Chemistry Reference
In-Depth Information
Harada and co-workers reported the use of cyclodextrins as initiators
for the ring-opening polymerization of various lactones including
b-butyrolactone, d-valerolactone and e-caprolactone (Scheme 5).
27-31
The
polymerization yield was found to rely on the size of the lactone and the
cavity of the cyclodextrin, indicating that the initiating step takes place
via inclusion of the lactone. Quantitative yields were reported after 48 to
96 hours at 100 1C for the polymerization of d-valerolactone in the pres-
ence of b-cyclodextrin, with dispersities around 1.8 and number-average
degrees of polymerization up to 10. The importance of the formation of
inclusion complexes between the lactone (guest molecule) and the
cyclodextrin (host molecule) was highlighted by the following finding:
the polymerization activity of d-valerolactone can be suppressed by using
a b-cyclodextrin-adamantane inclusion complex instead of native
b-cyclodextrin as initiator under the same conditions. It is noteworthy
that the reaction is regioselective, the polymerization occurring on a
secondary alcohol (OH group on C3) which is usually less reactive than a
primary alcohol (OH group on C6).
Based on the above finding, the same group designed a dissymmetric
cyclodextrin based dimer including a b-CD unit and a a-CD linked by
terephthalamide (Scheme 6).
32
It was anticipated that an artificial
Scheme 5 Functionalization of
a
,
b
and
g
-cyclodextrins via self-catalyzed ring-opening
polymerization of
b
-butyrolactone,
d
-valerolactone and
e
-caprolactone in bulk.
27,28
48-96 h reaction at 100
1
C. 3-20% cyclodextrin.
Scheme 6 Polymerization of
d
-valerolactone initiated by the
a
,
b
-TPA-dimer linked with
terephthalamide between the
a
-CD (green) and
b
-CD (blue).
32
120 h at 100
1
C.
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