methods of forming polymer particles in situ from polymer precursors such as monomers or oligomers , comprising exposing a composition comprising at least one polymer precursor , a solvent or solvent mixture , and an antisolvent or antisolvent mixture to photoradiation under conditions whereby particles are formed are provided . the polymer precursor may be photosensitive , or a separate polymerization initiator may be used . in a preferred embodiment , the polymer precursor is insoluble in the antisolvent or antisolvent mixture and the solvent or solvent mixture is soluble in the antisolvent or antisolvent mixture at the concentrations used . polymer particles comprising a polymer and a bioactive material are also provided . the polymer may be erodable , and the polymer particles formed may be used in a variety of applications , including controlled release of bioactive materials such as drugs . polymer particles formed using the methods of the invention have low residual solvent levels and high additive encapsulation efficiencies . the processes of the invention allow control of particle size and morphology , use low operating temperatures and are useful for efficient bulk production .