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.