The polymerization of styrene in THF initiated by the various isomers of butyllithium was studied at low temperature utilizing the rapid-injection NMR technique. The order of reactivity was found to be sec-BuLi > tert-BuLi > n-BuLi. The reactivity order is different than that observed in hydrocarbon solvents : sec-BuLi > n-BuLi > tert-BuLi. These reactivity differences are explained by noting the different aggregation states of the butyllithium reagents. A complexation mechanism is proposed where the styrene coordinates with the lithium of the BuLi and then undergoes subsequent reaction. MNDO calculations support this behavior and predict the relative reactivities found in the RINMR experiments.