Polymerizations of Lactams

Lactams are cyclic amides formed by the intramolecular amidation of amino acids. The polymerization of lactams [Eq. (10.48)] 

Hydrolytic polymerization [12,13] of e-caprolactam to form nylon-6 [m = 5 in Eq. (10.48)] is carried out commercially in both batch and continuous processs by heating the monomer in the presence of 5-10% water to temperatures of 250-270°C for periods of 12 hr to more than 24 hr. In the first step, the lactam is hydrolyzed to e-aminocaproic acid  

This is followed by step polymerization of the amino acid with itself 

The initial ring-opening [Eq. (10.49)] and subsequent propagation steps [Eqs. (10.50) and (10.51)], which include both condensation and stepwise addition reactions, constitute the principal mechanism of the polymerization and may be represented by the following three equilibria  

Employing the usual simplifying assumption, that the reactivity of the end groups are equal and independent of the chain length of the respective molecules, K2 my be expressed as 

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). 

Generally, the initiators activate the inactive amide groups, causing them to react with other lactams through successive transamidations that result in formations of polyamides. Both acids and bases catalyze the transamidation reactions. The additions of electrophiles affect increases in the electrophilicity of the carbonyl caibon of the acylating lactam. The nucleophiles, on the other hand, increase the nucleophilic character of the lactam substrate (if they are bases). 

All initiators can be divided into two groups. To the first belong strong bases capable of forming lactam anions by removing the amide proton. This starts the anionic polymerization reaction. To the 

The polymerization of lactams is distinguished by pronounced polymerization equilibria, i.e., -h M j. The equilibrium position var- 

Lactams can be polymerized anionically, cationically, or hydrolytically. Anionic polymerizations are started by sodium or Grignard compounds. They are used to produce large, molded articles in industry, since the polymerization rate is fast at relatively low temperatures and no volatile by-products are evolved. In the anionic polymerization, a nucleophilic lactam anion attacks a lactam molecule [see (18-33)]. Reaction of the newly produced anion with the large excess of lactam regenerates the actual initiator, the lactam anion  

The dimer anion which is formed attacks a further lactam molecule, etc. [cf. (18-34)]. 

With the cationic initiators known, only low yields and degrees of polymerization are obtained with a-caprolactam. Therefore the cationic polymerization process is not used commercially. Low yields and low degrees of polymerization presumably result from formation of amidine structures  

Commercially, the hydrolytic polymerization is the most important. With caprolactam, it is carried out at 250-260°C with water or water-producing substances as initiators. In principle, three different reactions can take place that is, hydrolysis of the lactam. 

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Anionic polymerization of lactams was shown to proceed according to what is called the activated monomer mechanism. With bischloroformates of hydroxy-terminated poly(tetramethyleneglycol) and poly(styrene glycol) as precursors for a polymeric initiator containing N-acyl lactam ends, block copolymers with n-pyrrol-idone and e-caprolactam were obtained by bulk polymerizations in vacuum at 30 and 80 °C, respectively361. ... 

It is well known that the anionic polymerization of lactams in general is markedly facilitated by the addition of N-acetylated lactams60-62. The polymerization of 61 is also accelerated by the addition of its N-acetylated compound 65 (Table 8). 

Azolides used as promotors in the anionic polymerization of lactams,1231,1241 and as plasticizers for PVC 1251... 

K. Hashimoto, Ring-Opening Polymerization of Lactams. Living Anionic Polymerization and Its Applications , Prog. Polym. Sci. 2000, 25, 1411-1462. 

The synthesis of polyamides follows a different route from that of polyesters. Although several different polymerization reactions are possible, polyamides are usually produced either by direct amidation of a diacid with a diamine or the self-amidation of an amino acid. The polymerization of amino acids is not as useful because of a greater tendency toward cycliza-tion (Sec. 2-5b). Ring-opening polymerization of lactams is also employed to synthesize polyamides (Chap. 7). Poly(hexamethylene adipamde) [IUPAC poly(iminohexanedioylimi-nohexane-l,6-diyl) or poly(iminoadipoyliminohexane-l,6-diyl)], also referred to as nylon 6/6, is synthesized from hexamethylene diamine and adipic acid [Zimmerman, 1988 Zimmerman and Kohan, 2001]. A stoichiometric balance of amine and carboxyl groups is readily obtained by the preliminary formation of a 1 1 ammonium salt (XU ) in aqueous solution at a concentration of 50%. The salt is often referred to as a nylon salt. Stoichiometric... 

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. 

