New homoleptic zinc and magnesium
complexes containing constrained
reduced Schiff base ligands based on substituted 7-hydroxy-1-indanone
were successfully synthesized and used as a catalyst for the polymerization
of lactide. The ligands contain a side arm having different basicity
because dimethylamino, pyridyl, and furfuryl groups are shown to greatly
affect the polymerization rates. The homoleptic zinc complex containing
constrained reduced Schiff base ligands and a dimethylamino side arm
was highly active, giving a 92% conversion of l-lactide in
3 min using [LA]:[Zn]:[BnOH] = 500:1:2 at room temperature. The polymerization
is pseudo-first-order dependent on the LA concentration. Well-controlled
and living behavior of the zinc complex was observed and demonstrated
in the preparation of stereodiblock and triblock copolymers of l
-, d-, and rac-lactide
in a one-pot sequential synthesis with a predictable block length,
block sequence, and narrow dispersity rapidly in 10 min. Stereocomplex
formation was observed for PLA made sequentially from 100 l-LA, 100 rac-LA, and 100 d-LA having a
high T
m of up to 220 °C.
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