Catalytic Polymerization of the Conjugated Dienes Using Quinolyl-Based <i>NNN</i> -Pincer Rare-Earth Metal Chlorides

Strong demand for rubber, coupled with limited natural rubber supply, has accelerated the development of synthetic alternatives. Metal-catalyzed polymerization is a key route to highly stereoregular rubber materials. Among the developed catalysts, rare-earth metal catalytic systems have attracted attention because of their relatively low toxicity, high catalytic activity, and pronounced stereoselectivity. A series of quinolyl-based pincer rare-earth chlorides was synthesized through the reactions of quinolyl-based lithium of the ligands with RECl3. The products obtained from reactions depend on the ionic radius of the rare-earth ion: rare-earth metal ions with larger ionic radii afford (κ3-L)RECl2(THF)2 (1 = 1Y, 1Dy, 1Gd) (L = 8-(2,6-iPr2C6H3N)-2-[CH═N(2,6-iPr2C6H3)]-C9H5N), whereas those with smaller ionic radii yield (κ3-L)(Cl)RE(μ-Cl)2Li(THF)2 (2 = 2Lu, 2Er, 2Yb). By contrast, the reactions of quinolyl-based dilithium of the ligands with RECl3 are insensitive to ionic-radius variation and consistently yield (κ3-L′)RE(THF)(μ-Cl)2Li(THF)2 (3 = 3Lu, 3Y, 3Dy) (L′ = 8-(2,6-iPr2C6H3N)-2-[(Me3SiCH2)CH-N(2,6-iPr2C6H3)]-C9H5N). In catalysis, complexes 1 exhibit high activity and excellent 1,4-cis selectivity in the polymerization of conjugated dienes such as isoprene (IP), 1,3-butadiene (BD), and myrcene (MY), whereas complexes 2 and 3 show substantially lower activity or are inactive under the conditions examined.