Caprolactam, polymerization

Figure 3.16 Effect of initiator on caprolactam polymerization at 260°C (o) water 0.01 mol % (x) aminocaprolic acid 0.01 mol %.28...

The cyclic oligomers are only slightly soluble in water and dilute solutions of caprolactam. They tend to separate out from die extracted waste during die process of concentration and chemical purification of die caprolactam. The cyclic oligomers tend to form on the walls of the equipment used in die process equipment. 6-Aminocaproic acid or sodium 6-aminocaproate may also be found in die oligomeric waste, especially if sodium hydroxide is used to initiate die caprolactam polymerization. 

Step growth polymerization can also take place without splitting out a small molecule. Ring-opening polymerization, such as caprolactam polymerization to nylon 6, is an example. Polyurethane formation from a diol and a diisocyanate is another step growth polymerization in which no small molecule is eliminated. 

The base catalysed caprolactam polymerization differs substantially in various aspects from the wellknown industrial polymerization process, which is introduced by water, and the mechanism of which is governed by endstanding carboxylic and amino groups. Therefore it is evident, that the broad experience gathered in the hydrolytic polymerization of caprolactam cannot be applied to the base polymerization, which was developed separately and which requires an independent theoretical treatment. 

The strong catalytic activity of bases in the caprolactam polymerization was recognized about as soon (44—47) as the "normal hydrolytic process was, but nevertheless, the fundamental informations about the basic process were disclosed only during the last few years. It was shown quite recently, that the base induced polymerization consists of an extremely complex action of the strong base (75, 31, 66—68) which is connected with the presence and/or formation of certain additional components. The base plays not only a role in the initiation and propagation reaction but it also involves important side reactions which in turn affect the active centers of the polymerization (67, 96). 

The mechanism of the caprolactam polymerization, i. e. the transamid-ation reaction catalysis by the system imide + salt can be interpreted by a nucleophillic attack of the amide anion on the carbonyl group of the imide which represents the strongest electrophillic reagent in the polymerizing system. 

Because the reactions (b) and (c) cannot proceed, if there is no hydrogen at the amide group, N-alkylated caprolactams failed to polymerize by basic catalysts. Indications of N-methyl caprolactam polymerization and copolymerization are to be revised experiments describing such polymerizations couldn t be reproduced and the authors mentioned seem to be mislead by results obtained by the polymerization of samples of N-methylcaprolactam containing appreciable amounts of unsubstituted caprolactam. 

During a study of the e-caprolactam polymerization mechanism in isothermal conditions,1,2 it was found that there was a distinct induction period at the beginning of the process when sodium caprolactam salt was used as a catalyst. The addition of the necessary quantity of an activator into the reactive mixture leads to a reduction of the induction period and thus allows us to regulate the process rate. 

The use of various thermal methods to investigate e-caprolactam polymerization allowed us to determine the expression for the kinetic function f(P) for this monomer 29 32... 

It is worth emphasizing that deceleration in anionic activation polymerization of co-dodecal-actam is the reverse of self-acceleration in e-caprolactam polymerization. 

Non-isothermal acceleration due to the enthalpy of the co-dodecalactam polymerization reaction is much less than that of e-caprolactam polymerization. Indeed, the total heat effect in polymerization of co-dodecalactam is 43 J/g, and it results in a maximum increase in temperature in adiabatic conditions of 18 - 20°C (2.5 times less than for e-caprolactam). 

Table 2.2. Values of the constants co and ci = K(1 + co) in the kinetic equation for E-caprolactam polymerization...

The parameters in Eq. (2.59) are usually determined from the condition that some function mean-square deviations between the experimental and calculated curves (the error function). The search for the minimum of the function Nelder-Mead algorithm.103 As an example, Table 2.2 contains results of the calculation of the constants in a self-accelerating kinetic equation used to describe experimental data from anionic-activated e-caprolactam polymerization for different catalyst concentrations. There is good correlation between the results obtained by different methods,as can be seen from Table 2.2. In order to increase the value of the experimental results, measurements have been made at different non-isothermal regimes, in which both the initial temperature and the temperature changes with time were varied. 

Figure 4.1. Flowsheet of production of a reactive mixture by activated anionic s-caprolactam polymerization (explanations of numbers are in the text).

