Amorphous saturated hydrocarbon polymers containing main-chain cyclic units have potential as novel optoelectronic materials for plastic lenses and optical storage media owing to their high glass transition temperature, nonhygroscopic nature, transparency, and low birefringence. Here, we report the synthesis of block copolymers (BCPs) incorporating a branched polyethylene glycol (PEG) hydrophilic block and randomly copolymerized styrene and isoprene units (p(St-co-Ip)) of different molecular weights as the hydrophobic component. Amorphous polymers bearing cyclic structures along with hydrophobic chains were prepared via the cationic cyclization of the unsaturated hydrocarbon polymers under acidic conditions. We suggest that the morphological transitions of self-assembled BCP structures are dependent on changes in the chain length of the hydrophobic block, which is determined by the regiospecificity of the isoprene units. The self- assembled structures could be covalently stabilized via intermolecular cyclization between the hydrophobic blocks. Notably, the cross-linked structures displayed a reversible swelling/deswelling ability in response to the type of solvent medium.