Blowing agent-containing polymeric MDI compositions

In a process for the preparation of a polyurethane or polyisocyanurate foam wherein a polyol component and an isocyanate component are reacted in the presence of a blowing agent, with an isocyanate index of from 70 to about 900, the improvement comprising

• a) selecting monochlorodifluoromethane as at least one portion of said blowing agent, and
• b) dissolving at least a portion of said monochlorodifluoromethane into an isocyanate component comprising in major portion a medium viscosity polymeric methylenediphenylenediisocyanate having a viscosity between 400 centipoise and 1200 centipoise when measured at 25.degree. C., prior to reaction of said isocyanate component with said polyol component.