Metal-catalyzed stereoselective ring-opening polymerization of functional β-lactones: methylene-alkoxy-fluorinated polyhydroxyalkanoates unveil the role of non-covalent interactions

Abstract

Non-covalent interactions (NCIs) can play a major role in the stereoselective ring-opening polymerization (ROP) of racemic β-lactones mediated by achiral metal catalysts, towards the formation of the corresponding polyhydroxyalkanoates (PHAs) with a specific microstructure. Our longstanding endeavor to better understand the factors governing the stereoselective ROP of 4-substituted β-propiolactone monomers rac-BPL^FGs (resulting in PBPL^FGs; FG = (non)functional group), from catalyst systems based on diamino- or amino-alkoxy-bis(ortho,para-substituted)phenolate rare earth complexes associated with an exogenous alcohol as an initiator, indicated that the formation of iso-enriched polyesters requires the use of ortho-halogen-substituted phenolate ligands (halogen = F, Cl, and Br) along with a methylene alkoxide pending substituent (FG = –CH₂OCH₂R*) on the lactone. Any other ligand/β-lactone combination resulted in either syndiotactic or atactic PBPL^FGs. We report herein the controlled ROP of 4-methylene-alkoxy-fluorinated substituted β-propiolactone, rac-BPL^{CH₂OCF₂CHF₂}, catalyzed by diamino-bis(ortho,para-R,R-substituted)phenolate yttrium catalyst systems Y{ONNO^R2}/iPrOH (with R = Me, Cl, tBu, cumyl; namely 1a–d/iPrOH, respectively). This monomer does not feature any outer methylene hydrogen in the alkoxide moiety (–CH₂O₂CHF₂vs. –CH₂O₂R*), thus enabling the evaluation of the role of outer CH₂O(R*)₂⋯R NCIs in the stereocontrol. The tBu catalyst 1c showed the highest ROP activity (TOF up to 4650 h⁻¹) out of the four catalyst systems investigated. The fluoroalkyl PHAs were recovered with molar mass values up to M_n,NMR = 106 000 g mol⁻¹ and narrow dispersity (Đ_M = 1.02–1.24). Detailed NMR and mass spectrometry characterization studies supported the formation of α-isopropoxy,ω-hydroxy telechelic PBPL^{CH₂OCF₂CHF₂} chains. Catalysts 1c and 1d flanked with bulky phenolate substituents (R = tBu, cumyl) and the chloro-substituted one 1b all returned syndio-enriched PBPL^{CH₂OCF₂CHF₂}s with P_r up to 0.87, as assessed by ¹³C NMR spectroscopy. Only the methyl substituted catalyst 1a gave atactic PHAs. Eventually, these findings support the hypothesis that both inner and outer methylene hydrogens within an alkoxymethylene exocyclic group on the rac-β-lactone monomer (₂O₂R*) are required, along with an ortho-chloro-substituted yttrium bisphenolate catalyst (Y{ONNO^Cl₂}), to induce NCIs (Cl⋯₂-O-(R*)₂⋯Cl), ultimately resulting in unique isotactic synthetic PHAs.