Statistical and amphiphilic block copolymers bearing cinnamoyl groups were prepared by ring opening metathesis polymerization (ROMP). The UV-induced [2 + 2] cycloaddition reaction of polymer bound cinnamic acid groups was studied in polymer thin films as well as in block copolymer micelles. In both cases, exposure to UV-light for 10 min led to a crosslinking conversion of about 60%, as determined by FT-IR spectroscopy and LTV-vis absorption measurements. Time based IR-spectroscopy revealed a maximum conversion of 78% reached after an irradiation time of about 16 min. For micelles obtained from polymers bearing 5 mol % or more cinnamoyl groups, the crosslinking reaction proceeded smoothly, yielding in crosslinked particles which were stable in a non-selective solvent (CHCl3). Diameters determined by dynamic light scattering in the selective solvent (MeOH) were similar for both, non-crosslinked and crosslinked. micelles, whereas diameters of crosslinked micelles in the non-selective solvent (CHCl3) were significantly larger compared to MeOH samples. This strategy of direct self assembly of block-copolymers in a selective solvent followed by "clean" crosslinking, without the need for additional crosslinking reagents or crosslinking initiators, provides a straight forward approach toward ROMP-based polymeric nano-particles. (C) 2008 Wiley Periodicals, Inc.