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)Ph2}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 η3-π-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 η3-π-syn-allyl 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 ...