A new method for preparation of crown ether-based polyrotaxanes is
described; stirring a
solution of the melt of an aliphatic crown ether and a polyurethane
afforded the corresponding
polypseudorotaxane with hydrogen bonding between the NH groups of the
polymer and the ether oxygen
atoms of the cyclic species as the driving force. The degree of
threading (m/n, the average number
of
cyclics per repeat unit) increased with increasing molar feed ratio of
crown ether to polymeric NH groups.
The degree of threading was reduced by simple dilution with a
cosolvent, or threading was prevented by
use of a competitive hydrogen-bonding solvent such as DMSO. The
polyrotaxanes had higher intrinsic
viscosities but lower shape-dependent Huggins constants than the
backbone; this was attributed to their
greater hydrodynamic volumes and more expanded chain conformations.
Moreover, all the polyrotaxanes
had only one glass transition temperature; the values depending on
m/n agreed with the Fox equation
using a T
g of −22 °C for threaded crown
ether instead of using the T
g (−68 °C) of
the free counterpart.
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