Inimer-Promoted Synthesis of Branched and Hyperbranched Polylactide Copolymers

Abstract

A series of (hyper)branched poly(l-lactide)(PLLA) copolymers has been prepared by ring-opening multibranching copolymerization of l-lactide with a hydroxyl-functional (ABB′) lactone inimer, 5HDON (5-hydroxymethyl-1,4-dioxane-2-on). Polymerization was conducted in bulk and solution and catalyzed either by stanneous-2-ethyl hexanoate (Sn(Oct)2) or an organic base, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD). Precise structural characterization of the resulting branched copolyester structures was accomplished by a combination of 2D NMR techniques, relying on the comparison with model compounds. The 5HDON inimer was employed in 1% to 20% fractions and is incorporated either as a dendritic unit or as a focal structure, but hardly in the linear mode. A detailed reaction mechanism was derived from kinetic investigation of the polymerization via NMR spectroscopy, preparative and analytical SEC and MALDI-TOF MS. The evolution and the extent of branching have been monitored and quantified. Both the degree of branching (DB = 2D(HDON)/2D(HDON) + L(lactide); DB = 0.02−0.22) and the molecular weight (MN = 1200−34000 g/mol) could be tailored by variation of the monomer/inimer ratio. For Sn(Oct)2 catalyzed polymerization approximately 50% of the inimer is transformed into dendritic units. In the case of TBD catalysis, the formation of dendritic units was suppressed at room temperature, resulting in linear poly(lactide) functionalized with a lactone end group. The fozcal 5HDON unit of the branched structures is susceptible to further functionalization, for example, by reaction with primary hydroxyl groups, leading to branched polylactide functionalized with precisely one single dye label at the focal moiety. The formation of previously absent linear repeat units from the addition of terminal lactide units to focal 5HDON units was observed when heating the polymers above Tg for prolonged times. This reaction was accompanied by a further increase in the molecular weight of the branched copolyesters.