Preferably, solvents such as tetrahydro furane, dioxane, or polymerizable monomers such as triethylenglycol bismethacrylate, diethylenglycol bismethacrylate, dioxolan bismethacrylate, vinyl-, vinylen- or vinyliden-, acrylate- or methacrylate substituted spiroorthoesters and 2,2-Bis[p-(acryloxyethoxy)phenyl]propane are present during esterification of the macromonomers.