Aqueous polymer dispersions are commodity materials produced on a multimillion-ton scale annually. Today none of these materials are biodegradable because the process by which they are made is not compatible with the synthesis of biodegradable polymers. Herein, we report a droplet microfluidic encapsulation strategy for protecting a water incompatible ring-opening polymerization (ROP) catalyst from the aqueous phase, yielding biodegradable polymer particles dispersed in water. Polymerization yields 300 mu m sized particles comprised of biodegradable poly(delta-valerolactone) with molecular weights up to 19.5 kg mol(-1). The success of this approach relies on simultaneous precise control of the kinetics of polymerization, the rate of mass transfer, and fluid mechanics. The power of this methodology was demonstrated by the synthesis of cross-linked polymer particles through the copolymerization of bis(ecaprolactone-4-yl)propane and d-valerolactone, producing cross-linked polymer particles with molecular weights reaching 65.3 kg mol(-1). Overall, this encapsulation technique opens the door for the synthesis of biodegradable polymer latex and processable, biodegradable elastomers.