Taking advantage of a chiral cobalt catalyst, Geoffrey W. Coates and coworkers at Cornell University have 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).
The team’s new polymer forms a semicrystalline stereocomplex, meaning it is a material made from combining right- and left-handed polymer chains. In addition to starting from commodity and biobased monomers and being inherently biodegradable, the polymer has an ability to form a stereocomplex, providing it with a melting point comparable to that of low-density polyethylene. These properties could one day make the new polymer competitive to polyethylene and isotactic polypropylene, the two most widely produced polymers in the world.
“Stereocomplexes in general spark the interest of chemists who enjoy structure,” says Kenneth B. Wagener, senior member of the George & Josephine Butler Polymer Research Laboratory at the University of Florida. “This new work is typical of the Coates group—making something new of chemistry well-known, epoxides and anhydrides in this case.”
Coates and graduate students Julie M. Longo and Angela M. DiCiccio first designed a chiral cobalt catalyst featuring N,N′-bis(salicylidene)cyclohexanediimine as a ligand. Using either the (R,R) or (S,S) version of the catalyst, the Cornell researchers copolymerized (R)- or (S)-propylene oxide with succinic anhydride to produce (R)- or (S)-poly(propylene succinate).
When right- and left-handed polymers are combined, they typically mix to form a random amorphous material or form a semicrystalline material with segregated right- and left-handed regions. The two polymer chains could also crystallize together in ways they can’t do alone, such as forming paired helices or interdigitated sheets to make a stereocomplex. This structural feature gives polymer chemists better control over the thermal properties and biodegradability of polymers. Stereocomplexes are exceedingly rare, however, with only about a dozen examples known.
When the Cornell team mixed right- and left-handed poly(propylene succinate), they found that the chains snuggled together to form a stereocomplex, the first known example for a an epoxide-anhydride copolymer. The stereocomplex has a melting point of about 120 °C, which is 40 °C higher than either of the polymers individually.
With the wide range of epoxides and cyclic anhydrides available, chemists should be able to create a broad class of new polymers, Coates notes. Currently, his team uses enantiopure propylene oxides, which are pretty expensive. “We are actively looking for a catalyst that will make the stereocomplex from racemic propylene oxide, which is considerably cheaper,” he says. Potential uses for poly(propylene succinate) include biomedical applications and large-scale packaging applications where biodegradability is needed. Cornell has patented the technology but has not yet licensed it for commercial development.
“This development bears the stamp of thorough expertise in homogeneous polymerization catalysis,” says Eric P. Wasserman, a senior research scientist at Dow Chemical. “It is relevant to the world of industrial polymers because it addresses issues with biodegradation, renewable raw materials, and the demands placed on modern plastics. In this case, that is the ability to crystallize quickly from the melt and have a melting point above 100 °C. In principle, this discovery could be a keystone of a new line of thermoplastic polymers.”