A selected monomer containing a photosensitizer flows along a capillary at a defined rate into a suitable receiver. Very high concentrations of free radicals (up to 10−4mole/l.) are produced at a selected point by focusing into it a large proportion of the radiation from a capillary are lamp. These polymer radicals grow to a chain length determined by the time spent subsequently in the capillary and it has been demonstrated by a utilization of the theory of the nonstationary phase in chain reactions that most of the radicals can be introduced into a second monomer contained in the receiver. That their continued growth in the new environment leads to the synthesis of block copolymer has been proved by examination of the ultraviolet spectra of the products.