Hydrolytic polymerization of lactams

This polymerization can be looked upon as a special case of cationic polymerization. It is particularly true when weak acids are added to the reaction mixture, as is often the case in industrial practice. In hydrolytic polymerizations of lactams initiated by water, the hydrolysi ondensation equilibria determine the concentrations of the amine and the carboxylic acid groups. Both functional groups participate in the propagation reactions  

The concentration of these groups also determines the molecular weights of the final products. This type of equilibria also occurs in polymerizations initiated by amino acids or by salts of carboxylic acids formed with primary and secondary amines. In the hydrolytic polymerizations of caprolactam the above reactions involve only a few percent of the total lactam molecules present. The predominant propagation reaction is a step-growth addition of lactam molecules to the end groups. It is acid-catalyz  

The exact mechanism of this addition is uncertain. It was postulated that the addition steps are through reactions of neutral lactam molecules with ammonium cations. Others felt, however, that the lactam molecules add to the undissociated salts. 

In industrial preparations, most of the water used to initiate the polymerizations is removed after conversions reach 80-90% in order to attain high molecular weights. The final products contain about 8% of caprolactam and about 2% of a cyclic oligomer. These are removed by vacuum or hot-water extraction. The material is then dried under vacuum at 100-200 °C to reduce moisture to about 0.1%. 

The polymerizations of these anhydrides (or substituted oxazolidine-2,5-diones) can be carried out with basic catalysts to yield polyamides  

Hydrolytic polymerizations are the smoothest of all three types of polymerization reactions because the growing species are less activated than in either cationic or anionic polymerizations. Many commercial processes utilize it in s-caprolactam polymerizations. Formation of irregular structures, however, and even crosslinked material was detected. In addition, at elevated temperatures deamination and decarboxylation of polycaprolactam can take place [150] Such reactions can result in formations of ketones and secondary amine groups. The ketones, in turn, can react with amines and form Schiff bases. This leads to branching and crosslinking. 

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Amidines are formed in hydrolytic polymerizations of lactams but do not limit the polymer molecular weight. Molecular weight buildup is not impeded since the carboxyl end groups of growing polymer are quite reactive toward amidine groups [Bertalan et al., 1984]. 

Compare cationic, anionic, and hydrolytic polymerizations of lactams by writing out all three modes of polymerization side by side and discuss and show the side reactions that take place in each of them. 

The values of the Huggins coeffici t k determined in cr l for low-molecular weight samples of nylon 6 and nylon 12 prepared by hydrolytic polymerization of lactams, were extremely high. The phenomenon has been interpreted as association of carboxyl and amine end groups. The assodation constants of the two nylons were calculated from the k values ... 

Since their discovery by Carothers [181], aliphatic polyamides such as nylon-6,6 and nylon-6 are important textile fibers and plastics. Similar polyamides produced by melt reaction of aliphatic diacids - - diamines, or by hydrolytic polymerization of lactams, have some interest as engineering plastics, and will also be discussed in this section. 

After purification by recrystallization from toluene, monomer was polymerized hydrolytically. Polymerization of lactam 49, either cationically or anionically, might result in epimerization of the cis-tetrahydrofuran ring junction which would result in a polymer containing a mixture of cis and trans tetrahydrofuran rings (Fig. 22). The importance of tetrahydrofuran ring geometry has... 

Polyamides (PAs, except for those that are fully aromatic) represent a versatile group of plastics that have been successful for 70 years in the market of fibers, engineering plastics and specialties. Aliphatic polyamides and copolyamides (Scheme 7.1), obtained by a classical polycondensation of diamines and diacids, or a-amino acids (PA 66, PA 612, PA 610, PA 46, PA 11) or by the hydrolytic polymerization of lactams (PA 6, PA 12), belong to the group of highly valued semicrystalline plastics that are widely used in most branches of industry in some applications they are, as yet, irreplaceable. 

The polymerization of lactams (cyclic amides) can be initiated by bases, acids, and water [Reimschuessel, 1977 Sebenda, 1976, 1978 Sekiguchi, 1984]. Initiation by water, referred to as hydrolytic polymerization, is the most often used method for industrial polymerization of lactams. Anionic initiation is also practiced, especially polymerization in molds to directly produce objects from monomer. Cationic initiation is not useful because the conversions and polymer molecular weights are considerably lower. 

Hydrolytic polymerization of e-caprolactam to form nylon 6 (Sec. 2-8f) is carried out commercially in both batch and continuous processes by heating the monomer in the presence of 5-10% water to temperatures of 250-270°C for periods of 12 h to more than 24 h [Anton and Baird, 2002 Zimmerman, 1988]. Several equilibria are involved in the polymerization [Bertalan et al., 1984 Sekiguchi, 1984]. These are hydrolysis of the lactam to e-amino-caproic acid (Eq. 7-56), step polymerization of the amino acid with itself (Eq. 7-57), and initiation of ring-opening polymerization of lactam by the amino acid. The amino acid is... 

