Stereoselective ring-opening polymerization of functional β-lactones: influence of the exocyclic side-group

Abstract

Polyhydroxyalkanoates (PHAs) are a unique class of polyesters especially due to their chemical diversity imparted by their side-chain substituent that provides a handle to tune their properties, such as their thermal, mechanical and (bio)degradability signature. Here, we report that some functional PHAs, namely PBPL^FGs (FG = functional group), were synthesized by the controlled stereoselective ring-opening polymerization (ROP) of racemic 4-substituted-β-propiolactones, namely rac-BPL^CH₂ZPhs with Z = O, S, CH₂OCH₂, catalyzed by diamino- or amino-alkoxy-bis(phenolate) yttrium amido complexes (1a–1g) in the presence of isopropanol. Unprecedented syndio-enriched (P_r up to 0.87) PBPL^CH₂ZPhs of high molar mass (M_n,NMR up to 86 400 g mol⁻¹, Đ_M < 1.23) were thus typically prepared under mild operating conditions (toluene, 20 °C). Catalyst systems with smaller “uncrowded” Me or Cl ortho-substituents installed on the yttrium phenolate ligands (1a–1c) typically revealed less active than those with larger bulkier tBu or CMe₂Ph groups (1d–1g), regardless of the monomer (TOF_1a–1c = 0.48–15 h⁻¹, TOF_1d–1g = 450–1840 h⁻¹). Irrespective of the catalyst system, exchanging oxygen with sulphur in BPL^CH₂ZPhs, with Z = O, S, did not affect the stereochemistry, always affording syndiotactic PBPL^CH₂ZPhs. On the other hand, either atactic or syndiotactic PBPL^{CH₂CH₂OCH₂Ph}s were formed upon tuning the catalyst from 1a–1c or 1d–1g, respectively. The intimate relationship, through “non-covalent” interactions, between the chemical nature of the exocyclic functional side-group on the β-lactone and the stereoelectronic tuning arising from the phenolate ligand ortho-substituents within the yttrium coordination sphere, modulated the stereocontrol of the ROP. The thermal behavior of these original functional PHAs depended closely on their side-chain substituent (T^onset_d = 226 to 272 °C; T_g = −15 to +40 °C).