Stereoselective Alkali-Metal Catalysts for Highly Isotactic Poly(<i>rac</i>-lactide) Synthesis
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Abstract
A high degree of chain end control
in the isoselective ring-opening polymerization (ROP) of <i>rac-</i>lactide is a challenging research goal. In this work, eight highly
active sodium and potassium phenolates as highly isoselective catalysts
for the ROP of <i>rac-</i>lactide are reported. The best
isoselectivity value of <i>P</i><sub>m</sub> = 0.94 is achieved.
The isoselective mechanism is chain-end control through the analysis
of the stereoerrors in the microstructure of a final polymer; thus,
isotactic multiblock structure polymers are obtained, and the highest
melt point can reach 192.5 °C. The donating group in phenolate
can clearly accelerate the ROP reaction, potassium complexes are more
active than the analogous sodium complexes, and the big spacial hindrance
of the ligand can decrease the activity. The high isoselectivities
of these complexes mostly result from their sandwich structure constructed
by the plane of the crown and the plane of the anthryl group
