The preparation of polystyrene-b-poly(ethylene oxide)-b-poly(ε-caprolactone) triblock copolymers using three different strategies is described. Experimentally, a method we have called “thermal”
is the simplest one of the three employed. However, it presents serious limitations for the preparation of
high molecular weight triblock copolymers with low polydispersity. The titration method is a more elegant
alternative to prepare the PS-b-PEO-b-PCL triblock copolymers. Nevertheless, the stoichiometric titration
of the precursor terminal hydroxyl groups requires much expertise, and it becomes more difficult as the
concentration of such terminal groups decreases when the precursor molecular weight increases. Finally,
the sequential method may be regarded as the most convenient one. However, the experimental steps
involved a solvent exchange during polymerization that must be performed under conditions of strict
purity and high vacuum. Several triblock and pentablock copolymers of varying compositions and molecular
weights were successfully prepared. The fact that these copolymers contain two crystallizable blocks,
i.e., PEO and PCL, makes them very interesting from a structural point of view. The copolymers were
found to be phase segregated according to evidence provided by DSC and WAXS. The crystallization and
melting behavior of both PEO and PCL blocks is greatly affected by composition. Fractionated
crystallization phenomena were detected when either one or both blocks constituted the minor components
within the block copolymers.
Summary: A first attempt was made to produce novel ABC triblock terpolymers with three potentially crystallisable blocks: polyethylene (PE), poly(ethylene oxide) (PEO), and poly(e-caprolactone) (PCL). Polybutadiene-b-poly(ethylene oxide) diblock copolymers were synthesized by living anionic polymerization. Then, a non-catalyzed thermal polymerization of e-caprolactone from the hydroxyl end group of the PB-b-PEO diblock precursors was performed. Finally, hydrogenation by Wilkinson catalyst produced PE-b-PEO-b-PCL triblock terpolymers. Side reactions were detected that lead to the formation of undesired PCL-b-PEO diblock copolymers, however, these impurities were successfully removed by purification. A range of triblock terpolymers with PCL and PEO minor components were prepared. Topological restrictions on the PEO middle block prevented this block from crystallizing while the complex crystallization behavior of the PE and PCL blocks was documented by DSC and WAXS measurements.
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