Anionic ring opening polymerization of lactams to generate polyamides has been studied quite extensively by Sebenda [8-10], Sekiguchi [11], and Wichterle [12-13], among others, in academia, and by Gabbert and Hedrick [14] and by us [23-25] in industry. By far, caprolactam is the most studied lactam and the nylon 6 prepared by this route compares favorably in properties with that prepared by conventional hydrolytic polymerization. 

Although several cyclic amides (lactams) can be polymerized by cationic mechanism, this method of polymerization is of little practical importance because the anionic or hydrolytic polymerization provides much more convenient route to corresponding polyamides. Polyamides obtained by cationic polymerization of lactams are less stable and oxidize faster than those obtained by anionic polymerization [213). 

Evaluation of the equilibrium concentrations of water, aminocaproic acid, chains and lactam in the hydrolytic polymerization of caprolactam [2, 3, 5, 8, 12, 221, 235, 238] resulted in the following values of Af/j and... 

Polyamides are produced commercially by two routes. First is the polycondensation of aminoacids or diamines with diacids, and the second route is the ring-opening polymerization of lactams l. The ring-opening process can be accomplished with anionic or cationic initiators and also in the presence of water by a hydrolytic process. Nylon-6 (poly-e-caprolactam) and Nylon-12 (poly-co-dodecanelactam) are commercially produced mostly by the hydrolytic process, in which the reaction control is easier and the properties of the product better than in polycondensation. 

This reaction can be initiated by strong bases (metal hydrides, alkali metals, metal amides, metal alkoxides, and organometallic compounds), protonic acids, or by water. Water is often used as the initiator (cf. Problem 10.7) for industrial polymerization of lactams and the process is referred to as hydrolytic polymerization. Anionic initiation with bases is preferred when polymerization is to be carried out in the mold itself for converting monomer directly into a molded object. Cationic initiation with acids, however, is not so useful because both the extents of conversion and polymer molecular weights are significantly lower (Odian, 1991). 

The polycondensation equilibrium, as represented by Equation 2.5, is also part of the more complex mechanism of the water-initiated polymerization of lactams (Equation 2.3), which entails hydrolytic ring opening (Equation 2.11), condensation (Equation 2.12), and addition (Equation 2.13), as the principal equilibrium reactions [15]. 

Polymerizations of lactams produce important commercial polymers. The polymerization reactions therefore received considerable attention. Lactam molecules polymerize by three different mechanisms cationic, anionic, and a hydrolytic one (by water or water-releasing substances). 

The anionic polymerization of lactams (mainly, CL) is less suitable than the hydrolytic process for the in situ preparation of their composites and nanocomposites. This is particularly the case when clays are used as reinforcing agents due to the presence of sizable amounts of water associated with day as well as of mobile cations in most day galleries and surfaces. Both agents contribute to the deactivation of the anionic active groups of the growing polyamide chains or, at least, exert a negative interference with them. In the field of day-based... 

The corresponding polymerizations have, therefore, been the subject of many excellent reviews, books, encyclopedias and monographs [5-11]. At present, these activities are focused on improving the step polymerization (polycondensation) that yields PA 66, together with its modifications. The hydrolytic polymerization of CL has also been studied in detail, and current research in this area is predominantly appHcations-oriented. From both theoretical and practical standpoints, the anionic polymerization of lactams continues to show great promise, due to its... 

Cationic polymerization is the only route to the polymerization of A-alkylated lactams. Both the hydrolytic and anionic routes require that a lactam have a hydrogen on the nitrogen. However, there are no commercial applications for A-alkylated polyamides, probably because their lack of hydrogen bonding results in lower melting points than for polyamides without an A-alkyl group. 

Frequently, the order of rates of polymerization of various lactams is different from that of the thermodynamic parameters for polymerization. For example, the initial rates of hydrolytic polymerization were practically the same for capro-, enantho- and capryllactam [25—27], whereas the corresponding heats of polymerization differed significantly for these monomers [27] —AH = 3.3, 5.3 and 7.8 kcal mole", respectively). Similarly, for substituted caprolactams the sequence of free energies of polymerization is just opposite to the order of rates of anionic polymerization (Fig. 1). 

The example in Scheme 1.29 of the ring-opening polymerization of s-caprolactam is given since it has also been exploited in reactive processing (Brown, 1992). In the example given the initiator is 0.2%-0.3% sodium lactamate. The anionic polymerization (such as that in Scheme 1.29) is faster than cationic or hydrolytic polymerization. 

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