Likewise, Ti/Zn , La/Mg , Li/Mg complexes have also been studied to compare their activity in the polymerization. Thus far, experimental observations suggest that metals in multi-metallic catalytic systems have different functions: larger and more electropositive metals (e.g. potassium, lithium, sodium) act as monomer coordination sites; (2) smaller and less electropositive metals (e.g. magnesium, zinc, aluminum, indium, yttrium, germanium, tin.) act as the source of metal-alkoxide nucleophiles .