In some cases, the heat source can include both the heat of polymerization and the heat output of crystallization of the newly formed products. This is the case in anionic activated e-caprolactam polymerization. This dual heat source must be included in the energy balance equation. As was discussed above, the temperature dependence of the crystallization rate is somewhat complicated. Nevertheless, the propagation of the heat wave is analogous to other well-known cases of wave propagation from consecutive reactions. 

Theoretical modelling and analysis of the results for the superimposed processes of polymerization and crystallization was carried out for wave propagation in anionic activated reaction of e-caprolactam polymerization.258 In the steady situation, the process is described by the system of differential equations ... 

It can be shown that three different modes of front propagation during the formation of polycaproamide can be observed, depending on the relationship between the process parameters. In the first mode, which was found experimentally, the zones of polymerization and crystallization coincide. In the second mode these zones are separated in space. The third mode, which was predicted theoretically, is characterized by a non-monotonic distribution of the degree of crystallinity. However, it is not clear whether this situation can actually be observed in anionic e-caprolactam polymerization because even slight variations in parameters transfers the system into another regime. 

For the initiation of caprolactam polymerization, Grignard reagents [173] have been used as a source of lactamate salt. Tsuchiya and Tsuruta have described an interesting case of methyloxirane polymerization initiated with... 

Initiation of e-caprolactam polymerization in the presence of acyllactam can be described by the scheme... 

Many authors have observed an acceleration of caprolactam polymerization in the presence of A-acetyl-e-caprolactam, isocyanates, thiolactones, organic sulphates, aromatic amides, benzyldioxime carbonate, arylenedicarbamoyllactams, phosphorus oxychlorides and phosphorus pen-tachloride. Activator effectiveness increases with growing substituent electronegativity in N-substituted lactams. 

Decay of imide groups (acyllactam and diacylamine structures) during anionic caprolactam polymerization 1125]... 

Fig. 20. Concentration of caprolactam hydrochloride during cationic caprolactam polymerization [186]. For conditions see Fig. 19.

The specific role of either amine or carboxyl groups in the polyaddition reaction has been established by Heikens et al. [216] by following the incorporation of added amine or carboxylic acid during the hydrolytic caprolactam polymerization. These experiments revealed that the lactam is added at the amine end group and that the reaction is catalysed by carboxyl groups. 

Heikens and Hermans [215] further improved the E eement between the experimental and calculated course of caprolactam polymerization by taking into consideration the volume contraction (ca. 9%) occurring during the polymerization. 

H. K. Hall (3) of DuPont subsequently proposed an identical mechanism which was published in December, 1958. The mechanism was tested by the simple procedure of adding a pre-formed acyllactam to caprolactam containing sodium caprolactam at 160°C. Very rapid polymerization and a solid nylon casting resulted in four to five minutes. In the absence of the acyllactam initiator, polymerization did not occur. The mechanism for the acyllactam initiated caprolactam polymerization shown in Figure 2 produces a nylon chain with an acylamino end group and without the amino end group of Figure 1. 

The most commonly-used catalysts for caprolactam polymerization are sodium caprolactam and caprolactam magnesium bromide. The latter catalyst can be made by reacting a Grignard reagent with caprolactam. 

Nylon block copolymers were previously synthesized from the anionic polymerization of caprolactam in the presence of polyurethane prepolymers. (11) The prepolymers, prepared from the reaction of diisocyanates with polyether glycols, contained Isocyanate end groups which initiated caprolactam polymerization. Sodium caprolactam was used to catalyze the reaction. This copolymer system is the basis for some current areas of nylon 6 RIM research. (12) NYRIM nylon block copolymers are formed from stoichiometric mixtures of polymeric polyols and caprolactam using poly acyllactam initiation which was described previously. The reactions are as follows ... 

POLYESTERAMIDE PREPOLYMER In the prepolymer reaction, the multifunctional acyllactam which normally acts as initiator for caprolactam polymerization, also functions to combine the polymeric polyol moeities. An excess of acyllactam is used so that the resulting prepolymer is terminated by acyllactam. The reaction occurs slowly with heat (13), but in the presence of an alkaline catalyst is completed within seconds. The prepolymer may be prepared in mass or in the presence of inert organic solvents, or in caprolactam as part of the total copolymerization reaction. See Reaction B. 

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