Journal of the American Chemical Society
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PLA obtained from (rac)-lactide because heterotactic PLA re-
ASSOCIATED CONTENT
Supporting Information
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quires the catalyst to alternate between pro-(R) and pro-(S)
diastereomeric sites. To accommodate this requirement, we
propose that epimerization of the iron stereogenic center oc-
curs after every inserted (rac)-lactide monomer (Figure 5).
The mechanism for this site epimerization is unclear, but sev-
eral possibilities are presented in the Supporting Information.
To explain why site epimerization occurs in preference to con-
secutive lactide enchainments for (rac)-lactide but not
Full experimental procedures, DFT calculations, and further
discussion of stereocontrol mechanism. “This material is avail-
able free of charge via the Internet at http://pubs.acs.org.”.
AUTHOR INFORMATION
Corresponding Author
(
meso)-lactide, a diastereomeric matched-mismatched situa-
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*Tel:
617-552-6725.
E-mail:
jeffery.byers@bc.edu.
tion is proposed. We hypothesize that insertion of (rac)-lac-
tide leads to a chiral intermediate whose stereogencity at the
polymer chain end is mismatched with the stereogenicity at
iron (Figure 5). To alleviate this energetically unfavorable sit-
uation, site epimerization occurs in preference to insertion of
a lactide monomer. As a result, insertion alternates with site
epimerization and heterotactic PLA is formed.
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Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
The authors thank Prof. Geoffery W. Coates, Prof. Jun Okuda,
and Catherine Hermans for useful discussions and Nature
Works, Inc. and Purac America, Inc for the generous donation
of meso and rac lactides, respectively.
In conclusion, we have demonstrated a novel approach to
form stereoregular PLA, which is based on the in situ genera-
tion of a chiral catalyst. The catalyst precursors are achiral and
the methods for their preparation neither require laborious
synthetic sequences nor the purchase of expensive chiral start-
ing materials. This method is conceptually similar to work
from Walsh, who has shown that enantiomerically enriched
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4
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Figure 5. Mechanism for heterotactic polymerization of (rac)-
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