Lactams, polymerization

Lactam polymerization represented by reaction 5 in Table 5.4 is another example of a ring-opening reaction, the reverse of which is a possible competitor with polymer for reactants. We shall discuss this situation in Sec. 5.10. 

Another method is to start with lactams. The cyclic lactams have a lower melting temperature compared to co-ammo acids and are therefore more easily purified and easier to handle. e-Caprolactam has a melting temperature of 69°C and can be transported in the molten state in heated tanks. The energy consumption of the lactam polymerization is also low as little water is added by the polymerization process and therefore there is little to evaporate. 

The initiation of the lactam polymerization with water is directly dependent on the water concentration ... 

Lactam polymerizations (nonassisted as well as assisted) are usually complicated by heterogeneity, usually when polymerization is carried out below the melting point of the polymer [Fries et al., 1987 Karger-Kocsis and Kiss, 1979 Malkin et al., 1982 Roda et al., 1979]. (This is probably the main reason why there are so few reliable kinetic studies of lactam polymerizations.) An initially homogeneous reaction system quickly becomes heterogeneous at low conversion, for example, 10-20% conversion (attained at a reaction time of no more than 1 min) for 2-pyrrolidinone polymerization initiated by potassium t-butoxide and A-benzoyl-2-pyrrolidinone. The (partially) crystalline polymer starts precipitating from solution (which may be molten monomer), and subsequent polymerization occurs at a lower rate as a result of decreased mobility of /V-acyl lactam propagating species. 

A large number of compounds used as catalysts in acid-ion lactam polymerization are known. These include alkalis, alkali-earth metals, hydrates, Grignard reagents, lithium oxide, various hydroxides and carbonates, sulfates, halides, sodium zincate, alkaline salts of different acids, i.e., compounds that cause the formation of lactam acid ion in the reactive medium. The mechanism of polymerization in the presence of sodium-lactam- salt compounds is largely known. 

A large number of inorganic compounds can be used as the activators of acid-ion (anionic) activated lactam polymerization. The activator in anionic activated lactam polymerization not only increases the process rate, but also changes the structure and functionality of a polymer formed, and, as a result an activator can regulate the properties of the end-product.1... 

Except for lactam n-acyl derivatives, compounds such as esters, anhydrides and halogen anhydrides of carboxylic acids, which can activate lactam polymerization, can also be used as activators (promoters). 

It is interesting to note the following results cited in these publications. First, the sum of the exponents m + n = 2 which diminishes the number of arbitrary constants. Second, m and n do not depend on temperature (changes of both constants with temperature were mentioned only in74). Third, while ki and k2 are strongly temperature dependent functions, their ratio which characterizes the effect of self-acceleration is almost independent of temperature. This was also mentioned above in the discussion of self-accelerating kinetics equations for lactam polymerization. Values of the constants m and n according to the experimental data from several publications are listed below ... 

Although the polymerization of e-caprolactam was described above, there is no difference in principle from the process flow sheets for centrifugal molding of items from other polymers and oligomers. Nevertheless, in most cases, the high temperatures used in lactam polymerization are not required, and the flow sheet as a whole is simplified. In industrial practice, poly(methyl methacrylate) pipes,172 and sheets of polyurethanes and unsaturated polyesters are obtained by centrifugal casting. 

The interpretation of the mechanism of anionic lactam polymerization based on the conventional scheme (ionic active centre with approaching monomer) could not exhaustively explain all the observed effects. Agreement could only be obtained when the acido-basic properties of lactams and polyamides had been respected. The equilibrium... 

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 undesirable side reaction is avoided in the polymerization of N-substituted lactams i.e., N-alkyl, N-aryl, or N-acyl lactams. Polymerization of N-substituted lactams proceeds differently, thus in the presence of hydrogen chloride the following sequence of reactions takes place [217]. 

Taking into account the equOibriunuiature of lactam polymerization, there will be always enough monomer avaUable for protonation. However, the proportion of protonated monomer will decrease with conversion. 

Most of the transacyiation reactions proceeding during lactam polymerizations are reversible and cyclization is competitive with linear polymeriz-... 

Lactam polymerizations are usually carried out at high temperatures in the absence of solvents and the problem of volume contraction is circumvented by expressing the concentrations in moles or equivalents per kg. 

In homogenous media, most of the transacylation reactions are reversible and as soon as the first polymer amide groups are formed, the same kind of reactions can occur both at the monomer and at the polymer amide groups. Unless the active species are steadily formed or consumed by some side reaction, a set of thermodynamically controlled equilibria is established between monomer, cyclic as well as linear oligomers and polydisperse linear polymer. The existence of these equilibria is a characteristic feature of lactam polymerizations and has to be taken into account in any kinetic treatment of the polymerization and analysis of polymerization products. The equilibrium fraction of each component depends on the size of the lactam ring, substitution and dilution, as well as on temperature and catalyst concentration. 

Lactam polymerization comprises the conversion of a cyclic lactam unit into a linear one without the formation of any new chemical bonds. The term polymerizability involves both the thermodynamic feasibility and a suitable reaction path to convert the cyclic monomer into a linear polymer. Sometimes, a slight confusion arises when the term polymerizability is used as a synonym for both the rate of polymerization and the thermodynamic instability of the lactam. Due to the reversible nature of the polymerization of most lactams, eqns. (1)—(3), their polymeriz-abilities cannot be expressed in terms of the rate of polymerization only, but the rate of both polymerization and monomer reformation must be compared. 

Homogenous lactam polymerizations usually proceed with a decrease of volume. At atmospheric pressure the value of the pAV term is negligible and the heat of polymerization can be considered as a measure of the difference in internal energy between the linear and cyclic monomer unit. At very high pressures, however, the effect of volume contraction during polymerization on the monomer-polymer equilibrium cannot be neg-... 

The most important features of the anionic lactam polymerization are that ... 

As a matter of fact, structures (XVI) and (XVII) represent only a minor fraction of polymer molecules. Due to the great number of irregular structures which may be incorporated into the polymer molecules (Section 4.3), a great variety of types of macromolecules can be present in anionic polymers [95]. The nature of irregular structures formed during anionic lactam polymerization is primarily determined by the type and concentration of catalytic species and temperature, as well as by ring size and substitution of the lactam. Only polymers of o ,a-disubstituted lactams are free of irregular structures, and should be composed only of macromolecules of type (XVI) and (XVII). 

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. 

In the anionic lactam polymerization it is always the lactam anion which is incorporated into the polymer molecules either in reaction (24)... 

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-... 

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