Polymerizability of cyclic monomers

Cyclic monomers with five- and six-membered ring atoms exist in strainless puckered conformations their heats of polymerization are either negative or have small positive values due to the repulsion of eclipsed hydrogen atoms. Because the nthalpy and entropy contributions are comparable, the free energies of polymerization are either positive or may become positive at high temperatures. 

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The size of substituents may also influence the polymerizability of cyclic monomers. This was shown for the cationic (and anionic) polymerization of cyclic esters of phosphoric acid 11... 

Most cyclic monomers of interest in the field of composites happen to be heterocyclic in nature. Polymerizability of some monomers is summarized in Table 1.1. Ring opening polymerizations invariably follow ionic mechanisms, although a few are known to proceed via the free radical route and some via metathesis involving metallocarbene intermediates. 

Polymerizability of bicyclic monomers is generally higher than the polymerizability of disubstituted cyclic monomers. This is because closure of an additional ring usually increases the angular strain moreover, conformational strain may also be increased due to the forced unfavourable conformation in the monomer molecule. In 1,4-epoxycyclohexane (7-oxabicyclo 2,2,l heptane) the 6-membered ring has to adopt the boatlike conformation while in the chain segment a more stable chair conformation may be attained (cf. Sect. 2.6.1) ... 

Figure 5.1 Cyclic monomers capable of anionic polymerization. Table 5.1 Anionic polymerizability of vinyl monomers.

THF as a five-membered cyclic ether is only weakly strained. Substitution decreases thermodynamic polymerizability of heterocyclic monomers thus substituted THFs do not polymerize... 

In contrast to polymerization of a large majority of unsaturated monomers, ROP of cyclic monomers is often accompanied by the presence of a relatively high concentration of the unreacted monomer when the process comes to equilibrium. This feature is related to pronounced reversibility of the propagation step (i.e., relatively high fed in comparison to fep, eqn [2]). Thus, the value of the equilibrium monomer concentration ([M]eq) at a given temperature is usually taken as a measure of the thermodynamic polymerizability of the monomer. The conesponding thermodynamic formalism has been developed by Dainton and Ivin in 1948-1958 and then by Tobolsky and Eisenberg. ... 

Vinyl monomers included in channels in compounds such as urea [24], thiourea [25], perhydrotriphenylene [26], desoxycholic acid [27], and so on, are polymerizable. In these reactions the walls of the channel provide the control which leads to stereoregularity of the polymers formed. The second type of system consists of cyclic monomers, such as trioxan, which polymerize by a process involving opening of the monomer ring followed by coupling with neighbors [12]. 

Table 1.1 Polymerizability of Some Unsubstituted Cyclic Monomers...

Several factors are known to affect the ROP of cyclic esters. The main factors are the reaction conditions, i.e., the nature of the initiator, type of solvent and reaction temperature, and also the ring size of the monomer used and the substituents on the monomer ring [105-107] Cyclic esters of four-, seven-, and eight-membered rings polymerize, whereas the five- membered ester does not. In the case of six-membered rings, the polymerizability depends on the substituents [105],... 

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. 

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. 

Problem 8.18 (a) Consider the cationic polymerizability of vinyl ethers, cyclic ethers (like tetrahydrofuran), cyclic acetals (like trioxane), and N-vinyl car-bazole. (b) Why do these monomers not copolymerize cationically with olefins like styrene or isobutene ... 

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