6040
Macromolecules 2007, 40, 6040-6047
Synthesis of Polymandelide: A Degradable Polylactide Derivative with Polystyrene-like Properties Tianqi Liu,† Tara L. Simmons,† David A. Bohnsack,‡ Michael E. Mackay,‡ Milton R. Smith, III,† and Gregory L. Baker*,† Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48824, and Department of Chemical Engineering and Materials Science, Michigan State UniVersity, East Lansing, Michigan 48824-1226 ReceiVed August 11, 2006; ReVised Manuscript ReceiVed June 12, 2007
ABSTRACT: Polymandelide, an aryl analogue of polylactide, was synthesized by the ring-opening polymerization of mandelide, the cyclic dimer of mandelic acid. The poor solubility of rac-mandelide limited the synthesis of polymandelide via solution polymerization, but polymerization of mandelide at 70 °C as a heterogeneous slurry in acetonitrile yielded first-order kinetic plots and polydispersities 150 °C exhibit properties that mimic those of polystyrene. Polymandelide is a glassy amorphous polymer with a Tg of 100 °C, with rheological properties comparable to polystyrene, and thermal gravimetric analyses under nitrogen show that the polymer is stable to ∼300 °C. Racemization during polymerization precluded formation of a crystalline polymer. Degradation of polymandelide in pH 7.4 buffer at 55 °C is consistent with a bulk erosion model and, due to its high Tg, proceeds at ∼1/100 the rate of polylactide under similar conditions.
Introduction From an environmental perspective, degradable polymers based on renewable resources are attractive alternatives to materials derived from petroleum. Poly(hydroxybutyrate)s and poly(lactide)1-4 are among the most widely studied degradable polymer systems, and polylactide is now commercialized as a commodity polymer for high volume applications such as fibers and packaging materials.5 While a broad range of physical properties is available from these two polymer families, a glassy, amorphous, degradable polymer with a high glass transition temperature (Tg) is unavailable. The backbone of polylactide is relatively flexible, resulting in Tgs ranging from 30 to 60 °C, depending on the degree of crystallinity and molecular weight of the polymer. Since adding bulky groups to the polymer backbone should increase rotational barriers along the polymer backbone and increase Tg, we initially synthesized poly(phenyllactide),6 an aromatic derivative of polylactide. The disappointingly low Tg of 50 °C for poly(phenyllactide) can be explained by the flexible methylene group that links the ring to the polymer backbone. Removing the methylene and substituting an aromatic ring for the methyl group of polylactide should, by analogy to the structure of polystyrene, result in a polymer with a significantly higher Tg. Thus, mandelide, the dimer of mandelic acid (2-hydroxyphenylacetic acid), is a particularly intriguing monomer for ring-opening polymerization. In this report, we describe the preparation of high molecular weight polymandelide, a polymer that shares many of the physical properties of polystyrene, but with the added feature of being degradable. Prior to our work, several groups explored the preparation of the mandelic acid homopolymer, but the schemes reported to date resulted in polymers with number-average molecular weights
