Block copolymers of allyl methacrylate and N-(2-hydroxypropyl) methacrylamide (HPMA) with different block lengths have been synthesized by reversible additionfragmentation chain transfer polymerization. Allyl groups were modified with cysteamine, via a thiol-ene photoreaction, with a high efficiency ( 100 ) as evidenced by NMR spectroscopy, yielding cationic copolymers of HPMA. Polyelectrolyte complexes of small interfering RNAs (siRNA) and the cationic block copolymers were then formed at an N/P ratio between 1 and 4 depending on the block length of the copolymers. Increasing the length of the hydrophilic block was found to decrease the efficiency of siRNA complexation. The hydrodynamic diameter of the polyplexes in 130 mM buffer solution was less than 100 nm.