Anionic Polymerization of Styrene and 1,3-Butadiene in the Presence of Phosphazene Superbases

The anionic polymerization of styrene and 1,3-butadiene in the presence of phosphazene bases (t-BuP(4), t-BuP(2) and t-BuP(1)), in benzene at room temperature, was studied. When t-BuP(1) was used, the polymerization proceeded in a controlled manner, whereas the obtained homopolymers exhibited the desired molecular weights and narrow polydispersity (Ð < 1.05). In the case of t-BuP(2), homopolymers with higher than the theoretical molecular weights and relatively low polydispersity were obtained. On the other hand, in the presence of t-BuP(4), the polymerization of styrene was uncontrolled due to the high reactivity of the formed carbanion. The kinetic studies from the polymerization of both monomers showed that the reaction rate follows the order of [t-BuP(4)]/[sec-BuLi] >>> [t-BuP(2)]/[sec-BuLi] >> [t-BuP(1)]/[sec-BuLi] > sec-BuLi. Furthermore, the addition of t-BuP(2) and t-BuP(1) prior the polymerization of 1,3-butadiene allowed the synthesis of polybutadiene with a high 1,2-microstructure (~45 wt %), due to the delocalization of the negative charge. Finally, the one pot synthesis of well-defined polyester-based copolymers [PS-b-PCL and PS-b-PLLA, PS: Polystyrene, PCL: Poly(ε-caprolactone) and PLLA: Poly(L-lactide)], with predictable molecular weights and a narrow molecular weight distribution (Ð < 1.2), was achieved by sequential copolymerization in the presence of t-BuP(2) and t-BuP(1).