Exoentropic polymerization

In exoenthalpic and exoentropic polymerizations, AGP° eventually becomes positive above a temperature known as the ceiling temperature (AGP° = AHP°/TCASP° > 0). For example, the ceiling temperature of bulk styrene is =400° C, whereas that of methyl methacrylate is =200° C. In contrast, AGP° becomes positive below a floor temperature when the polymerization is endoenthalpic and endoentropic (AGP° = AHp°/TfASp° > 0), such as in the polymerization of Sg (T/ = 160° C). 

Chain polymerizations of alkenes are exothermic (negative AH) and exoentropic (negative AS). The exothermic nature of polymerization arises because the process involves the exothermic conversion of re-bonds in monomer molecules into CT-bonds in the polymer. The negative AS for polymerization arises from the decreased degrees of freedom (randomness) for the polymer relative to the monomer. Thus, polymerization is favorable from the enthalpy viewpoint but unfavorable from the entropy viewpoint. Table 3-15 shows the wide range of... 

The ceiling temperature phenomenon is observed because AH is highly exothermic, while AS is mildly exoentropic. The opposite type of phenomenon occurs in rare instances where AS is endoentropic (AS = +) and AH is very small (either + or —) or zero. Under these conditions, there will be a floor temperature Tf below which polymerization is not possible. This behavior has been observed in only three cases—the polymerizations of cyclic sulfur and selenium octamers and octamethylcyclotetrasiloxane to the corresponding linear polymers (Secs. 7-lla). AH is 9.5, 13.5, and 6.4 kJ mol-1, respectively, and AS is 27, 31, and 190 J K-1 mol-1, respectively [Brandrup et al., 1999 Lee and Johannson, 1966, 1976]. 

In these systems a linear correlation has been observed between enthalpy and entropy of polymerization (compensation plot) (Fig. 2.3). For a small exocyclic group (R=CH3) polymerization is exothermic and exoentropic, thus, the driving force for the polymerization comes from the gain of energy djie to release of ring strain. 

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