Cationic polymerization of lactams

The ring-opening polymerization reactions depend upon thermodynamic and kinetic factors, and on the total molecular strain energies of the particular ring stmctures. Six-membered 8-valerolactam is the most stable ring stmcture and most difficult to polymerize. Also, presence of substituents increases the stability of the rings and decreases the ability to polymerize. 

The catalysts for cationic polymerization can be strong anhydrous acids, Lewis acids [115], salts of primary and secondary amines, carboxylic acids, and salts of amines with carboxylic acids that split off water at elevated temperatures [114]. The initiators react by coordinating with and forming rapid pre equilibrium lactam cations. These cations are the reactive species in the polymerizations. Initiations of this type are also possible with weakly acidic compound, but such compounds are not able to transfer protons to the lactam. They are capable, however, of forming hydrogen bonds with the lactams. The high reactivity of the lactam cations may be attributed to the decreased electron density at the carbonyl carbon atoms. This makes them more subject to nucleophilic attacks [114]. 

Protonations of the amides occur at the oxygens [116], but small fractions of N-protonated amides are also presumed to exist in tautomeric equilibrium. To simplify the illustrations, all lactams will be shown in this section as  

while the above stmcture commonly represents propiolactam, in this section it can mean any lactam, like a caprolactam, valerolactam, etc. Thus, the equilibrium can be shown as follows  

In a reaction mixture where the initiators are strong acids the strongest nucleophiles are the monomers. Acylations of the monomers with the amidinium cations result in formations of aminoacyllactams [113]  

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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... 

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 ... 

Cationic polymerization of lactams shows a number of pecularities, not covered in Vol I (Adv. Pol. Sci. 37). Therefore, relatively more attention is paid in this section to the polymerization mechanism. These pecularities are discussed separately for different initiating systems. 

The mechanism of the cationic polymerization of lactams formally resembles the activated monomer mechanism, better known for the anionic process. In this media-... 

Cationic polymerization of lactams is a complex process consisting of several elementary reactions which influence the kinetics of polymerization and product properties (molecular weights, structure of end-groups). 

This process for cationic polymerization of lactams has been studied extensively. Figure 2.1 shows the principal reactions of monomer conversion and chain growth [27]. Mechanistically, chain growth can commence on both the ammo-terminal end via acylation and the carboxy-terminal end via aminolysis of the polymer molecule. High extents of polymerization are rarely attained because of the occurrence of side-reactions. As shown in Figure 2.2 [27] these side-reactions result in terminal amidine groups that are incapable of adding further lactam. The cationic polymerization process has therefore not attained any practical importance. 

FIGURE 2.2 Formation of amidine functions during cationic polymerization of lactams. (From Reimschuessel, H.K., J. Polym. ScL, Macromol. Rev., 1977, 12, 65. With permission.)... 

In the cationic polymerization of lactams the ammonium and amidinium groups form N-terminal chain ends. The C-terminal chain ends are in the form of carboxylic acid groups or alkylamide residues. This is important, because the nature of the end groups and their reactivity determine the following step in the polymerizations. This means that the different types of cationic polymerizations of lactams are the results of the different end groups that form during the initiation steps. Formation of amidines increases with increasing acidity and concentration of the initiator and with an increase in the temperature ... 

Describe the catalysts that are useful in cationic polymerizations of lactams and the mechanism of polymerization. 

Certain heterocycles polymerize by very peculiar reaction pathways. For example, the cationic polymerization of lactams proceeds by activation of the monomer, as for their anionic polymerization ... 

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