| This research concerned the preparation of novel, well-defined, three-armed, star-branched, styrene/butadiene (S/B) tapered block copolymers by a one-step procedure based on a hydrocarbon-soluble, trifunctional organolithium initiator (TLi). The well-defined, stable, TLi was prepared by the stoichiometric addition of 3 moles of sec-butyllithium to 1,3,5-tris(1-phenylethenyl)benzene in benzene at ambient temperature. The synthetic objective was accomplished by copolymerizing styrene and butadiene with TLi in the presence of lithium 2,3-dimethyl-3-pentoxide (R′OLi) at 60°C overnight in benzene. It was necessary to add several equivalents of R′ OLi (i.e., [R′OLi]/[Li] = 2–5) to overcome the difficulties due to the strong aggregation of this trilithium initiator, including the elimination of the characteristic gel-like state throughout the polymerization. UV-vis spectroscopy revealed that the favorable effect of the bulky R′OLi on the molecular weight distribution was mainly attributable to the decrease of the rate of propagation rather than to the increase of the rate of initiation.; The effects of molecular weight and microstructure on the physical properties of these new thermoplastic elastomers, star S/B tapered block copolymers, were studied by DSC, DMTA, TEM, SAXS, stress-strain behavior and melt behavior. In general, similar trends were observed by DSC and DMTA, i.e., closer and broader transitions for the tapered block copolymers relative to the pure block copolymers. TEM and SAXS experiments showed that both tapered and pure block copolymers [i.e., 29–35 wt % block polystyrene (PS) content] formed cylindrical morphologies. In contrast to the stress-strain behavior of S/B pure block copolymers, the star S/B tapered block copolymers exhibited strong molecular weight dependence of the tensile properties within a similar range of PS block molecular weight (i.e., 9,700–25,000 g mol−1 ) and composition (i.e., 29–35 wt % block polystyrene content). It was necessary to increase the molecular weight of the PS block above the typical limit (i.e., 20,000 g mol−1) for a star S/B tapered block copolymer to show the excellent tensile strength at break (i.e., 30–35 MPa) characteristic of S/B pure block copolymers. The star S/B tapered block copolymers exhibited strong non-Newtonian melt behavior, and lower melt viscosities relative to the star S/B pure block copolymer. |
