Six trimers with biphenyl mesogenic groups and polymethylene spacers were synthesized. From these trimers and biphenol diglycidyl ether (BPGE), a new series of combined liquid crystalline polymers (LCPs) with polyether backbone were synthesized by the polyaddition with quaternary phosphonium chloride as catalyst. The effects of flexible spacer length on the phase transition and mesomorphism of the resulting trimers and combined LCPs were studied using differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction. The trimers with 1,4-propylene and 1,4-butylene spacers were absent of mesophase, and the trimers with odd-numbered spacers, 1,5-pentylene and 1,7-heptylene, were nematic. In contrast, the trimers with even-numbered spacers, 1,6-hexylene and 1,10-decylene, were smectic within very narrow temperature ranges. All the combined LCPs were thermotropically nematic, though the crystalline behavior was complex with varying sidechain spacer length. The combined LCPs with 3, 5, and 10 methylene units (n) of side-chain spacers were semicrystalline at room temperature while other polymers were glassy. The combined LCPs showed general odd-even effects of side-chain length on T-CP and isotropization entropy in which the polymers with even numbers of methylene units in side-chain spacer showed higher values.