Stereochemical control in lactide polymerization: from polymer stars to catalyst design

Poly(lactic acid) (PLA), a renewable, biodegradable, bioassimilable polymer derived from plant sources has become the focus of intensive research over the past 15 years. PLA technology has evolved greatly, with the development of metal catalysts that can control the insertion of lactide monomers, to prepare stereoregular PLAs. The emergence of more complicated brush, dendrimer and star-like polymer architectures provide novel platforms to which this stereocontrol has not been applied. By combining the concepts of stereocontrol and star polymer architecture valuable information regarding the structure property relationships of these PLAs can be learned.;Efforts to improve the stereocontrolled polymerization of lactide have been expanded to include the design and synthesis of novel aluminum anilido-aldimine catalysts incorporating diisopropylphenyl and cyclohexyl substituents on the anilido donors. These complexes have been screened in lactide polymerization to prepare PLA, and have been subjected to kinetic analyses.;We have prepared the first series of stereoregular PLA polymer stars utilizing known aluminum catalysts and dipentaerythritol, a multi-functional initiator. Stars bearing atactic, heterotactic, isotactic-rac and isotactic-L microstructures have been prepared. The materials have been characterized by gel permeation chromatography, 1H NMR spectroscopy, thermogravimetric analysis, differential scanning calorimetry and powder X-ray diffraction to elucidate the effect that stereocontrol has on the properties of these novel star polymers. The physical properties of these materials were found to be enhanced through the inclusion of stereoregularity. The isotactic samples are especially tunable by varying the percent tacticity. By changing the microstructure of the star PLAs, the glass transition temperatures (Tg) and melt temperatures (Tm) can be tuned by 10°C, while the thermal onset stability of the samples can be improved by 40°C through microstructure control. The dependence of observed properties on tacticity and molecular weight have been examined through the preparation of higher molecular weight isotactic stars, which exhibit 13°C and 25°C improvements in Tg and Tm respectively when accompanied by a 5 fold increase in molecular weight. These materials have also been subjected to degradation studies to assess the quality of these materials in sustained release for drug delivery applications. Microstructure control has afforded these samples huge improvements in solution stability, with a 3-fold increase in lifetime observed between atactic and isotactic-rac stars.