A variety of protonic and Lewis acids initiate the cationic polymerization of lactams [Bertalan et al., 1988a,b Kubisa, 1996 Kubisa and Penczek, 1999 Puffr and Sebenda, 1986 Sebenda, 1988]. The reaction follows the mechanism of acid-catalyzed nucleophilic substitution reactions of amides. More specibcally, polymerization follows an activated monomer mechanism. Initiation occurs by nucleophilic attack of monomer on protonated (activated) monomer (XXIV) to form an ammonium salt (XXV) that subsequently undergoes proton exchange with monomer to yield XXVI and protonated monomer. The conversion of XXIV to XXV involves several steps—attachment of nitrogen to C+, proton transfer from... 

Various side reactions greatly limit the conversions and polymer molecular weights that can be achieved in cationic polymerization of lactams. The highest molecular weights obtained in these polymerizations are 10,000-20,000. The most significant side reaction is amidine (XXXI) formation [Bertalan et al., 1984]. Propagation of the polymer chain... 

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]. 

The anionic polymerization of lactams proceeds by a mechanism analogous to the activated monomer mechanism for anionic polymerization of acrylamide (Sec. 5-7b) and some cationic polymerizations of epoxides (Sec. 7-2b-3-b). The propagating center is the cyclic amide linkage of the IV-acyllactam. Monomer does not add to the propagating chain it is the monomer anion (lactam anion), often referred to as activated monomer, which adds to the propagating chain [Szwarc, 1965, 1966]. The propagation rate depends on the concentrations of lactam anion and W-acy I lactam, both of which are determined by the concentrations of lactam and base. 

Polymerizations initiated by strong bases (R-, IIO, RO-) and tertiary amines (which are poor nucleophiles) proceed at much faster rates than do polymerizations initiated hy primary amines. Also, unlike the latter, where each polymer chain contains one initiator fragment (i.e., RNH—), these polymerizations do not result in incorporation of the initiator into the polymer chain. Polymerization proceeds by an activated monomer mechanism similar to that in the anionic polymerization of lactams. The reacting monomer is the NCA anion XLIV... 

What are the roles of an acylating agent and activated monomer in the anionic polymerization of lactams ... 

It has become the custom to name linear aliphatic polyamides according to the number of carbon atoms of the diamine component (first named) and of the dicarboxylic acid. Thus, the condensation polymer from hexamethylenedi-amine and adipic acid is called polyamide-6,6 (or Nylon-6,6), while the corresponding polymer from hexamethylenediamine and sebacoic acid is called polyamide-6,10 (Nylon-6,10). Polyamides resulting from the polycondensation of an aminocarboxylic acid or from ring-opening polymerization of lactams are indicated by a single number thus polyamide-6 (Nylon-6) is the polymer from c-aminocaproic acid or from e-caprolactam. 

Chemistry of Anionic Ring Opening Polymerization of Lactams. 8... 

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. 

Polymerization of lactams in reactive processing proceeds with the involvement of a catalyst and direct or indirect activators. A mathematical model of the process must be a kinetic equation relating the rate of conversion of a monomer to a polymer to the reagent concentrations and temperature. The general form of the model is... 

The batch process equipment used for preparing the components is essentially a set of reactors equipped with heaters and agitators. They operate under vacuum or in an inert gas atmosphere. One of the main requirements of the chemical molding process is the production of pore- and defect-free articles. The volatile products and moisture must be thoroughly removed from the reactant mixture. Moisture imparts porosity to the final articles due to evaporation and the chemical interaction of water with the components of the reactant system, for example, with isocyanates in case of polyurethane formulations. In some cases, moisture can also inhibit the polymerization process, for example, anionic-activated polymerization of lactams. Many monomers, particularly acrylic compounds, require removal of die inhibitors to increase their shelf-life. 

Semicrystalline polyamide fine powders have been used as toughening agents for epoxy networks. The powders can be obtained by grinding granules, or directly by anionic polymerization of lactams, 6 or 12, in an organic solvent from which the formed semicrystalline polymer precipitates. Microporous powders with an average particle size in the range of 10 pm and a narrow particle-size distribution, are obtained. 

Anionic polymerization of 58 activated by N-benzoyl lactam proceeds without side reactions. Since side reactions in the anionic polymerization of lactams are mainly caused by protonabstraction, the pKa value for the bridge-head methine proton adjacent to the lactam-carbonyl group in 58 must be higher than that for the a-methylene protons in 2-pyrrolidone. This is because the former monomer has a rigid bicyclic structure. 

