In this formula, each A is a divalent linking group as is typically derived from the core of the diisocyanate used to form the isocyanurate, and may be either an aliphatic (e.g., hexamethylene), cycloaliphatic (e.g., residue of isophorone diisocyanate or ##STR7## or aromatic group (e.g., residue of tetramethylxylylene diisocyanate), preferably up to 20 carbon atoms.