A homologous series of new spiro-orthocarbonate monomers 4, containing biphenyl mesogens at C-3 and C-9 positions of the six-membered spirocyclic ring through the alkylene spacers of different lengths, were prepared by condensation reaction of the corresponding biphenyl mesogenic 1,3-propanediol 3 with tetramethyl orthocarbonate. By opening of the spirocyclic ring with boron trifluoride etherate, spiro-orthocarbonate monomers 4 afforded a novel class of side-chain thermotropic LC polymer with poly(ether carbonate) as the main chain 6. Comparison of the thermal transitions of polymers 6 with those of the corresponding monomers 4 showed that the monomers exhibit far higher isotropic temperatures than the polymers. The ability of the tail groups (NO2, F, BuO and CN) to stabilize the mesophases in side-chain liquid-crystalline poly(ether carbonate)s closely parallels the effect of the tails for spiro-orthocarbonate monomers.