Selective transesterification mediated by lanthanum complexes in the copolymerisation of lactide and δ-valerolactone

Poly(lactide-co-valerolactone) copolymers were prepared via the one-pot copolymerisation of rac-lactide or S,S-lactide with δ-valerolactone at ambient temperature, mediated by bis(dimethylsilyl)amido lanthanum complexes supported by ligands derived from a salan framework (salan = N,N'-bis(o-hydroxy, m-di-tert-butylbenzyl)-1,2-diaminoethane), which incorporate either benzyl or 2-pyridyl groups at the tertiary amine moieties. Poly(δ-valerolactone)s were also prepared by the ring-opening polymerisation of δ-valerolactone and high molecular weight polymers (up to 83.6 kg mol^-1) with narrow dispersities were obtained. The poly(lactide-co-valerolactone) copolymers were fully characterized using ¹H and ¹³C NMR spectroscopy, gel-permeation chromatography and differential scanning calorimetry. Both the reaction solvent (toluene or THF) and the number of 2-pyridyl groups the complex possesses affect the complex activity and copolymer microstructure. The use of a non-coordinating solvent and presence of at least one 2-pyridyl group is required for high conversion of both monomers. Variation of the monomer feed ratio allowed copolymers across the full compositional range to be prepared. The copolymers are formed via a transesterification mechanism whereby all of the lactide undergoes rapid polymerisation in the early stages of the reaction and the δ-valerolactone is subsequently incorporated into the polymer. The rate and extent of δ-valerolactone polymerisation increases with the number of 2-pyridyl groups in the catalyst in toluene and is more rapid in non-coordinating solvent (toluene) than coordinating solvent (THF). Only low levels of the T_II mode of transesterification occur, with the T_I transesterification mode dominating, leading to the formation of copolymers with intact lactidyl units.