Disclosed is a process for preparing polymers and copolymer blocks of conjugated dienes with high amounts of branching, commonly referred to as vinyl content, comprising the use of at least two microstructure control agents. One of the microstructure control agents can be a bulk microstructure control agents such as diethyl ether. The other microstructure control agent is a specific microstructure control agent such as 1,2-diethoxypropane. The use of a bulk and a specific microstructure control agent together can allow for the production of high levels of vinyl content at high polymerization rates without the undesirable affects experienced with either one alone.