Tapered Multiblock Star Copolymers: Synthesis, Selective Hydrogenation, and Properties

Two series of well-defined multiblock four-arm tapered star copolymers with "arms"of the type poly(styrene)-block-poly(isoprene-grad-styrene), (SI/S)4, were synthesized on a multigram scale with short reaction times. Targeted molecular weight (M) values ranged from 80 to 240 kg mol-1 with polystyrene (PS) compositions of 40 and 60 wt %. Narrowly distributed (A= 1.04-1.12) copolymers were obtained regardless of M and without additional purification beyond precipitation. At sufficiently high M, members of the 40 wt % PS series possessed either a cylindrical or bicontinuous morphology, whereas the 60 wt % PS series yielded lamellae. The star copolymers outperformed their corresponding linear analogues with respect to ultimate toughness and elongation at break. Young's moduli of up to 304 ± 11 MPa were reached, and the overall toughest (83 ± 2 MJ/m3) star copolymer exhibited the highest strain at break (1198 ± 34%). Catalytic hydrogenation of the PI segments was achieved selectively without saturation of styrene units at conversions of ≥98%. More pronounced strain hardening occurred upon hydrogenation. Modulus, toughness, and ultimate strength of the linear tapered copolymer increased substantially upon hydrogenation to poly(styrene)-block-poly((ethylene-alt-propylene)-grad-styrene), SEP/S. However, these mechanical properties for the star copolymer were found to decrease upon hydrogenation to (SEP/S)4. These materials, particularly the unsaturated multiblock stars of the highest M, are promising candidates for future advanced materials that require extremely tough elastomers.