Abstract: Provided herein are methods utilizing microfluidics for the oxygen-controlled generation of microparticles and hydrogels having controlled microparticle sizes and size distributions and products from provided methods. The included methods provide the generation of microparticles by polymerizing an aqueous solution dispersed in a non-aqueous continuous phase in an oxygen-controlled environment. The process allows for control of size of the size of the aqueous droplets and, thus, control of the size of the generated microparticles which may be used in biological applications.