Unveiling
the Detailed Mechanism and Origins of Chemo‑,
Regio‑, and Stereoselectivity of Rare-Earth Catalyzed Alternating
Copolymerization of Polar and Nonpolar Olefins
posted on 2024-02-03, 14:31authored byNi Zhang, Zuqian Xue, Lei Shi, Gen Luo
The direct copolymerization of polar and nonpolar olefins
is of
great interest and significance, as it is the most atom-economical
and straightforward strategy for the synthesis of functional polyolefin
materials. Despite considerable efforts, the precise control of monomer-sequence
and their regio- and stereochemistry is full of challenges, and the
related mechanistic origins are still in their infancy to date. Herein,
the mechanistic studies on the model reaction of Sc-catalyzed co-syndiospecific
alternating copolymerization of anisylpropylene (AP) and styrene were
performed by DFT calculations. The results suggest that the subtle
balance between electronic and steric factors plays an important role
during monomer insertions, and a new amino-dissociated mechanism was
proposed for AP insertion at chain initiation. AP insertion follows
the 2,1-si-insertion pattern, which is mainly controlled
by steric factors caused by the restricted MeO···Sc
interaction. As for styrene insertion, it prefers the 2,1-re-insertion manner and its regio- and stereoselectivities
are influenced by steric repulsions between the inserting styrene
and the polymer chain or the ligand. More interestingly, it is found
that the alternating monomer-sequence is mainly determined by the
“steric matching” principle, which is quantitatively
expressed by the buried volume of the metal center of the preinserted
species. The concept of steric pocket has been successfully applied
to explain the different performances of several catalysts and other
alternating copolymerization reactions. The insightful mechanistic
findings and the quantitative steric pocket model present here are
expected to promote rational design of new rare-earth catalysts for
developing regio-, stereo-, and sequence-controlled copolymerization
of specific polar and nonpolar olefins.