In a study of the polymerization of lactams it was noted that 156 (R = benzenesulfonyl) did not polymerize.183... 

The equilibrium in these cases is shifted to the right the monomer yields a formation carrying charge (energy), and only the monomer activated in this way is added to the end of the (usually) neutral chain. This type of addition is typical for the ionic polymerization of lactams. 

This statement has its limitations. Ionic polymerization in hydrocarbons is always kinetically complicated, it often starts only after the addition of a polar compound (co-initiator), and it is affected by the aggregation of initiating and propagating particles. In strongly polar media, activation of initiator by dissociation of acids and bases is easy. Such solvent is simultaneously a reactive transfer agent. Propagation usually does not occur, and only low molecular products are formed. Exceptions can, of course, be found. During anionic polymerization of lactams in DMF, the solvent only increases the amount of dissociated initiator [27]. 

Polymerization of oxirane (and of its derivatives) by the mechanism of activated monomer is so far exclusively cationic and can be represented by schemes (27) and (28) of Chap. 4. In contrast to the ring-opening polymerization of lactams, both the classical and the activated monomer mechanisms are operating in this case. Conditions can be found where one or the other mechanism predominates [339]. 

In his review, Reimschuessel described [353] isomerization polymerizations of lactams containing a carboxyl group as a substituent or part of a substituent. Whereas normal lactam ring opening leads to polyamides, isomerization polymerization produces polyimides in a complicated way, with proton transfer and through bicyclic intermediates... 

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). 

There are essentially two mechanisms of chain growth in the cationic polymerization of lactams. Either the cationically activated monomer reacts with neutral growth center or the neutral monomer molecule reacts with cationic active centers located at the end of the growing chain [214) ... 

Polyamides prepared by cationic polymerization of lactams contain a significant fraction of cyclic oligomers. Reactions leading to formation... 

Back-biting reaction occurring during cationic polymerization of lactams is detrimental to preparation of block copolymers of two different lactams by sequential polymerization. Block copolymers can be obtained only in those systems in which the rate of polymerization of the second monomer is much higher than the rate of chain transfer to polymer resulting in transamidation [219]. 

One of the few systems for which the initiation with protonic acid has been comprehensively studied is the cationic polymerization of lactams Initiation of the cationic polymerization of lactams with HCl gives predominantly a nwnomer molecule protonated at the oxygen atom but a small amount of N-protonated lactam is assumed to be present in the tautomeric equilibrium ... 

In the cationic polymerization of lactams, initiated with protonic acids, macromolecules are formed having two ends (A and B in the structure below) potentially active (cf. Sect. 3.1) ... 

Termination involving intramolecular proton shift or intermolecular proton transfer, both followed by elimination of a molecule of water has been reported for the high temperature cationic polymerization of lactams a) intramolecular ... 

The formation of polyamides by ring-opening polymerization of lactams can proceed through several types of reversible transacylation reactions in which the lactam amide bond is cleaved while a polymer amide bond is formed, viz. 

Irrespective of the reaction mechanism, the polymerization of lactams leads to an equilibrium between monomer, cyclic oligomers and polymer. Tobolsky and Eisenberg [9] showed that the thermodynamic parameters are independent of the reaction mechanism, so that the polymerizability may be rationalized in terms of the ease of formation of the cyclic monomer, or, its opening into a linear chain unit. The simple relation between the equilibrium monomer concentration [L]e, temperature, and standard heat and entropy of polymerization. 

The accumulation of a great number of small particles into one polymer chain is an aggregation process resulting in a decrease in the translational entropy of the system. In the polymerization of cyclic monomers, the decrease of translational entropy is partially counterbalanced by the increase in rotational and vibrational entropy resulting from the conversion of a more or less rigid cyclic monomer into a flexible monomer unit inside a polymer chain. Thus the net entropy of polymerization of lactams is more positive (e.g. —3 eu for seven-membered lactams) than the entropy of polymerization of vinyl monomers (—25 to —30 eu). 

In this connection it has to be stressed that the ceiling temperature concept should be applied to the polymerization of lactams very carefully. At elevated temperatures, the reversible transacylation reactions are accompanied by decomposition of the polymer and/or monomer and by other side reactions. With respect to the low ASp values (as compared to vinyl monomers), very high ceiling temperatures can be expected for most lactams. Therefore, the calculated values of lie high above the decomposition temperature of the polymer and monomer except for five-, six- and substituted seven-membered lactams. In addition, for very large rings the heat of polymerization approaches zero and the polymerization entropy becomes positive, so that the value of T<, = AHp/ASp becomes meaningless. 

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