Chemistry Reference
In-Depth Information
molecular CD clamp attached to the active site of b-CD could play an
important role in the polymerization by holding the polymer chain and
consequently securing the active site. The polymerization reaction was
performed in bulk condition at 100 1C for 120 hours using this a,b-TPA-
dimer and d-valerolactone as monomer ([d-VL]/[CD unit]
=
50, Scheme 6).
A relatively high number-average molecular weight was obtained
(Mn
=
11 000g/mol). More polymerizations using other cyclodextrins and
cyclodextrin derivatives as initiators such a-CD, b-CD, mixture of a-CD
and b-CD, a,a-TPA-dimer were performed in the same conditions. It was
discovered that by itself, a-CD is not able to initiate the polymerization.
b-CD, mixtures of a-CD and b-CD, a,a-TPA-dimer show certain
reactivity in the applied conditions, but leading to much lower mole-
cular weight compared to a,b-TPA-dimer initiated polymerization
[b-CD (Mn
=
2300 g/mol), mixture of a-CD and b-CD (Mn
=
2900 g/mol),
a,a-TPA-dimer (Mn
=
2500 g/mol)]. These results furthermore confirmed
that the linkage of a suitable artificial molecular CD clamp (a-CD for the
polymerization of d-valerolactone) to active cyclodextrin (b-CD for the
polymerization of d-valerolactone) is able to accelerate the corresponding
ring-opening polymerization.
Cyclodextrin can also be functionalized using lactide as monomer, as
demonstrated by Hao et al.
33
The reaction was conducted in dimethyl-
formamide at 80-85 1C (Scheme 7). Relatively small amounts of lactide
were introduced in order to improve the solubility of the resulting
compound in view of the drug delivery applications targeted by the
authors. The reaction resulted in oligolactides on the C6 carbon
of cyclodextrin with number-average degrees of polymerization
around 2 and an average degree of substitution of 1.5 for 56% yield after
6 hours.
In the aforementioned studies, but also when the functionalization is
conducted with metal-based catalysts,
34,35
native cyclodextrins are only
partially functionalized. Zinck et al.
25
recently reported an easy access
to a fully functionalized b-cyclodextrin. DMAP was used as organo-
catalyst for the ring-opening polymerization of D,L-lactide initiated by
b-cyclodextrin (Scheme 8). The initiation eciency was found to be
quantitative at 120 1C in bulk. Narrow molecular weight distribution
(Ð
M
1.09) and a number-average degree of polymerization of 10 per arms
were reported in the absence of side reactions, leading to a cyclodextrin
core star polylactide with 21 arms. A perbenzylated b-cyclodextrin diol
could also be functionalized using this strategy (number-average degree
of polymerization per arms of 25, Ð
M
1.09 after 40 h reaction in
dichloromethane at 35 1C).
Scheme 7
b
-cyclodextrin functionalization using lactide in DMF.
33
6 h at 80-85
1
C.
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