Mechanism for Coordination Quasi-Depolymerization of Polydiolefins with Cp₂ZrHCl

The coordination depolymerization of poly(di)olefins via transition-metal complexes is significant but greatly challenging with respect to controllability and selectivity. Herein, we report the efficient, selective, and controllable coordination quasi-depolymerizations of cis-1,4-polybutadiene (cis-PB), polycyclooctene, cis-1,4- and trans-1,4-polyisoprene (cis-PI and trans-PI), and so on with Cp2ZrHCl (ZrH) under mild conditions. The molecular weights of the final chain-scission products vary with polymer precursors and have bottom values. By means of analyses such as deuterated ZrD and 2H NMR spectroscopy, the mechanism is revealed. Hydrozirconation between the C═C double bonds in the polymers and ZrD gives terminal and internal alkylzirconium intermediates (TAKI and IAKI). IAKI is highly active to induce β-alkyl or β-H chemoselective elimination reaction depending on the steric repulsion between polymer chains and the Zr center. The β-alkyl elimination arouses chain cleavage, which is regioselectively determined by the hydrozirconation mode and chemical environment of C═C bonds. Zr-right-β-alkyl elimination is more preferred for cis-PB, while for cis-PI, the agostic interaction between the methyl substituent and Zr-center facilitates the Zr-left-β-alkyl elimination.