Silk as a biomaterial

23 Stereocontrol of Lactide ROP 6164 231 Isotactic Polylactides 6164 232 Syndiotactic Polylactides 6166 233 Heterotactic Polylactides 6166 3 Anionic Polymerization 6166 4 Nucleophilic Polymerization 6168 41 Mechanistic Considerations 6168 42 Catalysts 6169 421 Enzymes 6169 422 Organocatalysts 6169 43 Stereocontrol of Lactide ROP 6170 44 Depolymerization 6170 5 Cationic Polymerization 6170 6 Conclusion and Perspectives 6171 7 Acknowledgments 6173 8 References and Notes 6173

Controlled ring-opening polymerization of lactide and glycolide.

Poly(lactic acid) modifications

Recent Developments in Ring Opening Polymerization of Lactones for Biomedical Applications

Biodegradable polymers have significant potential in biotechnology and bioengineering. However, for some applications, they are limited by their inferior mechanical properties and unsatisfactory compatibility with cells and tissues. A strong, biodegradable, and biocompatible elastomer could be useful for fields such as tissue engineering, drug delivery, and in vivo sensing. We designed, synthesized, and characterized a tough biodegradable elastomer from biocompatible monomers. This elastomer forms a covalently crosslinked, three-dimensional network of random coils with hydroxyl groups attached to its backbone. Both crosslinking and the hydrogen-bonding interactions between the hydroxyl groups likely contribute to the unique properties of the elastomer. In vitro and in vivo studies show that the polymer has good biocompatibility. Polymer implants under animal skin are absorbed completely within 60 days with restoration of the implantation sites to their normal architecture.

A tough biodegradable elastomer

Synthesis of polylactides in the presence of co-initiators with different numbers of hydroxyl groups

Cross-linked e-caprolactone (CL) and D,L-lactide (DLLA) copolymers with elastic properties were synthesized in three steps. First, the monomers were copolymerized in ring-opening polymerization to obtain telechelic star-shaped oligomers with almost completely random monomer distribution. The oligomers were methacrylated with methacrylic anhydride in the second step and cured in a third. Molar CL/DLLA compositions of 30/70, 50/50, 70/30, 90/10, and 100/0 were used to obtain elastic structures with a wide range of properties. The effect of the average length of the copolymer block on the properties of the networks was evaluated with three different co-initiator contents (0.5, 1.0, and 2.0/100) in the oligomer synthesis. The oligomers were characterized by 13 C NMR spectroscopy, size-exclusion chromatography (SEC), and differential-scanning calorimetry (DSC). The formation of elastic networks was confirmed by the absence of a flow region in dynamic mechanical analysis (DMA), the increase in T g in DSC, and the full recovery of the sample dimensions after tensile testing. In addition, gel contents were high and the samples swelled in CH 2 Cl 2 . The networks possessed break stresses from 0.7-9.7 MPa with elongations from 80-350%. Networks with 100 or 90% of e-caprolactone retained their form in vitro for 12 weeks, but an increase in lactide content made the networks more vulerable to hydrolysis.

Cross-linked poly(ε-caprolactone/D, L-lactide) copolymers with elastic properties

Chain extending of lactic acid oligomers. Effect of 2,2′-bis(2-oxazoline) on 1,6-hexamethylene diisocyanate linking reaction

Biodegradable crosslinked polymers based on triethoxysilane terminated polylactide oligomers

The effect of molecular architecture on the properties of crosslinked polylactides was studied. D,L-Lactide-based telechelic oligomers with different numbers of arms were functionalized with methacrylic anhydride. Functionalized oligomers were crosslinked with thermal initiation at 90°C, and the mechanical and thermal properties, as well as the gel content, were evaluated. The crosslinking density increased with the number of arms in the oligomers, and the compressive yield strength was built up at the same time. The best mechanical properties (compressive yield strength = 120 MPa, modulus = 2800 MPa, and strain = 5.0%) were obtained with cured polymers prepared with a 100 : 8 ratio of lactide to pentaerythritol. When dimethacrylated butanediol was used as a reactive monomer in curing, polymers with high crosslinking density and mechanical strength were obtained at 37, 60, and 90°C. The use of the reactive monomer also enabled high conversions with photoinitiated crosslinking. Finally, hydrolytic degradation and strength retention were demonstrated. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3616–3624, 2002

Antti Helminen

Papers

Synthesis of polylactides in the presence of co-initiators with different numbers of hydroxyl groups

Cross-linked poly(ε-caprolactone/D, L-lactide) copolymers with elastic properties

Chain extending of lactic acid oligomers. Effect of 2,2′-bis(2-oxazoline) on 1,6-hexamethylene diisocyanate linking reaction

Biodegradable crosslinked polymers based on triethoxysilane terminated polylactide oligomers

Structure modification and crosslinking of methacrylated polylactide oligomers