The synthesis and characterization of the hemiphasmidic monomers 4-[3,4,5-tri(p-n-dodecan-1-yloxybenzyloxy)benzoate]-4′-(p-allyloxybenzoate)biphenyl (10) and 4-[3,4,5-tri(p-n-dodecan-1-yloxybenzyloxy)benzoate]-4′-(p-allyloxybenzoate)thiodiphenyl (11) and of the polymethylsiloxanes and copolymethylsiloxanes [13(X/Y)] containing side groups derived from10and11are described. Both10and11are only crystalline. However,13(100/0),13(0/100), and the corresponding copolymers exhibit enatiotropic, most probably phasmidic mesopha