When it comes to making an environmentally friendly polymer, chemists tend to think that starting with a renewably sourced monomer is all it takes. But according to Eugene Y.-X. Chen of Colorado State University, a biobased monomer is only one of three criteria needed to build such a material: “A green synthesis and product recyclability are important too.”
To emphasize that point, Chen and postdoctoral researcher Miao Hong have developed a new synthetic process to create what they believe is a “truly sustainable polymer.” They polymerize a biobased monomer using a metal-free organocatalyst to make a fully recyclable material. Chen and Hong described their approach in a pair of presentations during a Division of Polymer Chemistry session Sunday at the American Chemical Society national meeting in San Diego.
The Colorado State researchers started out by designing the first practical ring-opening polymerization of γ-butyrolactone, a sugar-derived compound that is used as a solvent and building block in fine and specialty chemical applications. Although ring-opening polymerization of cyclic lactones is a common way to make biodegradable polyesters, γ-butyrolactone is an exception. Chemists had labeled the compound “nonpolymerizable,” Chen noted, because the ring is too stable except under extreme pressure and moderate temperature.
Nevertheless, after probing an array of thermodynamic, kinetic, and processing conditions, Chen and Hong discovered lanthanum and yttrium catalysts that polymerize γ-butyrolactone via a coordination-insertion mechanism. The team figured out how to control the reaction equilibrium and vary the amount of catalyst to produce high-molecular-weight linear and cyclic polyesters. They also discovered that heating the bulk materials above 200 °C completely recycles the polymers back to the γ-butyrolactone monomer (Nat. Chem. 2015, DOI: 10.1038/nchem.2391).
But that still wasn’t good enough, Chen said. The team took the polymerization to the next level by discovering an organocatalyst to sidestep toxicity and availability issues associated with metal catalysts. The researchers again probed an array of reaction conditions to zero in on a polyaminophosphazine catalyst with alcohol initiators. The phosphazine is a superstrong base capable of deprotonating the γ-butyrolactone ring to generate the active species that propagates the polymerization (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201601092).
“This is a real tour-de-force study that combines mechanistic understanding, sustainability, and practical importance,” commented Cornell University’s Geoffrey W. Coates, whose group develops catalysts for preparing biodegradable polymers. “These new phosphazene initiators, coupled with polymer recycling, constitute an exciting advance in the development of new biodegradable polyesters.”
“Chen and Hong convincingly demonstrate that what was thought to be an ‘impossible’ hurdle can be overcome by a smart selection of appropriate catalysts and reaction conditions,” added Jean-François Carpentier, a polymerization catalysis expert at the University of Rennes 1. Although the catalytic rate and other improvements are needed to make the process cost effective, and the health and safety of the organocatalyst must be considered, the achievement “will surely launch new interest in this field,” Carpentier noted. “The ability to start with biomass-derived cyclic lactones creates a nice playground for future investigations and developments.”
Other Related Stories: