Synthesis and characterization of polystyrene-b-poly(ethylene oxide)-b-poly(epsilon-caprolactone) block copolymers

The preparation of polystyrene-b-poly(ethylene oxide)-b-poly(epsilon -capro lactone) triblock copolymers using three different strategies is described. Experimentally, a method we have called "thermal" is the simplest one of t he three employed. However, it presents serious limitations for the prepara tion 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 th e precursor terminal hydroxyl groups requires much expertise, and it become s more difficult as the concentration of such terminal groups decreases whe n 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 u nder conditions of strict purity and high vacuum. Several triblock and pent ablock copolymers of varying compositions and molecular weights were succes sfully prepared. The fact that these copolymers contain two crystallizable blocks, i.e., PEO and PCL, makes them very interesting from a structural po int 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. Fractionate d crystallization phenomena were detected when either one or both blocks co nstituted the minor components within the block copolymers.