A series of novel mesogen-jacketed liquid-crystal polymers, poly[2,5-bis(4'-alkoxyphenyl)-styrene] (P-n, n = 1-11), were prepared via free-radical polymerization of newly synthesized monomers, 2,5-bis(4'-alkoxyphenyl)styrene (M-n, n = 1-11). The influence of the alkoxy tail length on the liquid-crystalline behaviors of the monomers and the polymers was investigated with differential scanning calorimetry (DSC), then mogravimetry, polarized optical microscopy (POM), and wide-angle X-ray diffraction (WAXD). The monomers with n = 1-4, 9, and 11 were monotropic nematic liquid crystals. All other monomers exhibited enantiotropic nematic properties. Their melting points (T-m's) decreased first as n increased to 6, after which T-m increased slightly at longer spacer lengths. The isotropic-nematic transition temperatures decreased regularly with increasing n values in an odd-even way. The glass-transition temperatures (T-g's) of the polymers first decreased as the tail lengths increased and then leveled off when n greater than or equal to 7. All polymers were thermally stable and entered the mesophase at a temperature above T-g. Upon further heating, no mesophase-to-isotropic melt transition was observed before the polymers decomposed. WAXD studies indicated that an irreversible order- order transition for the polymers with short tails (n less than or equal to 5) and a reversible order- order transition for those with elongated tails (n greater than or equal to 6) occurred at a temperature much higher than T-g. However, such a transition could not be identified by POM and could be detected by DSC only on heating scans for the polymers with long tails (n greater than or equal to 7). (C) 2003 Wiley Periodicals,