Ah Initio Quantum-mechanical Calculations of the Propagation Enthalpies for the Radical and Anionic Polymerizations of Formaldehyde and Methanimine

The gas phase enthalpies of formation for oligomeric radicals and anions H(CH2NH)(n)* and H(CH2O)(n)* were theoretically determined by ab initio quantum-mechanical calculations with n in the range 1 to 6. From these results, the reaction enthalpies for each of the first five propagation steps of the polymerization were estimated for methanimine (H2C=NH) and formaldehyde (H2C=O). At the same step of oligomerization, enthalpies associated with anionic polymerizations are always more negative than enthalpies corresponding to radical polymerizations, but the difference between them decreases with increasing n. Both Delta H(propagation) vs. n curves tend rapidly, particularly for radical polymerizations, towards an asymptotic value independent of the mode of polymerization and equal to -12 kcal/mol for formaldehyde and -14 kcal/mol for methanimine. Experimental data for the gas phase polymerization of formaldehyde are in good agreement with our theoretical value. These results demonstrate that heals of polymerization can be reasonably estimated by intensive calculation methods if a careful choice of the reaction mimicking the propagation step is done