Mechanistic Study of the Stereoselective Polymerization of d,l-Lactide Using Indium(III) Halides

We report the results of a comprehensive investigation of the recently discovered stereoselective and controlled polymerization of racemic lactide (d,l-LA) using an initiator prepared in situ from indium(III) chloride (InCl3), benzyl alcohol (BnOH), and triethylamine (NEt3). Linear relationships between number-average molecular weight (Mn) and both monomer to alcohol concentration ratio and monomer conversion are consistent with a well-controlled polymerization. Studies on polymerization kinetics show the process to be first-order in [InCl3]0 and zero-order in both [BnOH]0 and [NEt3]0. The rate of d,l-LA conversion is also dependent on the indium(III) halide (i.e., t1/2(InCl3) ≈ 43 min versus t1/2(InBr3) ≈ 7.5 h, 21 °C, CD2Cl2, [d,l-LA]0/[BnOH]0 ≈ 100, [d,l-LA]0 = 0.84 M, [InX3]0/[BnOH]0 = 1) and lactide stereoisomer (i.e., kobsd,l-LA ≈ kobsmeso-LA > kobsl-LA). A model system that polymerizes d,l-LA with the same high degree of stereoselectivity was developed using 3-diethylamino-1-propanol (deapH) in lieu of BnOH and NEt3. The product of the reaction of deapH with InCl3 was identified as [InCl3(deapH)(H2O)]2 by elemental analysis, X-ray crystallography, and NMR and FTIR spectroscopies. An anhydrous version of the complex was also isolated when care was taken to avoid adventitious water, and was shown by pulsed gradient spin−echo (PGSE) NMR experiments to adopt a dinuclear structure in CD2Cl2 solution under conditions identical to those used in its stereoselective polymerization of d,l-LA. The combined data suggest that the initiating species for the InCl3/BnOH/NEt3 system is similar to [InCl3(deapH)(H2O)]2 and of the type [InCl(3−n)(OBn)n]m. With this information we propose a mechanism that rationalizes the observed stereocontrol in d,l-LA polymerizations. Finally, in an exploration of the scope of the InCl3/BnOH/NEt3 system, we found this system to be effective for the polymerization of other cyclic esters, including ε-caprolactone and several substituted derivatives.