Catalyst for ring expansion metathesis polymerization of cyclic monomers

A tetraanionic pincer ligand metal-oxo-alkylidene complex of the structure:

• where:
• Z is independently O or S;
• R comprises, independently, H, methyl, ethyl, n-propyl, i-propyl, n-butyl, t-butyl, or alkyl containing 5-22 carbon atoms;
• R′ is methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, alkyl containing 5-22 carbon atoms, phenyl, naphthyl, or aryl containing 13-22 carbon atoms;
• X is O, N, S, P, or Se;
• R″, independently, is methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, alkyl containing 5-22 carbon atoms, phenyl, naphthyl, aryl containing 13-22 carbon atoms, or two R″ are an alkylene containing 4-6 carbon atoms combined with a single X to form a heterocycle;
• n is 1 to 3;
• and M is a group 5-7 transition metal.