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 FG s (FG = functional group), were synthesized by the controlled stereoselective ring-opening polymerization (ROP) of racemic 4-substituted-β-propiolactones, namely rac -BPL CH 2 ZPh s with Z = O, S, CH 2 OCH 2 , 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 CH 2 ZPh s of high molar mass ( M n,NMR up to 86 400 g mol −1 , 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 t Bu or CMe 2 Ph 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 CH 2 ZPh s, with Z = O, S, did not affect the stereochemistry, always affording syndiotactic PBPL CH 2 ZPh s. On the other hand, either atactic or syndiotactic PBPL CH 2 CH 2 OCH 2 Ph 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). 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.