To obtain a soluble rigid polymer by selecting a polymer which comprises a rigid main chain and a solubilizing organic side groups having a specified or higher average molecular weight and in which the solubilizing side groups are flexible ones that permit the linear polymer to remain dissolved in the polymerization solvent system.

There is provided a linear polymer comprising at least 25 covalently bonded monomer units, wherein at least 95% of the bonds comprise a rigid main chain and solubilizing organic side groups having an average molecular weight of 300 or above and bonded in a rate of at least one per 100 monomer units. The solubilizing organic side groups constitute 30-95 wt.% of the total polymer and are flexible groups selected among alkyls, aryls, aralkyls, etc., and permitting the polymer to remain dissolved in the polymerization solvent system. The rigid main chain desirably has an aspect ratio of 100 or above and comprises at least one type of monomer units such as p-phenyl, p-biphenyl and p-terphenyl.