A method for improving the initiation efficiency during the anionic polymerization of 15 to 100 percent weight of a vinyl aromatic hydrocarbon monomers and 0 to 85 percent by weight of a conjugated diene monomer by conducting the polymerization in the presence of a catalytically effective amount of: (a) an organolithium initiator and (b) a 2,5-substituted tetrahydrofuran modifier having the structural formula: ##STR1## wherein R.sub.1 and R.sub.1 ' are independently H or an alkyl group containing 1 to 4 carbon atoms and R.sub.2 and R.sub.2 ' are alkyl group containing 1 to 4 carbon atoms.