Of lactams, anionic

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. 

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. 

Substituted keto amides (X)—(XIV) having one a-hydrogen are much more acidic than polymer or lactam amide groups and they lower the concentration of lactam anions through the equilibria... 

The pattern of side reactions in the polymerization of a-substituted lactams is slightly different. The keto timide structures (XV) formed in the Claisen type condensation have no acidic a-hydrogen so that the concentration of lactam anions cannot decrease so dramatically as with the acidic keto amides (X)—(XIV). Consequently, the position of the condensation equilibrium is shifted in favour of the initial components. Also the main products formed from af,a -disubstituted keto amides [136], scheme (52), are different from those of the monosubstituted keto amides, scheme (45). 

As expected, the molecular weight distribution also depends strongly on the concentration of amide anions. In the polymerization of caprolactam around 200°C with an activator/initiator ratio [A]o/[I]o <2.5, the concentration of lactam anions is lowered from the very beginning of polymerization by the value A[I] = [A]o/2.5. The remaining initiator ([I] = [I]o [A]o/2.5) produces additional growth centres entering successively into the polymerization. Similarly, as in the non-activated... 

Due to the relatively fast side reactions consuming both initiator and growth centres, the evaluation of the kinetics of anionic polymerization becomes very difficult. We are dealing with a system of varying concentration of both active species which, according to schemes (45), (51) and (52), can be not only consumed but also regenerated in the complicated set of side reactions. Hence, the key problem of the anionic lactam polymerization consists in the determination of the instantaneous concentrations of lactam anions and growth centres. 

At the beginning of polymerization, the concentration of lactam anions is determined by the dissociation of the lactam salt added as initiator, viz. 

The acidity of cis amide groups (of small and medium lactams) is much lower than that of the trans amide groups in the corresponding linear polymer. As soon as polymer amide groups are formed, the concentration of lactam anions is lowered due to the equilibrium... 

However, the total concentration of anions is increased appreciably (Table 8) and an increasing fraction of anions arises from trans amides whereas the concentration of lactam anions decreases steeply. It has been estimated [164], that the formation of 2.5% of polymer lowers the concentration of lactam anions by 30% whereas the total concentration of amide anions doubles. 

When the metal lactamate (I) is not dissociated completely, then the concentration of lactam anions calculated from... 

Was found to proceed lO times faster (at 30°C, feo = 10 1 mole sec ) than the propagation [166]. Hence, the rate determining step of polymerization is the acylation of lactam anions (24). At the beginning of polymerization and at low temperatures, the depolymerization can be neglected for most lactams (except the five- and six-membered) and the rate of polymerization is given by [94]... 

As the concentration of polymer amide groups increases during the polymerization, the equilibrium (25) decreases the concentration of lactam anions and increases the concentration of polymer amide anions (P ). Hence, an increasir fraction of polymer is formed in the sequence of reactions (30) and (31) whereas the contribution of reaction (24) to the chain growth decreases. Similarly, the contribution of the bimolecular depolymerization reaction (29) will be increasing as compared with the monomolecular depolymerization in reaction (24). The rate of the monomolecular depolymerization is proportional to the concentration of amide anions in the vicinity of the acyllactam, viz. 

In the copolymerization of lactams of different ring size, the relative rate of incorporation of the two lactams is not necessarily determined by the reaction in which the lactam ring is cleaved. Vofsi et al. [167] showed that in the anionic copolymerization of caprolactam and pyrrolidone (Table 9), the acylation of lactam anions with the exocyclic carbonyl of the growing acyllactam structure (i.e. exchange of monomer units) occurs faster than acylation with the cyclic carbonyl (propagation), viz. 

Therefore, the copolymer composition is determined by the relative acidities of both lactams and by the nucleophilicities of the corresponding anions in the transacylation reactions. The distribution of lactam anions is given by the equilibrium constant... 

Preliminary data on MMD of our samples are given in Table IV. It is evident that equimolar concentrations of activator and initiator produce PCL polymers characterized by a regularly decreasing polymolecularity index Q, from ca. 2.6 to 2.0. In Figure 1 the number of polymer molecules formed per acyllactam molecule is plotted as a function of initiator concentration. The actual values should be compared to the theoretical value of 1, which corresponds to the assumption that the number of macromolecules would be equal to the number of acyllactam molecules (26J, as in the ideal case of a step-addition of lactam anions to a constant number of growth centers. 

The polymer amide anions can undergo acylation by acyllactam groups with accompanying ring opening or with formation of lactam anions. In the first instance, it is an alternate path of propagation with formation of imide groups ... 

At higher polymerization temperatures, however, side reactions occur. Among them are Claisen-type condensations. They lead to two types of N-acylated j -keto imide structures and take place readily above 2(X) C. Formation of these imides decreases the concentration of lactam anions ... 

Cyclic keto imides as well as linear ones can yield active species through acylation of lactam anions. This results in formations of growth centers and keto amides ... 

The acidity of keto amides with a>hydrogen atoms is much greater than that of the monomers or of polymer amide groups. Any formation of such structures therefore decreases the concentration of lactam anions. 

The presence of ketoimide/ketoamide groups has been demonstrated using modd experiments and also isolation of aminoketones formed by total hydrolysis of the polymer. It is noteworthy that a,a-disubstituted lactams, lacking a-hydrogens, cannot undergo this type of secondary reactions. In the case of a-monosubstituted lactams, the partem of side reactions is fairly different for example, the ketoimide sUuctures obtained by the Claisen-type condensation (see 32 and 35 in Schemes 15 and 16) have no acid hydrogens thus, the concentration of lactam anions is decreased at a lower rate. 

A peculiar situation is met in the polymerization at temperatures below the melting point of the polyamide when depolymerization and side reactions are largely reduced and the disproportionation reaction is appreciably slower. In this case, even at equimolar concentrations of initiator and activator, the polymerization proceeds essentially by the reaction of lactam anions with a constant number of growth centers, resulting in a narrower molar mass distribution (M /Mn<2). Moreover, since bifunctional activators may be safely used under specific conditions without any formation of cioss-linked structures, very high molar masses can he obtained. ° ... 

N-aqflated derivatives of 2-oxoamides (ketoimides) 9, and subsequently 2-oxoamides (ketoamides) 10, are formed these are the key derivatives and lower the concentration of the components of the initiation system. Here, each condensation step consumes two molecules of classical growth centers during the formation of one ketoimide structure. As its acidity is distinctly higher than that of the amidic group, this leads also to a decrease in the concentration of lactam anions, and hence in the concentration of the initiator. The ketoamides at higher temperatures decompose to yield isocyanates, to form uracil structures and water (the latter being an inhibitor of the polymerization process), and so on [9, 12]. 

The high activity of CLMgBr 29 can be explained as a consequence of the coordination of initiator to the growth centers (intermediate 28) [26], due to the coordination the electrophilicity and the reactivity of the growth center (the rate of addition of lactam anion to the nonionic growth centers) is increased. It is also necessary to take into account the so-called Schlenk equilibrium, which effect cannot be neglected ... 

---