By in situ thin-film polymerization conducted on a heating stage of a polarizing microscope, we have investigated the effects of monomer structures on the formation of liquid crystallinity. Three polymerization systems studied are 2,6-acetoxynaphthoic acid (ANA)/acetoxy acetanilide (AAA)/phthalic acid (PA), ANA/AAA/ isophthalic acid (IA) and ANA/AAA/terephthalic acid (TA). In the three systems, PA, IA, and TA may create an ortho, a meta, and a para linkage, respectively. The formation of liquid crystallinity was found strongly dependent on the straightness and configuration of monomeric units. For ANA/AAA/PA and ANA/AAA/IA systems, there exists the critical ANA content to yield the Liquid crystalline phase. Below this critical content, either amorphous phase forms or crystallization occurs during polymerization. Experimental data also indicate that defect density in the polymerization product reduces with increasing ANA content. Surprisingly, for the first time, we have observed that the ANA/AAA/PA system has a higher tendency to yield liquid crystallinity than the ANA/AAA/IA system. For the ANA/AAA/TA system, the polycondensation reaction is incomplete if the TA content is too high because of the low reactivity and the high melting point of TA. Polymerization of the ANA/AAA/TA system does not yield totally random copolymers because the liquid crystal phase appears before all TA crystals disappear during the polymerization.