A New Approach to Styrenic Thermoplastic Elastomers: Synthesis and Characterization of Crystalline Styrene−Butadiene−Styrene Triblock Copolymers

Crystalline styrene−butadiene−styrene (SBS) triblock copolymers consisting of elastic cis -polybutadiene ( cis -PB) chemically bonded with crystallizable syndiotactic polystyrene ( syn -PS) were synthesized through a stereospecific sequential triblock copolymerization of S with B catalyzed by C 5 Me 5 TiMe 3 /B(C 6 F 5 ) 3 /trioctylaluminum (Al(oct) 3 ) under controllable polymerization conditions. The first polymerization step was conducted at −25 °C, yielding syn -PS with controllable molecular weight by changing the polymerization time. The second one was started with the addition of excess B to the progressing syn -PS polymerization at −40 °C, yielding syn -PS/ cis -PB diblock copolymer with high block efficiencies. The third one was conducted at −25 °C just by removal of the unreacted B in order to enhance the reinsertion and propagation of the remained S, yielding crystalline SBS triblock copolymers with relatively high PS compositions. The livingness of the whole polymerization process was confirmed through a linear increase of the molecular weights of the polymer products obtained in each step against their polymer yields, while keeping the polydispersity values almost constant. The 13 C NMR analysis of the copolymer product proves that the PS blocks were highly syndiotactic ([rrrr] > 95%), and the PB block was primarily cis -1,4 structure (>70%). Cross-fractionation chromatography (CFC) was used to evaluate the contamination of the dead polymers in the crystalline SBS. Transmission electronic microscopy (TEM) was used to observe the phased separation morphology of the PS and PB blocks in the crystalline SBS. The existence of the syn -PS blocks in the crystalline SBS was further confirmed with the observation of a strong endothermic peak at 272 °C in their differential scanning calorimetry (DSC) curves. Preliminary results on the heat deformation stability as well as the chemical resistance property of this new material have also been evaluated in comparison with those of the corresponding anionic SBS materials.