Selective
Transesterification to Control Copolymer
Microstructure in the Ring-Opening Copolymerization of Lactide and
ε‑Caprolactone by Lanthanum Complexes
posted on 2023-12-21, 13:08authored byBette Beament, Daniel Britton, Thomas Malcomson, Geoffrey R. Akien, Nathan R. Halcovitch, Michael P. Coogan, Rachel H. Platel
A series of novel lanthanum amido complexes, supported
by ligands
designed around the salan framework (salan = N,N′-bis(o-hydroxy, m-di-tert-butylbenzyl)-1,2-diaminoethane)
were synthesized and fully characterized in the solid and solution
states. The ligands incorporate benzyl or 2-pyridyl substituents at
each tertiary amine center. The complexes were investigated as catalysts
in the ring-opening homopolymerization of lactide (LA) and ε-caprolactone
(ε-CL) and copolymerization of equimolar amounts of LA and ε-CL
at ambient temperature. Solvent (THF or toluene) and the number of
2-pyridyl groups in the complex were found to influence the reactivity
of the catalysts in copolymerization reactions. In all cases, complete
conversion of LA to PLA was observed. The use of THF, a coordinating
solvent, suppressed ε-CL polymerization, while the presence
of one or more 2-pyridyl groups promoted ε-CL polymerization.
Each copolymer gave a monomodal trace in gel permeation chromatography–size-exclusion
chromatography (GPC-SEC) experiments, indicative of copolymer formation
over homopolymerization. Copolymer microstructure was found to be
dependent on catalyst structure and reaction solvent, ranging from
blocky to close to alternating. Experiments revealed rapid conversion
of LA in the initial stages of the reaction, followed by incorporation
of ε-CL into the copolymer by either transesterification or
propagation reactions. Significantly, the mode of transesterification
(TI or TII) that occurs is determined by the structure of the metal complex
and the reaction solvent, leading to the possibility of controlling
copolymer microstructure through catalyst design.