Indium catalysts improve biopolymer synthesis
The chirality or “handedness” of its monomer strongly influences the properties of poly(lactic acid) (PLA), one of the world’s most popular renewable polymers. Both the random (atactic, top) and alternating (heterotactic, middle) patterns result in polymers that are amorphous, with relatively low melting points. In contrast, an isotactic stereoblock (bottom) polymer has a higher melting point. A new class of indium catalysts developed at the University of British Columbia could allow for faster synthesis of stereoblock PLA that is more tolerant to impurities like water.
By Tyler Irving
Posted September 2012
For years, starch-derived, biodegradable poly(lactic acid) has been a popular bioplastic, but its market penetration has been limited by undesirable mechanical properties and low heat tolerance. A series of indium catalysts developed at the University of British Columbia could provide a solution.
Poly(lactic acid) (PLA) is made by ring-opening polymerization of lactide, which itself is a condensed dimer of lactic acid. Because lactic acid is chiral, there are several forms of PLA. Both the left-handed L form (PLLA) and the right-handed D form (PDLA) have relatively low melting points, as do random (atactic) mixtures or alternating (heterotactic) mixtures of the two. However, if the polymer is made as a stereoblock - a chunk of PLLA followed by a chunk of PDLA - its heat tolerance increases significantly. UBC chemist Parisa Mehrkhodavandi has been studying the chiral catalysts needed to make stereoblock PLA. While certain tin and aluminum complexes have been shown to selectively form PLLA over PDLA, they have their drawbacks. “Lactide derived from biological sources will always have some water in it, but most known catalysts are decomposed by water,” says Mehrkhodavandi. They can also take days to react, and can be thrown off by any functional groups that might be added to the monomers to improve their properties. In contrast, Mehrkhodavandi’s group has developed unique catalysts based on indium. Not only are they more tolerant of water and functional groups, they are also much more active. “The aluminum system takes 12 days to do what we can do with indium in 30 minutes,” says Mehrkhodavandi. The key to this reactivity, as confirmed in a recent paper published in the Journal of the American Chemical Society, is that the indium complexes have two metal centres as opposed to one.
Despite these advantages, there is still work to be done; for example, the enantioselectivity is still not quite as high as with the slower-acting aluminum complexes. Nevertheless, the technology has been licensed by the commercialization organization GreenCentre Canada, which is working with Mehrkhodavandi and unnamed partners toward industrial application.
Photo Credit: Parisa Mehrkhodavandi
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