The acyclic diene metathesis (ADMET) polymerization of 6-hydroxy-1,10-undecadiene (M1) and 6-acetoxy-1,10-undecadiene (M2) by the action of two different catalysts, i.e., the second-generation Grubbs-Hoveyda system ([RuCl2(IMesH(2))(CH-2-(2-PrO-C6H4)]) (1) and the dicationic ruthenium alkylidene [Ru(DMF)(3)(IMesH(2))(CH-2-(2-PrO-C6H4)] (2, IMesH(2) = 1,3-dimesitylimidazolin-2-ylidene) is reported. Biphasic conditions using 1-butyl-2,3-dimethylimidazolium tetrafl uoroborate ([BDMIM+BF4-]) and 1,2,4-trichlorobenzene (TCB) are applied. Under the chosen conditions (T = 75 degrees C, 20 mbar), the use of catalyst 1 results only in the formation of low-molecular-weight polymers ((M) over bar (n) <= 10 000 g mol(-1)), while catalyst 2 allows for the high yield synthesis of high-molecular-weight polymers ((M) over bar (n) <= 40 000 g mol(-1), yields <= 99%). Irrespective of the catalyst used, all polymers display a high trans-content (>95%). Notably, Ru-contamination of the target polymers without any additional purification is as low as 1.2 ppm with catalyst 2. Together with the high yields and high molecular weights, the low Ru-contaminations clearly illustrate the advantages of the biphasic setup.