Cationic polymerization, lactams

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... 

For cationic polymerization with an acid whose anion Z is nucleophilic, initiation involves the sequence described by Eq. 7-51 plus the formation of XXIX. XXIX propagates by a sequence similar to that described by Eqs. 7-52 and 7-53 except that a growing polymer chain possesses a Z—CO— end group instead of a lactam end group. 

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... 

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. 

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. 

Being very strong bases, organometallic compounds are capable to start the polymerization (7, 12, 30, 51, 62). If organomagnesium compounds are used, the interaction of magnesium cations with lactam anions can probably reduce the dissociation of the salt analogous the basic character is certainly weakened in lactam complexes formed by addition of hydrated oxides of Ti, Zr, Hf, Th or Ce (48). 

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). 

Cationic polymerization is the only route, however, for the polymerization of iV-substituted (N-alkyl and Af-acyl) lactams the resulting polyamides have much lower melting points than their unsubstituted analogs due to the absence of hydrogen bonding. 

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 ... 

Volume 15 deals with those polymerization processes which do not involve free radicals as intermediates. Chapters 1 and 2 cover homogeneous anionic and cationic polymerization, respectively, and Chapter 3 polymerizations initiated by Zeigler-Natta and related organometallic catalysts. Chapters 4, 5 and 6 deal with the polymerization of cyclic ethers and sulphides, of aldehydes and of lactams, respectively. Finally, in Chapter 7 polycondensation reactions, and in Chapter 8 the polymerization of AT-carboxy-a-amino acid anhydrides, are discussed. 

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). 

Cationic polymerization proceeds faster with strained unsubstituted lactams 50% conversion is reached in bulk polymerization with 1 % of HC1 at 180 °C after 1 hour with 9-membered lactam and after more than 2 weeks with the less strained 13-membered lactam. With changing the ring size of lactams (e.g. 9 - 13) not only the ring strain is changed (AAHP = 7 kcal mol-1) but also their basicities and dielectric constant of the medium (Ae = 17)3). 

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.)... 

The catalysts for cationic polymerization can be strong anhydrous acids, Lewis acids, salts of primary and secondary amines, carboxylic acids, and salts of amines with carboxylic acids that split off water at elevated temperatures. The initiators react by coordinating with and fonrung 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 ate 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. Protonations of the amides occur at the oxygens, 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... 

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 ... 

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 ... 

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

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