SYNTHESIS OF HYPERBRANCHEND POLYLACTIDES BY SELF CONDENSING RING-OPENING BRANCHING POLYMERIZATION

. Since polymer degradation preferentially occurs in the amorphous region of PLA, an increase in the number of molecular branches (chain ends) for branched PLAs enhanced the enzymatic degradability and the alkaline hydrolyzability 9 . This allows a control of the degradation profile via the degree of branching. Therefore, our approach focuses on the introduction of branching into the PLA backbone. In recent works, we described enzymatic and metal-catalyzed copolymerizations, based on the combination of ROP and polycondensation of AB2 monomers (bishydroxy acids) 10 . In order to avoid some problematic issues related to the polycondensation like character of the reaction with increasing bishydroxy acid content, we were looking for an appropriate substitute for this class of AB2 comonomer. Several examples describing the synthesis of branched polyesters prepared from functionalised lactones can be found in the literature. e-Caprolactone derivates with ABB’ and AB2B’ like character, bearing additional hydroxyl groups attached to the lactone ring, have been homopolymerized to form hyperbranched polyesters in the late 1990ies 11,12 . Incorporation of this kind of cyclic monomer as a branching unit into the structure of conventional linear polyesters has only rarely been described. Ouchi et. al. applied this concept, utilizing mevalonolactone as a branching inimer in a one-pot copolymerization with Sn(Oct)2 as catalyst 8 . Although branching has been shown to occur, control over the extent of branching was rather difficult to accomplish. Knaus et. al. introduced branching points in via copolymerization of lactide with glycidol 13 . We are interested in control of the materials properties of poly(L-lactide) via the degree of branching. Hence we were looking for a reactive cyclic lactone with a pendant hydroxyl group, which would be polymerizable under conditions suitable for the lactide monomer. Rokicki et. al. have recently discovered the potential of the cyclic dimer of glycerol and glycolic acid, 5-hydroxymethyl-1,4-dioxane-2-one (5-HDON) in the synthesis of a hyperbranched, strictly alternating copolyester-copolyether 14 . High molecular weights were obtained, employing similar conditions as used for the bulk polymerization of lactide. Here we describe the combined AB/ABB’ ROP/ROMP copolymerization of L-lactide and 5-HDON in bulk with Sn(Oct)2 as catalyst. Since the ROP of lactide is known to be initiated by free hydroxyl groups by this catalyst system 15 , branching units are expected to result from a combination of ring opening of the lactone and the additional chain growth from its pendant hydroxyl group (B’). Further repeat units of the obtained poly(lactide) copolymer consist of glycerol and glycolic acid, rendering the material potentially biodegradable and biocompatible.