We report here an in-depth analysis of the reaction mechanisms involved in the formation of polymer dimers formed by the coupling of polystyryllithium (PSLi) with dichloromethane (DCM), dibromomethane (DBM), and diiodomethane (DIM) in tetrahydrofuran at -78 degreesC. The DBM-mediated reactions give a high degree of coupling but generate 1,2-diphenyl linkages in addition to the expected 1,3-diphenyl linkages and small amounts of P-substituted styrenic end groups that are detectable by fluorescence measurements. This is consistent with the formation of bromobenzyl end groups by lithium-bromine exchange and PSLi-mediated elimination. The formation of a-substituted styrenic end groups by conventional displacement and elimination is also possible, Although reactions of PSLi with DCM show no coupling at all, DIM is a much better coupling agent than DBM, significantly suppressing side reactions.