Process for oligomerization

A process for oligomerizing an olefin comprising

• contacting at least one olefin with a catalyst system under olefin oligomerization conditions sufficient to convert at least a portion of the at least one olefin to a product comprising at least one oligomer of the at least one olefin, the catalyst system comprising:
• a) a source of chromium selected from the group consisting of CrCl3(THF)3, CrBr3(THF)3, CrI3(THF)3, CrCl3, CrBr3, CrI3, CrCl2, CrCl2(THF)2, (chromium (III) acetylacetonate), Cr(acetate)3 (chromium (III) acetate), Cr(2-ethylhexanoate)3 (chromium (III) 2-ethylhexanoate), (THF)3CrMeCl2, (Mes)3Cr(THF), ((TFA)2Cr(OEt)2)2, (THF)3CrPh3, Cr(NMe3)2Cl3, Cr(neopentyl)3(THF)3, Cr(CH2—C6H4-o-NMe2)3, Cr(TFA)3, Cr(CH(SiMe3)2)3, Cr(Mes)2(THF)3, Cr(Mes)2(THF), Cr(Mes)2(THF)2, Cr(Mes)Cl(THF)2, Cr(Mes)Cl(THF)0.5, Cr(p-tolyl)Cl2(THF)3, Cr(diisopropylamide)3, Cr(picolinate)3, Cr(NO3)3, Cr(hexafluoroacetylacetonato)3, (THF)3Cr(η2-2,2″biphenyl)Br, Cr(CO)6, Cr(CO)3(THF)3, Cr(CO)3(NCCH3)3, (benzene)Cr(CO)3, (toluene)Cr(CO)3 and mixtures thereof;
• b) one or more activators selected from the group consisting of neutral Lewis acids comprising Group 13 metal and metalloid compounds;
• polymeric alumoxanes;
• oligomeric alumoxanes;
• (hydrocarbyl)zinc compounds;
• and non-polymeric, non-coordinating, ion-forming compounds;
• aluminates;
• and
• c) at least one phosphacycle-containing ligating compound of the formula:
• wherein:
• R5 attached to N is independently C1-40 substituted or unsubstituted alkyl, C1-40 substituted or unsubstituted aryl;
• each R5 attached to the phenyl group is independently halogen, C1-40 substituted or unsubstituted alkyl, C1-40 substituted or unsubstituted aryl;
• n is an integer from 0 to 5;
• and each X″ is independently hydrogen, halogen, C1-4 alkyl or substituted alkyl, C6-10 aryl or substituted aryl, cyano or nitro.