Isoprene Regioblock Copolymerization: Switching the Regioselectivity by the in Situ Ancillary Ligand Transmetalation of Active Yttrium Species
Xiaying Yu, Qing You, Xigeng Zhou, Lixin Zhang
Index: 10.1021/acscatal.8b00600
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Abstract
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 [LbYPIP3,4]+ (Lb = [PhC(NC6H4iPr2-2,6)2]−) changes to the tetramethylaluminate yttrium active center {LsYPIP3,4}+ (Ls = [AlMe4]−) in situ by amidinate ligand transfer in the presence of AlMe3. The transformation of active species switches the regioselectivity from 3,4- to cis-1,4 polymerization while the polymer chain keeps propagating. AliBu3 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.