Butyllithium is principally valued as an initiator for the anionic polymerization of dienes, such as butadiene.  The reaction is called "carbolithiation":
:CHLi + CH=CH−CH=CH → CH−CH−CH=CH−CHLi
Isoprene can be polymerized stereospecifically in this way.  Also of commercial importance is the use of butyllithium for the production of styrene-butadiene polymers.  Even ethylene will insert into BuLi.