Coordination-Induced Ring-Opening Cationic Polymerization of Bioderived 1,3-Diene by Cationic Rare-Earth Metal Alkyl Complexes

Cationic rare-earth metal alkyl complexes have proven
to be outstanding
single-site catalysts, particularly for the coordination polymerization
of 1,3-dienes with high catalytic activity and stereoselectivity.
Unprecedentedly, the polymerization of the novel bioderived 1,3-diene,
namely 2-allylidene-6,6-dimethylbicyclo[3.1.1]heptane (ADH), catalyzed by a cationic scandium alkyl complex, proceeded via
a ring-opening cationic mechanism rather than a coordination–insertion
pathway. Detailed studies indicated that during the initiation step,
the coordination-induced steric repulsion between the catalyst and ADH was critical for generating the active tertiary carbocation,
which was formed by the ring-opening of the fused cyclobutane unit
in ADH. Density functional theory (DFT) calculations
revealed that both coordination polymerization and ring-opening cationic
polymerization of ADH can be energetically competitive,
and the steric hindrance of the catalyst may play a critical role
in determining the polymerization outcome. The resulting poly(ADH) (PADH) exhibited a high glass transition temperature,
excellent thermostability, optical transparency, and potential for
postmodification.