A novel semi-interpenetrating polymer network (semi-IPN) with photomechanical switching ability was developed by a cationic copolymerization of azobenzene containing vinyl ethers in a matrix of a linear polycarbonate (PC). The semi-IPN film showed reversible deformation upon switching the UV irradiation on and off and responded with unprecedented rapidity. The photomechanical effect is attributed to a reversible change between the highly aggregated and dissociated states of the azobenzene groups.