crystalline polymer and the reaction in a melt consists of the absence of terminal groups in the macromolecules of translational mobility in the first case. For this reason, the reaction can essentially take place only because of the groups that formed contact pairs in crystallization of the melt.

From a kinetic point of view, this is equivalent to knowing that solid-phase polycondensation takes place as a pseudomonomolecular reaction of dehydration of such contact pairs, which can be complicated by the different "readiness" of the latter for carrying out the chemical act.

These reactions are called [7] polychronic and in the case of a first-order irreversible reaction, the kinetic curves are linearized in coordinates of the equation [8] , ln 1 ) 76 .

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“…As we noted previously in [3][4][5][6], the main difference between the solid-phase polycondensation that takes place in amorphous regions of the partially crystalline polymer and the reaction in a melt consists of the absence of terminal groups in the macromolecules of translational mobility in the first case. For this reason, the reaction can essentially take place only because of the groups that formed contact pairs in crystallization of the melt.…”