Abstract: The present invention provides a method of producing polymer particles by precipitation polymerisation, the method comprising: providing a reaction solution comprising (i) solvent, (ii) a photo-active compound that upon being subjected to a wavelength of light forms two o-quinodimethane structures, and (iii) a multi-dienophile compound; subjecting the reaction solution to the wavelength of light, which promotes step-growth Diels-Alder polymerisation of the photo-active and multi-dienophile compounds; wherein as a result of the step-growth Diels-Alder polymerisation polymer precipitates from the reaction solution and self-assembles into the polymer particles.