Ring-opening polymerization of six-membered cyclic esters catalyzed by tetrahydroborate complexes of rare earth metals

Abstract Catalytic behavior of tetrahydroborate complexes of rare earth metals, Ln(BH 4 ) 3 (THF) x ( 1 : Ln = La, x  = 3; 2 : Ln = Pr, x  = 2; 3 : Ln = Nd, x  = 3; 4 : Ln = Sm, x  = 3; 5 : Ln = Y, x  = 2.5; 6 : Ln = Yb, x  = 3), for ring-opening polymerization (ROP) of six-membered cyclic esters, δ -valerolactone (VL) and d , l -lactide ( d , l -LA), was studied. The controlled polymerization of VL with 1 – 6 proceeded in THF at 60 °C. The catalytic activities of these complexes for the ROP of VL were observed to be in order of the ionic radii of the metals: 1 (La) ≥  2 (Pr) ≥  3 (Nd) >  4 (Sm) >  5 (Y) >  6 (Yb). The obtained polymers were demonstrated to be hydroxy-telechelic by 1 H NMR and MALDI-TOF MS spectroscopy. The controlled ROP of d , l -LA also proceeded by these complexes. The activities of these complexes for the d , l -LA ROP were also in order of the ionic radii of the metals.