Ring-Opening Polymerization of Cyclic Esters Promoted by Phosphido-Diphosphine Pincer Group 3 Complexes

Phosphido-diphosphine Group 3 metal complexes 1–4 [(o-C6H4PR2)2P-M(CH2SiMe3)2; R ¼ Ph, 1: M ¼ Y, 2: M ¼ Sc; R ¼ iPr, 3: M ¼ Y, 4: M ¼ Sc] are very efficient catalysts for the ring-opening polymerization (ROP) of cyclic esters such as e-caprolactone (e-CL), L-lactide, and d-valerolactone under mild polymerization conditions. In the ROP of e-CL, complexes 1–4 promote quantitative conversion of high amount of monomer (up to 3000 equiv) with very high turnover frequencies (TOF) (4 104 molCL/molI h) showing a catalytic activity among the highest reported in the literature. The immortal and living ROP of e-CL and L-lactide is feasible by combining complexes 1–4 with 5 equiv of 2-propanol. Polymers with controlled molecular parameters (Mn, end groups) and low polydispersities (Mw/Mn ¼ 1.05–1.09) are formed as a result of fast alkoxide/alcohol exchange. In the ROP of d-valerolactone, complexes 1–4 showed the same activity observed for lactide (L- and D,L-lactide) producing high molecular weight polymers with narrow distribution of molar masses. Complexes 1–4 also promote the ROP of rac-b butyrolactone affording atactic low molecular weight poly(hydroxybutyrate) bearing unsaturated end groups probably generated by elimination reactions.