Enthalpy, compensation

Stereocontrol of free radical polymerization is influenced by monomer constitution, solventy and temperature. Most polymerizations seem to follow at least a Markov first-order one-way mechanism. Ratios of the four possible rate constants ki/iy ki/8, k8/i, and k8/8 can be calculated from the experimentally accessible concentrations of configurational triads and diads. With increasing temperature, more heterotactic triads are formed at a syndiotactic radical whereas the monomer addition at an isotactic radical favors isotactic and not heterotactic triads. Compensation effects exist for the differences of activation enthalpies and activation entropies for each of the six possible combinations of modes of addition. The compensation temperature is independent of the mode of addition whereas the compensation enthalpies are not. 

The search for stereocontrol parameters which depend on monomer constitution only has recently lead to the discovery of a compensation effect between the differences of the activation enthalpies and entropies, respectively (6). The calculated compensation temperature T0 and the compensation enthalpy can be used to evaluate the influence of the monomer constitution on the stereocontrol (6, 7, 8). The foundations and merits of this approach are discussed below. 

Within the limits of error, the slope of the function (AHf — A Hf) = /(ASf — ASf) is constant for a given monomer (Figure 3). The compensation temperature T0 is thus independent of the mode of addition. The compensation enthalpy AAHf differs however (Table W). 

Table IV. Compensation Temperatures (T0) and Compensation Enthalpies (AAIff) of Various Monomers0...

Table V. Compensation Temperature (T0) and Compensation Enthalpy (AAJff) for the Various Monomers...

Lee B, Graziano G. A two-state model of hydrophobic hydration that produces compensating enthalpy and entropy changes. J. Am. Chem. Soc. 1996 118 5163-5168. 

Figure 15-4. Compensation effect for various placement possibilities for first-order Markov statistics in the free radical polymerization of methyl methacrylate. For better clarity, some lines have been vertically displaced by -10.5 (for AHj/, - AH /,), 4.2 (for AH, - AH /,), 6.3 (for A H /, - A H /,), and 10.5 (for A H /, - A H /,) kJ/mol. The compensation temperature, but not the compensation enthalpy, is independent of the kind of placement occurring. (From data by H.-G. Elias and P. Goeldi.)...

The stereocontrol of most free radical polymerizations appears to be governed by an end-controlled mechanism. It generally follows first-order Markov statistics with respect to diads (see also Section 16.5.2.3). The tactic-ity of the formed polymer is also influenced by the solvent used. The cause of this solvent control effect is unclear, and possibly is due at least partly to different degrees of solvation. A compensation effect (see Section 16.5.4.) exists in the relationship between the activation entropies and enthalpies for diad formation in various solvents. The compensation temperature TJj varies with monomer constitution (Table 20-11). The compensation enthalpies AAHI vary strongly according to both monomer and placement type. 

Despite the fact that, in some cases, small differences in AH and A5 can be observed (for various solutes, but on the same column with the same mobile phase), these differences could be found to be essentially insignificant when compared to a change in stationary-phase bonding density using enthalpy-entropy compensation. Enthalpy-entropy compensation is a term used to describe a compensation temperature, which is system independent for a class of similar experimental systems.Melander et have used the enthalpy-entropy compensation method in studies of hydrophobic interactions and separation mechanisms in reversed phase HPLC. Mathematically, enthalpy-entropy compensation can be expressed by the formula 9 ... 

Calculated Compensation Temperatures To and Compensation Enthalpies AAHJ TABLE 5. ACTIVATION ENTROPY DIFFERENCES (AS - AS, , CALCULATED FROM VARIOUS LITERATURE DATA... 

TABLE 6. CALCULATED COMPENSATION TEMPERATURES To AND COMPENSATION ENTHALPIES AAhI FOR VARIOUS MONOMERS ... 

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