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

The organocatalyzed ring-opening polymerization (ROP) of various 4-alkoxymethylene-beta-propiolactones (BPL(OR)s; R = CH2CH=H-2 (All), CH2Ph (Bn), (CH2)(3)CH3 (Bu-n), (SiMe2Bu)-Bu-t (TBDMS)) towards the formation of the corresponding poly(hydroxyalkanoate)s (PHAs; poly(BPLOR)s (PBPL(OR)s)) is investigated under mild operating conditions (neat, 60 degrees 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 beta-lactones (especially the ubiquitous beta-butyrolactone (BL) and alkyl beta-malolactonates (MLA(R)s)), 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(OR)s 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.