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.