Isoprene Regioblock Copolymerization: Switching the Regioselectivity by the in Situ Ancillary Ligand Transmetalation of Active Yttrium Species

Regioblock copolymers of single alkenes hold great promise for modifying the properties of polymer materials but remain scarce due to the lack of viable synthetic methodologies. Here we describe a method for switching the regioselectivity of the cationic yttrium-catalyzed polymerization of conjugated dienes during chain growth, which leads to the formation of a series of di- and multiregioblock homo/mixed-copolymers with different properties from isoprene and myrcene. Mechanistic data demonstrate that the amidinate yttrium active species [L(b)YPIP3,4](+) (L-b = [PhC((NC6H4Pr2)-Pr-i-2,6)2](-)) changes to the tetramethylaluminate yttrium active center {(LYPIP3,4)-Y-s}(+) (L-s = [AlMe4](-)) in situ by amidinate ligand transfer in the presence of AIMe(3). The transformation of active species switches the regioselectivity from 3,4- to cis-1,4 polymerization while the polymer chain keeps propagating. (AlBu3)-Bu-i not only functions as a chain transfer agent but also plays a key role in preventing the chain termination during the amidinate transmetalation. These results highlight the versatility and potential utility of a strategy for the design and precision control of polymer structure and physical properties.