Effect of Polymer Composition on Rheological and Degradation
Properties of Temperature-Responsive Gelling Systems Composed of Acyl-Capped
PCLA-PEG-PCLA
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Abstract
In
this study, the ability to modulate the rheological and degradation
properties of temperature-responsive gelling systems composed of acyl-capped
poly(ε-caprolactone-<i>co</i>-lactide)-<i>b</i>-poly(ethylene glycol)-<i>b</i>-poly(ε-caprolactone-<i>co</i>-lactide) (PCLA-PEG-PCLA) triblock copolymers was investigated.
Eight polymers with varying molecular weight of PCLA, caproyl/lactoyl
ratio (CL/LA) and capped with either acetyl- or propionyl-groups were
synthesized by ring-opening polymerization of l-lactide and
ε-caprolactone in toluene using PEG as initiator and tin(II)
2-ethylhexanoate as catalyst, and subsequently reacted in solution
with an excess of acyl chloride to yield fully acyl-capped PCLA-PEG-PCLA.
The microstructure of the polymers was determined by <sup>1</sup>H
NMR, and the thermal properties and crystallinity of the polymers
in dry state and in 25 wt % aqueous systems were studied by differential
scanning calorimetry and X-ray diffraction. Rheological and degradation/dissolution
properties of aqueous systems composed of the polymers in 25 wt %
aqueous systems were studied. <sup>1</sup>H NMR analysis revealed
that the monomer sequence in the PCLA blocks was not fully random,
resulting in relatively long CL sequences, even though transesterification
was demonstrated by the enrichment with lactoyl units and the presence
of PEG–OH end groups. Except the most hydrophilic polymer composed
of acetyl-capped PCLA<sub>1400</sub>-PEG<sub>1500</sub>-PCLA<sub>1400</sub> having a CL/LA molar ratio of 2.5, the polymers at 25 wt % in buffer
were sols below room temperature and transformed into gels between
room temperature and 37 °C, which makes them suitable as temperature-responsive
gelling systems for drug delivery. Over a period of weeks at 37 °C,
the systems containing polymers with long CL sequences (∼8
CL) and propionyl end-groups became semicrystalline as shown by X-ray
diffraction analysis. Degradation of the gels by dissolution at 37
°C took 100–150 days for the amorphous gels and 250–300
days for the semicrystalline gels. In conclusion, this study shows
that changes in the polymer composition allow an easy but significant
modulation of rheological and degradation properties