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 seobutyllithium as initiators in benzene solution. The stressstrain 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. Stressstrain curves most closely resembling those of crosslinked elastomers are obtained in.genera1 from polymers containing about 3045% 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.
The advent of anionic polymerization techniques, particularly those arising from lithium-based initiators, has wade possible the preparation of many novel block
Recently, it has been demonstrated that ABA-type block polymers of styrene (monomer A) with butadiene or isoprene may exhibit behavior of crosslinked elastomers, even though the polymer molecules are perfectly linear and have no chemical crosslinks between them.5-9 Such polymers are thermoplastic and can be easily molded at elevated temperatures to form,intrjcate shapes, yet at ambient temperatures behave as though they were vulcanized rubbers. It was of interest in our laboratories to prepare block polymers of this type and study their physical properties as a function of their composition. This paper reports the stress-strain curves for a number of styrene-butadienestyrene and styrene-isoprene-styrene block polymers, which clearly show their behavior as pseudo-crosslinked elastomers.