Preparation and stress–strain properties of ABA‐type block polymers of styrene and isoprene or butadiene

Several series of ABA‐type “tapered” block polymers of styrene (monomer A) with isoprene or butadiene were prepared with the use of n ‐butyllithium or sec ‐butyllithium as initiators in benzene solution. The stress‐strain curves of the raw polymers are reported, showing that many of them behave at ambient temperature like vulcanized elastomers. These polymers consist, however, of completely linear chains, with no chemical crosslinks between them; they are thermoplastic, can be easily molded, and are soluble in common solvents. Hence, they may be classed as pseudo‐crosslinked elastomers. Better stress‐strain properties are obtained from polymers made with sec ‐butyllithium than with n ‐butyllithium; in turn, polymers from butadiene have better properties than those from isoprene. Stress–strain curves most closely resembling those of crosslinked elastomers are obtained in general from polymers containing about 30–45% styrene. It was found that AB‐type block polymers have very poor tensile strengths and low elongations. The microstructures of polybutadienes and polyisoprenes of various molecular weights, prepared in benzene solvent with alkyllithium initiators, were also determined and compared with literature data for like polymers prepared in cyclohexane solvent.