Polymer Science: Process marries right- and left-handed polymer chains to create a stereocomplex material with commodity potential
Taking advantage of a chiral cobalt catalyst, chemists this year copolymerized propylene oxide enantiomers and succinic anhydride to form poly(propylene succinate), the first member of a new class of thermoplastics (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja509440g). Geoffrey W. Coates and coworkers at Cornell University developed the new polymer. It forms a semicrystalline stereocomplex, meaning it’s a material made from combining right- and left-handed polymer chains. Combining the two polymer chains allows them to crystallize together in ways they can’t do alone, giving polymer chemists better control over the thermal properties and biodegradability of polymers. Stereocomplexes are exceedingly rare, however, with only about a dozen examples known. Coates and graduate students Julie M. Longo and Angela M. DiCiccio first designed a chiral cobalt catalyst, and then using either the (R,R) or (S,S) version of the catalyst, they copolymerized (R)- or (S)-propylene oxide with succinic anhydride to produce (R)- or (S)-poly(propylene succinate). In addition to starting from commodity and biobased monomers and being inherently biodegradable, the polymer has an ability to form a stereocomplex, which gives it a melting point of about 120 °C—40 °C higher than either of the polymers individually and comparable with that of low-density polyethylene. Potential uses for poly(propylene succinate) include biomedical applications and large-scale packaging applications where biodegradability is needed. “This development bears the stamp of thorough expertise in homogeneous polymerization catalysis,” commented Eric P. Wasserman, a senior research scientist at Dow Chemical. With the polymer’s ability to crystallize quickly from the melt and having a melting point above 100 °C, “this discovery could be a keystone of a new line of thermoplastic polymers,” Wasserman noted.