The present invention is directed to a catalyst system for synthesizing rubbery polymers, such as styrene-butadiene rubber, that can be utilized in tire tread rubbers. The catalyst system, in one embodiment, includes an organolithium compound and a potassium alkyl borohydride or potassium alkyl aluminum hydride defined by: (R)3A−(H)K+, wherein A is boron or aluminum, H is hydrogen, K is potassium, and R, which is the same or different, is either H, a C1-C10 alkyl, a C3-C10 cycloalkyl, phenyl, or phenyl substituted C1-C10 alkyl, or a C3-C10 cycloalkyl, with the proviso that at least one R is a C1-C10 alkyl, and further wherein any two R's may optionally be part of a carbocyclic ring. The rubbery polymers can have low to medium vinyl microstructure content, i.e., from about 10% to about 40%, and are useful in tire tread rubber compounds which can exhibit improved wear and tear characteristics.