of the invention include pressures less than 300 mm hg less than 200 mm hg less than 100 mm hg or less than 20 mm hg lower pressures are preferred in most cases because the reaction then can be run at a lower temperatures whereby fewer competing reactions are likely to take place the residence time in the reaction zone for reacting 1 3 propanediol and phosgene to form trimethylene carbonate and is typically a function of the starting reactant feed rates and the temperature the residence time can vary considerably and with some embodiments may range from about 1 to 600 seconds typically 1 to 30 seconds in the particular embodiments described herein and most typically 5 to 20 seconds the residence time can be shortened by employing a non reactive sweeping gas to carry the reactants and reaction products through the reactor suitable non reactive gasses include but are not limited to nitrogen and the inert gases neon argon krypton xenon and helium nitrogen is a preferred gas due to its low cost the trimethylene carbonate vapors in the reaction product stream may be condensed after leaving the reaction vessel the condensation is accomplished by exposing the trimethylene.