Preparation and stress‐strain properties of aba block copolymers of α‐methylstyrene and butadiene

ABA‐type “tapered” block copolymers of α‐methylstyrene (monomer A) and butadiene were prepared using four commercially available dilithio initiators. Polymerizations were run at 25°C in benzene solvent, or at 40°C with butadiene dissolved in neat α‐methylstyrene. Although α‐methylstyrene has a rather low ceiling temperature, triblock copolymers could be made at these temperatures by using α‐methylstyrene concentrations well in excess of the [ M ] e values at the respective temperatures. Its concentration was such that molecular weights of at least 15,000–20,000 for the A blocks could be attained. The course of the copolymerization at 40°C was followed, showing that copolymers containing about 40% α‐methylstyrene could be formed in 4–8 hr, depending on the initiator used. They showed the usual behavior of triblock thermoplastic elastomers, with tensile strengths > 3000 lb/in. 2 at 24°C. However, because of the high T g of poly(α‐methylstyrene) (172°C), they also had tensiles of several hundred lb/in. 2 at 100°C, unlike comparable polymers with polystyrene end blocks, which have practically no strength at this temperature. Microstructures of polybutadienes made with these initiators are also given.