Thermodynamic polymerizability

Thermodynamics determines whether or not a monomer will polymerize, to what extent it polymerizes, and what conditions such as solvent, temperature, and concentrations are required. As discussed in Chapter 1, the thermodynamic polymerizability of a monomer is independent of the mechanism and is therefore identical for radical, anionic, cationic, and coordinative mechanisms if structurally identical polymers are obtained. Although this requires that both the end groups and the microstructure are the same, the influence of regioselectivity and stereoselectivity on the enthalpy and/or entropy of polymerization has not been confirmed experimentally yet. 

The degenerative nature of propagation results in reformation of the same active species, but with monomer consumption and chain growth. Although the monomer s thermodynamic polymerizability is independent of the mechanism, the mechanism and structure of the active species determines the rate of monomer conversion. The structure of the active species involved in carbocationic polymerizations was discussed in Section II detailed information on the reactivities of model species was presented in Chapter 2, with the conclusion that covalent precursors do not react directly with alkenes, but must first ionize to sp2-hybridized carbenium ions. Only the resulting carbenium ions can add to double bonds. 

For unsubstituted repeating units (i.e., hydrogen atoms as substituents) these interactions are relatively weak however, they may become important when larger substituents are present. This is illustrated by the data of Table 3 where the calculated enthalpies of (hypothetical) polymerization of cyclopentanes are compared [64]. Thus, in general, substitution, decreasing the enthalpy of polymerization, decreases the thermodynamic polymerizability of heterocyclic compounds. 

Thermodynamical data are shown. Thermodynamical polymerizability is discussed using this data along with the strain energies. 

Similarities in the bond lengths and angles lead to similar thermodynamic polymeriz-abilities of cycloalkanes and heterocyclics. 

The high thermodynamic polymerizability of oxiranes (due to the relatively large ring strain) and the availability of monomers such as ethylene oxide (EO), propylene oxide (PO) or epichlorohydrin (ECH) led to considerable efforts directed toward the preparation of high molecular weight polymers. Both cationic and anionic polymerizations were explored and it soon became clear that for several monomers only anionic polymerization gives high polymers. 

The thermodynamic polymerizability of oxetanes, in regard of their ring strain, is high for unsubstituted oxetane AHp -80kJ/mol ( -20 kcal/mol) 6). Thus oxetanes like oxiranes may be polymerized to complete conversion ([M]e is very low) and in this respect differ considerably from the next group in the homologous series oxolanes (5-membered cyclic ethers), for which an equilibrium character of polymerization is clearly noticeable. 

Effect of ring size and substitution on thermodynamic polymerizability... 

The great number of possible elementary reactions in free radical polymerizations explains why only a fraction of the many thermodynamically polymerizable groups can be converted free radically to un-cross-linked high-molar-mass polymers. Vinyl, vinylidene, and acrylic compounds, as well as some strained saturated rings, belong to this fraction. Allyl compounds only polymerize to branched oligomers, but diallyl and triallyl compounds produce high-molar-mass cross-linked networks. 

Thermodynamic polymerizability of monomers may be described and compared in various ways. AabG at a given temperature is one of the possible approaches. However, the ceiling temperaorre of monomers in hulk (Tc(hulk)) appears as a useful candidate since this is the upper limit at which the polymer of a given monomer is thermodynamically stable. 

The higher the ceiling temjjerature of a given monomer, the higher its thermodynamic polymerizability. Tc could be calculated from AH and AS and the extensive scale of Tc for various monomers could thus be tabulated. 

Thermodynamic poiymerizabiiity is of major importance. If a monomer to be studied had (for AH< 0) a very low Tc(bulk), as determined thermochemically or calculated, then its polymerization is hopeless. If it looks thermodynamically polymerizable then it could be expected that the mechanism would be found for successful polymerization. The accumulated experience allows to reasonably choose the corresponding conditions of polymerization. 

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

An interesting example of the influence of the number and positions of substituents on the thermodynamic polymerizability was provided by studies of cationic polymerization of 3,4-dialkoxy-THF. When two substituents were in cis-position, polymers with DP up to 35 could be obtained while trans-monomer essentially did not polymerize. As shown in Scheme 34, if substituents in cyclic monomer are in ds-position, there is a considerable additional strain due to their steric repulsion this strain is partly released when a rigid cyclic monomer is converted to a polymer chain in which, due to free rotation around carbon-carbon and carbon-oxygen bonds, the strain can be minimized. In trans-monomers, steric repulsion is considerably lower thus the gain in enthalpy is less significant. 

For bicyclic (i.e., disubstituted) acetals, the thermodynamic polymerizability is usually enhanced due to additional strain introduced by the presence of the second ring thus, for example,... 

The influence of the additional strain on the thermodynamic polymerizability may be illustrated by the polymerization of... 

Initially, thermodynamic polymerizability of aliphatic cyclic esters was analyzed in qualitative terms by Hall and Schneider. Later, at the Gorki University, enthalpies and entropies of polymerization were determined for the most important cyclic esters. ... 

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. ... 

As it is already stressed in the Introduction, the fulfillment of thermodynamic requirements is a necessary but not sufficient prerequisite for a polymerization to occur. In general, the Thermodynamic polymerizability cannot be taken as a direct measure of monomer reactivity. For instance, the rate constants of alkaline hydrolysis of y-BL and CL at comparable conditions are close to each other (1.5 x 10" and 2.6 x lO lmol s" respectively) although the corresponding ring strains differ considerably (cf. Table 1). 

---