organic group, the .PHI..sub.3s including at least 0.1 molar equivalent of a quadrivalent hetrocyclic group selected from the following Group(a); .PSI..sub.3s may be the same or different and are individually a bivalent organic group, the .PSI..sub.3s including at least 0.1 molar equivalent of a bivalent hetrocyclic group selected from the following Group (b); and n is a positive integer).