Self-Activated Coordination Polymerization of Alkoxystyrenes by a Yttrium Precursor: Stereocontrol and Mechanism

Coordination-insertion polymerization of polar vinyl monomers has always been a challenging subject since the Lewis basic polar groups are usually poisonous to the Lewis acidic metal centers of the catalysts. In this paper, we report the coordination polymerization of the unmasked polar alkoxystyrenes (AOS), including o-methoxystyrene (oMOS), o-ethoxystyrene (oEOS), o-n-propoxystyrene (oPOS), o-n-butoxystyrene (oBuOS), o-benzyloxystyrene (oBnOS), and 5-phenyl-2-methoxystyrene (Ph-MOS), by using the cationic quinolyl anilido yttrium alkyl species that was inert to styrene polymerization. In addition, the styrenic monomer bearing two polar groups, 4-fluoro-2-methoxystyrene (FMOS), was also polymerized since the presence of the additional polar fluorine group did not block the interaction between the ortho-oxygen and yttrium center. In contrast, the o(tert-butyldimethyl)silyloxystyrene (oSiOS), where the ortho-oxygen atom was prevented from coordinating to the yttrium center by the sterically bulky -(SiMe2Bu)-Bu-t group, could not be polymerized. Thus, the o-alkoxy groups of AOS did not poison the rare-earth metal center but activated the polymerization through chelation to the active species. The stereoregularity of the resultant polymers could switch from syndiotactic to atactic by changing the ortho-oxygen substituent from methyl to benzyl, which was attributed to the weakening Y-sigma-0 coordination bond. The mechanism was elucidated by the DFT calculations.