Driving Force for the Thermally Induced Solid State Polymerization of Alkali Metal Halogenoacetates:  A Thermochemical Analysis

Reaction-solution calorimetry was used to determine the standard molar enthalpies of formation of a series of lithium and sodium halogenoacetates in the crystalline state at 298.15 K. The obtained values were as follows: Delta(f)H(m)(o)(ClCH2COOLi, cr) = -(763.6 +/- 1.3) kJ.mol(-1), Delta(f)H(m)(o)(BrCH2COOLi, cr) = -(723.7 +/- 1.7) kJ.mol(-1), Delta(f)H(m)(o)(ICH2COOLi, cr) = -(671.1 +/- 2.0) kJ.mol(-1), Delta(f)H(m)(o)(ClCH2COONa, cr) = -(740.9 +/- 0.7) kJ.mol(-1), Delta(f)H(m)(o)(BrCH2COONa, cr) = -(700.2 +/- 1.2) kJ.mol(-1), and Delta(f)H(m)(o)(ICH2COONa, cr) = -(642.0 +/- 1.6) kJ.mol(-1). Microcombustion calorimetric studies of a polyglycolide sample with the empirical formula C2.000H2.160O2.032Cl0.009243Na0.004404 (PGA) led to Delta(f)H(m)(o)(PGA, pol) = -(379.1 +/- 1.1) kJ.mol(-1). These results were used to discuss the energetics of the solid-state polymerization of alkali metal halogenoacetates leading to polyglycolide. In agreement with experimental observations, it is concluded that for a given metal, the reaction becomes less favorable when the halogen changes along the series Cl --> Br --> I. The reaction is also considerably less favorable for the lithium than for the homologue sodium salts. These differences in reactivity seem to be mainly determined by the lattice enthalpy of the alkali metal halide salt formed in each reaction.