Method of making alpha-olefin oligomers

The continuous process for polymerizing a 1-olefin to an oligomer mixture including trimer and tetramer which comprises continuously introducing a 1-olefin having from about five to about 14 carbon atoms or a mixture thereof into the first of a series of one or more tank reactors with mixing therein to provide a substantially homogeneous reaction mixture in the presence of a catalytic amount of boron trifluoride and a co-catalyst that complexes with boron trifluoride and continuously removing a partially oligomerized product stream from said series of one or more tank reactors and flowing said product stream through a tube reactor for further conversion of said 1-olefin to oligomer product, the length to diameter ratio of said tube reactor being at least about 15 to one and said series of tank reactors and said tube reactor being maintained at a same or different temperature suitable for the oligomerization reaction.