As an extension of our work on the elucidation of the mechanism and control of 3-dimensional network formation in the free-radical crosslinking polymerization and copolymerization of multivinyl compounds with. the aim to molecularly design vinyl-type network polymers, novel amphiphilic polymers were prepared as crosslinked polymer precursors. Thus, benzyl methacrylate, a nonpolar monomer, was copolymerized radically with 5 mol % of triicosaethylene glycol dimethacrylate [CH2=C(CH3)CO(OCH2CH2)(23)OCOC(CH3)=CH2], a polar monomer, in the presence of lauryl mercaptan as a chain transfer agent. The resulting prepolymers (i.e., vinyl-type network-polymer precursors or amphiphilic polymers) were characterized mainly by viscometry using t-butylbenzene (t-BB) and a t-BB/MeOH (80/20) mixture as solvents. The viscosities in the t-BB/MeOH (80/20) mixture were quite high compared with those in t-BB and completely reversed concentration dependencies were observed in the solvents. These are discussed by considering the difference in conformation and the shrinkage of polar, flexible polyoxyethylene units or the entanglement of nonpolar, rigid primary chains.