Hence R2 includes, but is not limited to C2 -C20 unsubstituted hydrocarbon, including alkyl, cycloalkyl, and alkaryl, C2 -C20 hydrocarbon partially substituted by halogen, or ##STR10## wherein m is an integer of 0 to 20, Y1 to Ym+2 are independently selected from divalent C2 -C20 hydrocarbon, and C2 -C20 hydrocarbon partially substituted by halogen; and X1 to Xm+1 are independently selected from t