Difluoroethylene as a chain transfer agent during ring-opening metathesis polymerization (ROMP) of norbornene by a ruthenium alkylidene complex: A computational study

The polynorbornene chain transfer reaction pathways to ethylene (2a), trans-1,2-difluoroethylene (2b) and trans-1,4-dichloro-2-butene (2c) by (1,3-diphenyl-4,5-dihydroimidazol-2-ylidene) (PCy₃)Cl₂RuCHPh (I) have been studied at B3LYP/LACVP^∗ level of theory. The calculations show that the free Gibbs activation energy of metathesis reaction is dependent on the volume of substituents directly linked to the double bond of an olefin. Highest activation energy is observed for 2c with highest molecular volume. The activation energy is lower for 2a with small molecular volume. Compared to 2a and 2c, fluorinated olefin 2b binds more strongly to the 14 electron Ru-alkylidene catalyst to form tighter transition state. Therefore, sterical factor is the most important contribution to the activation energy for Ru-alkylidene mediated olefin metathesis.