Metal salts reduction during parylenes polymerization

Abstract

Recently it was discovered that some metals and some metal salts quench the polymerization of poly-para-xylylenes (also known as parylenes) [Vaeth and Jensen, Chem. Mater. 12 (2000) 1305–1313]. The inhibitions were found to be dependent on the metal type. On the other hand, it is known that the polymerization mechanism of parylenes is of radical type [Smalara et al., J. Phys. Chem. A 114 (2010) 4296–4303, Errede and Szwarc, Q. Rev. Chem. Soc. 12 (1958) 301–320] and no catalyst or solvent is required in this process. In the present work we postulate that the reaction between parylene and metal salts can go through one-electron reduction of metal salts by radical parylene chains. We also consider the role of monomers in this type of reactions and propose a stoichiometric notation for the reductions. Thermodynamical barriers were found by means of DFT calculations with two different functionals and various basis sets and to estimate the changes of the enthalpy and Gibbs free energy we computed the translational, rotational and vibrational contributions to the partition functions of the substrates and products. Additionally, water solvent effects have been appraised in the PCM model and the energy relations were compared to those calculated for gas-phase reactions. It turned out that in the case of metal salts where an appropriate reduction of metal characterizes sufficiently high positive value of standard redox potential, the reaction is thermodynamically favorable. Moreover, one can find that within some domain there is a tendency that the higher the potential the more stable products.