Polymerizability of lactams

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. 

Frequently, the order of rates of polymerization of various lactams is different from that of the thermodynamic parameters for polymerization. For example, the initial rates of hydrolytic polymerization were practically the same for capro-, enantho- and capryllactam [25—27], whereas the corresponding heats of polymerization differed significantly for these monomers [27] —AH = 3.3, 5.3 and 7.8 kcal mole , respectively). Similarly, for substituted caprolactams the sequence of free energies of polymerization is just opposite to the order of rates of anionic polymerization (Fig. 1). 

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Even more complicated relations exist in the series of bridged bi-, tri-, and tetracyclic lactams. The extensive pioneering work of Hall [49, 69, 70] on the influence of structure on the polymerizability of lactams containing six-membered rings revealed that lactams in which the cyclohexane ring occurs in the boat form (I) polymerize [71]. On the other hand, most lactams with the amide group in a six-membered chair (II) failed to polymerize [70] and the same result can be expected for lactams in which two stable chair forms are fused together (HI), viz. 

In addition to the aspects discussed thus far, the polymerizability of lactams is also affected by the presence of heteroatoms in the ring moiety [19-21] and by substituents. The latter affect both enthalpy and entropy of polymerization mainly due to changes in the conformation on conversion of the cyclic structures into linear ones. The overall effects depend markedly on number, size, location, and nature of the substituents [22-26]. 

Block copolymer systems have aroused interest with reviews of the synthesis of nylon elastomers, thermoplastic polyether-polyamide elastomers, and thermoplastic cross-linked polyamides of 3,3 -bis(hydroxymelhyl) glutaric add. Block copolymers were also reported from poly(/n-phenylene isophthalamidc) and poly(ethylene oxide) or poly(dimethylsiloxane). The polycondensation of oco -dicarboxylic-poly(amide 11) and x -dihydroxy-polyoxyethylene has also been studied and rate constants and activation energies evaluated for the process. The polycondensation of axo -diacid and e9o> -diester-poly(amide 11) oligomers with cuco -dihydroxy-polyether oligomers has similarly been reported. Lactam Rli -opening Polymerization Routes.—The effects of ring size, substitution and the presence of heteroatoms on the polymerizability of lactams has been the subject of reviews. - In the field of lactam polymerization, two systems have evoked major interest, namely caprolactam and 2-pyrrolidone. Studies on caprolactam have reported the effect of water on the mechanism of polymerization and polymerization rate, where it was found that the process was... 

A few attempts have been made to describe the kinetic polymerizability of lactams, but rather poor results have been obtained and contradictory conclusions on reactivities and, in particular, on substituent effects have been published. Sekiguchi and Coutin proposed a... 

The condensation polymers (105) have been prepared (80JOC5325) by ring-opening polymerization of bicyclic lactam (101), the urethanes (102) and (103) and urea (104 Scheme 28). The polymerizability of these monomers, compared with the inertness of isomers (106)... 

The anionic polymerizability of 67 is much lower than that of 58. This difference is due not only to the lower ring-strain of 67 and but also to a greater steric hindrance that the lactam anion of 67 receives when it comes closer to the carbonyl... 

Irrespective of the reaction mechanism, the polymerization of lactams leads to an equilibrium between monomer, cyclic oligomers and polymer. Tobolsky and Eisenberg [9] showed that the thermodynamic parameters are independent of the reaction mechanism, so that the polymerizability may be rationalized in terms of the ease of formation of the cyclic monomer, or, its opening into a linear chain unit. The simple relation between the equilibrium monomer concentration [L]e, temperature, and standard heat and entropy of polymerization. 

Substitution at the nitrogen atom decreases the polymerizability of small and medium lactams much more than substitution at any other ring atom (see Section 3) and, therefore, only the highly strained four-, eight-and nine-membered N-substituted lactams have been polymerized so far [53,241—243] (Table 11). Because of the lack of a dissociatable hydrogen at the amide group, A/ -substituted lactams cannot form the chemically activated species required for the anionic polymerization, i.e. lactam anions and AT-acylated lactam an anionic propagation could proceed only... 

Although the general reaction mechanisms of polymerization are independent of the size of the lactam ring, quantities related to the ring size mainly determine the polymerizability of a particular unsubstantiated lactam containing only carbon atoms. 

It is readily seen that there is a reciprocal relationship between Ka and [M]e. The equilibrium constant Ka is therefore a direct measure of the polymerizability of a particular lactam, and AHp and ASp are thus the corresponding principal parameters. 

The chemical, thermodynamic, and kinetic aspects of lactam polymerizability have been the subjects of many papers mainly published in the 1960s and 1970s. " Remarkably elegant has been the study of Korshak et who intended to... 

N-substitution affects polymerizability of the ds-lactams much more than does C-substitution. The largely reduced reactivity given by substituents with +I effect on nitrogen has been attributed to the loss of resonance stabilization and the cis character of the amide bond. ... 

Syntheses of a number of bicyclic lactams and their qualitative polymerizabilities related to the steric strains were described and comprehensively summarized by Hall23>s4), as simply shown in Table 7. 

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

Similarly, substitution only changes the entropy of the lactam very slightly, but decreases considerably the entropy of the linear monomer unit inside a polymer chain by restricting its rotation. In this way, substitution shifts the enthalpy and entropy of polymerization in favour of the cyclic structure and decreased polymerizability. 

The effect of substitution on the heat of polymerization depends not only on the size and position of the substituent. It follows from Table 5 that the size of the lactam ring also plays an important role, so that substituents affect the polymerizability in a complex manner. Hence, in a general treatment, the effect of the substituent on the enthalpy and entropy of both the corresponding linear and cyclic unit must be taken into account. 

J.4.2.3 Heterocyclic Monomers A variety of heterocyclic monomers can be polymerized by anionic ring-opening polymerizations. The types of anionically polymerizable heterocyclic monomers include oxiranes (epoxides), thiacyclopropanes, thiacyclobutanes, lactones, lactides, lactams, anhydrides, carbonates, and silicones... 

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