Bulk Polymerization of l ‐Lactide with Dibenzoylmethane Sn( II ) Compounds

The bulk polymerization behavior of l ‐lactide monomer using dibenzoylmethane ( DBM )‐based Sn( II ) compounds ( DBM )Sn( OBu ) ( 2 ) and ( DBM ) 2 Sn ( 3 ) is investigated. Compounds 2 and 3 were prepared from the reaction of Sn( OBu ) 2 ( 1 ) with 1 or 2 equiv of DBM , respectively. X‐ray diffraction studies revealed that, while 2 exists as a dimeric form linked by 1‐butoxide bridges, 3 is monomeric in the solid state. The Sn( II ) centers in both compounds adopt distorted square‐pyramidal geometry. Ring‐opening polymerization of l ‐lactide with 1–3 efficiently produced poly( l ‐lactide)s with high molecular weight and narrow polydispersities ( PDI  < 2). The catalytic activity was found to be in the order 1  >  2  >  3 , indicating that the DBM ligand causes a sluggish coordinative insertion of the lactide monomer into the Sn( II ) center. In particular, the addition of dodecanol ( DDO , 0.1 mol%) substantially improved the catalytic activity for all compounds. The comparative polymerization using Sn(Oct) 2 (Oct = 2‐ethylhexanoate) in the absence and presence of DDO further showed that 1 is higher in activity than Sn(Oct) 2 and that Sn( II ) alkoxides are the actual active species in the polymerization of l ‐lactide.