C O M M U N I C A T I O N S
polymer attacks the carbonyl carbon of the included esters and
cleaves the C-O bond. In pathway B, a hydroxyl group at the end
of the poly(lactone) attacks the carbonyl carbon of the included
cyclic esters. Continuous inclusion and insertion of the cyclic esters
yield linear poly(lactone)s with CDs. However, pathway B is
unlikely, because mono-2-O-(6-benzoxypentanoyl)-â-CD was found
to initiate polymerization of δ-valerolactone even when there is
no hydroxyl end group (Table S1 and Figure S17). The integral
values of the glucose residue with poly(δ-VL) and the other natural
glucose residue showed a ratio of 1:6.
In conclusion, a series of CDs were found to initiate the
polymerization of cyclic esters selectively without any cocatalysts
and cosolvents. The products are polyesters with CD at the end of
the chain. Detailed investigation of the polymerization mechanism
is now under way using various CD derivatives. This study should
contribute to the development of environmentally benign polymers
and catalysts using CDs.
Acknowledgment. This work was partially supported by a Grant
in-Aid No. S14103015 for Scientific Research from the Ministry
of Education, Culture, Sports, Science and Technology, Japan.
Figure 3. The 1H NMR spectrum of â-CD attached to poly(δ-VL) obtained
from the mixture â-CD and δ-VL (a), and the spectrum of mono-2-O-
(6-benzoxypentanoyl)-â-CD (b) in DMSO-d6.
Supporting Information Available: Polymerization of ꢀ-CL and
â-BL with R-CD, â-CD, γ-CD, modified CDs, and methanol. MALDI-
TOF MS spectra of â-CD attached poly(δ-VL) and poly(ꢀ-CL), γ-CD
attached poly(δ-VL) and poly(ꢀ-CL), R-CD attached poly(â-BL), and
â-CD attached poly(â-BL). Experimental procedures and characteriza-
tion for mono-2-O-(6-benzoxypentanoyl)-â-CD and â-CD attached
poly(δ-VL). This material is available free of charge via the Internet
Scheme 1
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derivatives. First of all, methanol is inactive in polymerization under
the same conditions (Figures S10 and S11). 2,6-Di-O-methyl-â-
CD and 2,3,6-tri-O-acetyl-â-CD showed no reactivity for the
polymerization of lactones (Figure S12). These results indicate that
the inclusion of lactones in the CD cavity and C2 hydroxyl groups
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were found to form inclusion complexes at 100 °C without water.
Scheme 1 shows possible mechanisms for the polymerization of
lactones. The first step is the complex formation of cyclic esters
with CD to give a 1:1 inclusion complex. Then OH-2 of CDs should
attack the carbonyl carbon of the included cyclic esters, cleaving
the C-O bond to form a CD-O(2)-ester. The FT-IR spectra of
lactones showed that the guest molecules included were activated
by the inclusion in the CDs cavity.
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Two plausible routes for the propagation step are shown in
Scheme 1. In pathway A, O′-2 of the glucose unit attached to a
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