Theoretical Investigations of Isoprene Polymerization Catalyzed by Cationic Half-Sandwich Scandium Complexes Bearing a Coordinative Side Arm

Density functional theory studies have been conducted for isoprene polymerization catalyzed by the cationic half-sandwich scandium alkyl species containing a methoxy side arm [(C5Me4C6H4OMe-o)Sc-(CH2SiMe3)](+) (1) and that containing a phosphine oxide side arm [{C5Me4SiMe2CH2P(O)Ph-2}Sc(CH2SiMe3)](+) (2). It has been found that trans-1,4-polymerization of isoprene by species 1 prefers an insertion isomerization mechanism: (i) an insertion of cis-isoprene into the metal alkyl bond to give eta(3)-pi-anti-form, (ii) anti-syn isomerization of the resulting 1,2-disubstituted allyl complex to yield a syn-allyl form, (iii) repetitive insertion of cis-isoprene into the metal syn-allyl bond and subsequent anti-syn isomerization. The resulting eta(3)-pi-syn-ally] species is suitable for more kinetically favorable cis-monomer insertion. The stability of the key transition state involved in the most feasible pathway could be ascribed to the smaller deformation of cis-isoprene and stronger interaction between the cis-isoprene moiety and the remaining metal complex. The origin of experimentally observed inertness of 2 toward isoprene polymerization is that the steric hindrance derived from the crowding of eta(3)-pi-syn-allyl species hampers the insertion of the incoming isoprene monomer. The modeling of 2-mediated chain propagation also has a high energy barrier and is endergonic. To corroborate the steric effect on the kinetic and thermodynamic aspects, various analogue complexes with smaller hindrance have been computationally modeled on the basis of 2. Expectedly, lower energy barrier and favorable thermodynamics are found for the monomer insertion mediated by these complexes with less steric hindrance around the metal center.