Organocatalyzed ring-opening polymerization (ROP) of functional β-lactones: new insights into the ROP mechanism and poly(hydroxyalkanoate)s (PHAs) macromolecular structure

Abstract

The organocatalyzed ring-opening polymerization (ROP) of various 4-alkoxymethylene-β-propiolactones (BPL^ORs; R = CH₂CHCH₂ (All), CH₂Ph (Bn), (CH₂)₃CH₃ (ⁿBu), SiMe₂^tBu (TBDMS)) towards the formation of the corresponding poly(hydroxyalkanoate)s (PHAs; poly(BPL^OR)s (PBPL^ORs)) is investigated under mild operating conditions (neat, 60 °C), simply using basic organocatalysts of the guanidine (1,5,7-triazabicyclo[4.4.0]dec-5-ene, TBD), amidine (1,8-diazabicyclo[5.4.0]-undec-7-ene, DBU) or phosphazene (2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2diazaphosphorine, BEMP) type. The polymerization proceeds basically at the same rate as the alike organocatalyzed ROP of related β-lactones (especially the ubiquitous β-butyrolactone (BL) and alkyl β-malolactonates (MLA^Rs)), with BEMP being significantly more active than TBD and DBU. Insights into the polymerization mechanisms are gained through detailed 1D/2D NMR spectroscopy and MALDI-ToF mass spectrometry analyses of the resulting PBPL^ORs and in particular through the identification of the nature of the end-capping groups. Each of the three organobases promotes the polymerization in its own way, as dictated by either its basic, nucleophilic or dual behavior.