Calcium-magnesium systems as lithium-free initiators for the anionic polymerization of butadiene

Lithium-based initiators are commonly used for the polymerization of aromatic vinylic monomers and conjugated dienes. The corresponding polymers and copolymers are of huge importance in particular in the field of tire industry. Nevertheless, lithium is nowadays increasingly used for energy-related applications and alternatives has to be proposed in the field of anionic polymerization. A lithium-free anionic polymerization of butadiene using a new bimetallic initiator based on two alkaline earth metals, calcium and magnesium, is proposed and discussed. From two inactive initiators for butadiene polymerization, i.e [(THF)₂Ca{N(SiMe₃)₂}₂] and [(n,s)Bu₂Mg], an active bimetallic initiator complex is prepared by mixing the two in apolar medium at 40 or 60 °C. Polybutadienes are synthesized with controlled molar masses, a low dispersity, and a living character. Interestingly, unusual microstructure of 75 % of 1,4-trans units with low vinyl content (<10 %) could be reached.