Ring Opening Polymerization of Lactides and Lactones by Multimetallic Titanium Complexes Derived from the Acids Ph2C(X)CO2H (X = OH, NH2)

The reactions of the titanium alkoxide [Ti(OR)4] (R = Me, nPr, iPr, tBu) with the acids 2,2’-
Ph2C(X)(CO2H), where X = OH and NH2, i.e., benzilic acid (2,2’-diphenylglycolic acid, L1H2), and
2,2’-diphenylglycine (L2H3), have been investigated. The variation of the reaction stoichiometry allows
for the isolation of mono-, bi-, tri or tetra-metallic products, the structures of which have been
determined by X-ray crystallography. The ability of the resulting complexes to act as catalysts for
the ring opening polymerization (ROP) of ε-caprolactone (ε-CL) and r-lactide (r-LA) has been investigated.
In the case of ε-CL, all catalysts except that derived from [Ti(OnPr)4] and L2H3, i.e., 7,
exhibited an induction period of between 60 and 285 min, with 7 exhibiting the best performance
(>99% conversion within 6 min). The PCL products are moderate- to high-molecular weight polymers.
For r-LA, systems 1, 3, 4 and 7 afforded conversions of ca. 90% or more, with 4 exhibiting the
fastest kinetics. The molecular weights for the PLA are somewhat higher than those of the PCL, with
both cyclic and linear PLA products (end groups of OR/OH) identified. Comparative studies versus
the [Ti(OR)4] starting materials were conducted, and although high conversions were achieved, the
control was poor.