Calcium/lithium and calcium/magnesium systems for the anionic copolymerization of butadiene and styrene

The synthesis of styrene-butadiene copolymers is widely used in rubber materials. The microstructure of polybutadiene segments and the amount of styrene incorporated into it are key parameters to tune the rubber properties. Copolymerization using an anionic initiator such as an alkyllithium tends to produce block copolymers in an apolar medium. Polar additives can be used to increase the amount of styrene into the copolymer but with an increase amount of 1,2 units of the polybutadiene part. Herein, we report a controlled anionic copolymerization of butadiene and styrene using a bimetallic [(THF)₂Ca{N(SiMe₃)₂}₂]:[n-BuLi] initiator in cyclohexane. The insertion of styrene units into polybutadiene chains is increased due to a decrease of the butadiene reactivity ratio by the presence of the calcium complex at the chain ends. The polybutadiene part is mainly composed of 1,4-trans units and keeps low vinyl content. A less efficient lithium-free approach using [(THF)₂Ca{N(SiMe₃)₂}₂]:[(n,s)-Bu₂Mg] initiator for the butadiene and styrene copolymerization is also discussed.