Determination of Arrhenius Parameters for Propagation in Free-Radical Polymerizations: An Assessment of ab Initio Procedures

Ab initio calculations have been carried out for the addition of n -alkyl radicals to ethylene, with the aim of identifying levels of theory suitable for the reliable description of the mechanism and energetics of the propagation process in the free-radical polymerization of ethylene. For the calculation of absolute barriers, use of a method such as QCISD(T)/6-311G(d,p), together with MP2 basis set corrections to 6-311+G(3df,2p) and B3-LYP/6-31G(d) zero-point vibrational energy corrections, is recommended. For the calculation of Arrhenius frequency factors, the parameters required, i.e., geometries, fundamental frequencies, and rotational barriers, can be reliably obtained at much simpler levels of theory, such as UHF/6-31G(d) or even UHF/3-21G.