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Green Thermoplastic Elastomers

Recent Research from the groups of Professors Marc Hillmyer and William Tolman.

The development of new polymeric materials from renewable resources is gaining considerable attention. These materials can act as environmentally friendly replacements for commodity plastics that contain petrochemical starting materials. The low cost and ready availability of starting materials from renewable resources can allow biorenewable polymers to be competitive with current commercial plastics. Polylactide (PLA) is one of the more attractive biodegradable polyesters because of the availability of lactic acid and the nontoxicity, hydrolyzability, and biocompatibility of the homopolymer. Unfortunately, under tension PLA fractures at very low strains (ca. 3%), and is therefore unsuitable for use in numerous applications where elasticity and ductility are essential.

With the aim of enhancing the utility of polylactide, graduate student Carolyn Wanamaker and Professors Marc Hillmyer and William Tolman have been exploring all biorenewable ABA triblock copolymers as tough alternatives to PLA. To do this, they incorporated polymenthide (PM), a new biorenewable polymer developed at Minnesota derived from menthol (Zhang, C.; Hillmyer, M. A.; Tolman, W. B. Biomacromolecules 2005, 6, 2091–2095), as the soft B block in an ABA triblock copolymer with PLA (A) end blocks. A series of these triblock copolymers were prepared with varying composition by the ring-opening polymerization of menthide using a difunctional initiator, diethylene glycol, giving the difuctional macroinitiator HO-PM-OH, followed by the ring-opening polymerization of lactide to yield the desired PLA-b-PM-b-PLA triblock copolymer (see Figure). The all biorenewable ABA triblock copolymers exhibited both good strength and elasticity. Furthermore, these materials demonstrated impressive elongations (nearly 10x the original length) that were comparable to commercial, fossil fuel derived, thermoplastic elastomers (TPEs). These materials also showed impressive recovery or “true elasticity” upon subjecting the sample to multiple cycles of loading and unloading at a strain of 300%. These results demonstrate that PLA-b-PM-b-PLA triblock copolymers are potentially suitable for numerous applications in the biomedical and pharmaceutical fields.

Their results will soon appear in the American Chemical Society journal Biomacromolecules.

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