A series of polymethacrylates containing laterally methyl-substituted 4-alkoxy-4-nitrotolane, 4-alkoxy-4′-cyanotolane, and 4-alkoxy-4′-trifluoromethyltolane as mesogenic side groups are presented. The thermal behaviours of the prepared compounds were characterized by differential scanning calorimetry, optical polarized microscopy and X-ray diffraction. All of the obtained methacrylate monomers exhibit no mesophase, while most of the synthesized polymers reveal enantiotropic mesomorphism. The polymer containing 4-propyloxy-2′-methyl-4′-nitrotolane side groups was the only one to show no mesomorphic behaviour. The spacer length and the nature of terminal group have a profound influence on the type of mesophase formed and the thermal stability. Incorporation of a lateral methyl-substituent into the mesogenic groups reduces the thermal stabilities of the mesophases. X-ray diffraction reveals that polymers with a smectic A or smectic E phase have the inclination to form a bilayer structure. Increasing the temperature within the smectic phase range, the intermolecular spacing increases while the layer spacing decreases. On the other hand, introducing a lateral substituent into the mesogenic side groups leads to increase in both layer and intermolecular spacing.