Abstract
The monomer synthesis and ring-opening metathesis polymerization (ROMP) of cis-5-norbornene-exo-2,3-dicarboxylic anhydride (1a) and 7-syn-trimethylsilyl-cis-5-norbornene-exo-2,3-dicarboxylic anhydride (1b) mediated by ruthenium-alkylidene catalysts (I, II and III) were experimentally carried out. Metathesis reaction pathways of 1a and 1b monomers using II have been studied at PBE-D3(BJ)/def2-TZVP level of theory, employing the SMD model for simulation of 1,2-dichloroethane solvent. The calculations unravel that reactivity difference between 1a and 1b towards ruthenium alkylidene complex II is due to the fact that the intermediate π-complex formation was found in the 1a reaction pathway but was absent in the 1b one. Moreover, there are marked differences in the formation processes of the metallacyclobutane intermediaries 5a and 5b, the first is an exergonic process (−8.3 kcal/mol) and the last one is an endergonic process (2 kcal/mol), in addition to the high activation energy of the monomer 1b (15.8 kcal/mol) compared with 1a (5.5 kcal/mol). Such differences are attributed to the high steric impediment imposed by -Si(CH3)3 over the double bond (syn conformation). Using quantum theory of atoms in molecules (QTAIM) it was possible to analyze successfully the mechanistic pathway of metathesis reaction for both monomers, complementing the results obtained by DFT energetic analysis.
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