Branched and crosslinked resorbable polymers based on lactic acid, lactide and ε-caprolactone

Abstract

Branched and crosslinked degradable polyesters based on lactic acid, lactide and ε-caprolactone were prepared by utilizing different polymerization methods. Chain linking of hydroxyl telechelic lactic acid oligomers with 1,6-hexamethylene diisocyanate (HMDI) as a chain extender, yielded lactic acid based poly(ester-urethanes). When an excess of HMDI was used, polymers with broader molecular weight distribution and long-chain branching were obtained. Long-chain branching was confirmed by NMR and was seen in high shear thinning and melt strength in rheological measurements. Ring-opening polymerization of D,L-lactide, L-lactide and ε-caprolactone in the presence of stannous octoate and a co-initiator was employed as a method to prepare polyesters with controlled molecular structure. The initiation activity of different hydroxyl containing compounds was evaluated and new co-initiator compounds, polyglycerines, were shown to yield polylactides with up to 8 arms. Furthermore, by increasing the number of hydroxyls in the co-initiator, the polymerization rate increased and polylactides with higher molecular weight were obtained.

Crosslinked polymers with controlled molecular structure were prepared from end functionalized linear and branched polyester precursors. The effect of molecular architecture, i.e. branching, molecular weight and different monomer units in the precursors, on the curing and properties of the final networks was studied. Triethoxysilane and methacrylic double bond functionalization and polymerization of these moieties were utilized in the preparation of hydrolytically degradable networks with different properties. High strength was obtained with DLLA based networks, whereas the use of flexible CL/DLLA copolymers as network precursors yielded highly elastic crosslinked polymers with creep resistance.

In addition to the preparation and characterization, the degradability of the crosslinked polymers was evaluated. The polyester networks exhibited degradation similar to the corresponding thermoplastic polyesters. In general, the degradation occurred through bulk degradation where the mechanical properties of the polyesters deteriorated first and the mass loss followed. The degradation of the networks was enhanced by introducing labile anhydride bonds as weak linkages into the polyester network structure. This approach led to very rapidly degrading, crosslinked poly(ester-anhydrides) based on PCL, PDLLA, and PLLA. When a short PCL based precursor was used, the network showed characteristics of surface-erosion: the poly(ester-anhydride) network lost its mass and the dimensions of the specimen decreased at a steady rate over two days.