Lactam polymerizations, cationic

Unsubstituted lactams polymerize cationically with high activation energies, at elevated temperatures (about 200 °C). With increasing temperature the contribution of termination reactions (i.e. formation of non-reactive amidine end-groups) also increases, which leads to the incomplete conversion. 

Lactam polymerization with anionically activated monomer has its counterpart in the cationic processes of lactam polymerization. This type of mechanism has also been observed recently in some polymerizations of oxygen-containing heterocycles (see Chap. 4, Sect. 2.3)... 

This type of lactam polymerization is initiated under anhydrous conditions with acids or acid salts which do not split off water at the polymerization temperature (e.g., lactam or amine hydrochloride) as well as with some Lewis acids [176, 177]. The activated species is the monomer cation which takes part both in the initiation and propagation reactions. 

The consumption of one amine group in reaction (93) increases the acidity of the medium. In order to establish the equilibria (90) and (91), new amine groups and acyllactam structures are formed. As soon as at least one lactam molecule or lactam cation is involved in the disproportionation reaction (90), the sequence of disproportionation (90) and bimolecular aminolysis (93) results in the incorporation of one or two monomer units into the polymer molecule. The participation of this type of chain growth in cationic lactam polymerization, suggested by Doubravszky and Geleji [182—184], has been confirmed both for polymerization [185—188] as well as for model reactions [189, 190]. The heating of an equimolar mixture of acetylcaprolactam with cyclo-... 

Cationic process is the only route to the polymerization of N-alkyl lactams the resulting polymers have much lower melting points than their unsubstituted analogues, because of the absence of the H-bonding. N-alkyl lactams polymerize by mechanisms different from those established for their unsubstituted analogues. Their polymerization is discussed in a separate section. 

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. 

Therefore, all initiators used for lactam polymerization so far may be divided into two types 1. strong bases which are capable of removing the eunlde proton to form a lactam anion and thus can start an anionic polymerization (J ), and 2. compounds with active hydrogen which can protonate the amide bond so that cationic polymerizations become possible (2). An excellent... 

Table I. Endgroups formed in cationic lactam polymerizations.

These strongly basic groups bind the initiating acid very firmly. Amldinium salts Initiate the polymerization of lactcuns much less effectively than ammonium salts. Therefore, their formation leads to a high decrease of the polymerization rate (2o, 23, 24, 25) which is typical for all cationic lactam polymerizations. 

Table III. Comparison of the mechanisms of anionic and cationic lactam polymerizations.

At the end of the 1930s, the cationic polymerization as well has been attempted by Schlack, who discovered that anhydrous hydrogen chloride was capable of initiating the polymerization of CL. However, for reasons unknown at that time, the acid-initiated reartion did not yield high-quality PA6 and was therefore considered to be of no interest for commercial purposes. As a matter of fact, the cationic mechanism of lactam polymerization has always found very limited applications, if any, not only because of the low conversions and the low molar masses of the resultant polyamides but also for the extensive occurring of side reactions. 

Looking at these reactions, a specularity between the anionic and the cationic mechanism is observable. Different from conventional ionic polymerizations, the growth center in cationic and anionic lactam polymerizations is always neutral, and it is the lactam monomer that carries the ionic charge. Such polymerizations follow what is sometimes referred to as activated monomer mechanism, since the reaaive species are lactam cations/anions, thus activated monomers. However, in order to avoid misunderstandings, it must be well dear that this definition has nothing to do with the term activated... 

Consequently, different active species and more than a single polymerization mechanism are operating in the second stage of the cationic lactam polymerization. 

Lactam polymerizations initiated under anhydrous conditions by carboxylic adds and primary/secondary amines are referred to as addolytic or aminolytic polymerizations, respectively. Actually, these terms do not dearly identify different polymerization mechanisms, since thdr basic reactions simultaneously partidpate in the cationic and the hydrolytic processes. 

Lactams can also be polymerized under anhydrous conditions by a cationic mechanism initiated by strong protic acids, their salts, and Lewis acids, as weU as amines and ammonia (51—53). The complete reaction mechanism is complex and this approach has not as yet been used successfully in a commercial process. 

This is not the first time that the kinetics of bulk polymerizations has been analysed critically. Szwarc (1978) has made the same objection to the identification of the rate constant for the chemically initiated bulk polymerization of tetrahydrofuran as a second-order rate constant, k, and he related the correct, unimolecular, rate constant to the reported by an equation identical to (3.2). Strangely, this fundamental revaluation of kinetic data was dismissed in three lines in a major review (Penczek et al. 1980). Evidently, it is likely to be relevant to all rate constants for cationic bulk polymerizations, e.g., those of trioxan, lactams, epoxides, etc. Because of its general importance I will refer to this insight as Szwarc s correction and to (3.2) as Szwarc s equation . 

While lactams are often polymerized by anionic ROPs, A-carboxyl-alpha-amino acid anhydrides, called Leuchs anhydrides, can be polymerized by either cationic or anionic techniques (structure 5.29). These polypeptide products, which are called nylon-2 products. 

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... 

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. 

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