Ring-opening (co)polymerization of chiral seven-membered lactones mediated by achiral yttrium catalysts: insights into the catalyst stereocontrol by mass spectrometry

Abstract

Ring-opening polymerization (ROP) of cyclic esters is a preferred approach for the preparation of various polyesters with controlled microstructures. In this work, the ROP of chiral seven-membered substituted-ε-caprolactones, namely 1-methyl-ε-caprolactone (CL^Me) and 1-n-butyl-ε-caprolactone (CL^nBu, aka ε-decalactone), was investigated to assess the potential stereoregularity of the resulting polylactones. The reactions mediated by yttrium complexes Y{ON(N)O^R2} based on non-chiral diamino-bis(o,p-disubstituted-phenolate) ligands, associated with an exogeneous alcohol as a co-initiator, were effectively catalyzed – that is, with good control over molar mass values, narrow dispersity, and chain-end fidelity. However, the tacticity of the homopolymers obtained from racemic monomers rac-CL^Me or rac-CL^nBu could not be evidenced by NMR spectroscopy, as outlined in previous literature reports. Alternatively, the ring-opening copolymerization (ROCOP) of equimolar mixtures of (R)-CL^nBu/(S)-CL^Me enabled, indirectly, the assessment of the catalyst stereocontrol through the evaluation of the ultimate degree of alternation of the inserted units of each comonomer. While NMR spectroscopy again did not enable unambiguous evaluation of the copolymer topology/sequence, detailed MALDI-ToF and high-resolution ESI mass spectrometric analyses rewardingly revealed two major series of macromolecules, cyclic and linear ones. Both series of macromolecules showed randomly distributed units of both comonomers, thereby evidencing the absence of any significant stereocontrol from the yttrium catalyst over these large, seven-membered substituted ε-caprolactones. This latter lack of stereocontrol is assumed to arise from a too long range between adjacent chiral centers, preventing an effective chain-end stereocontrolled mechanism.