Making polycarbonate from sugar and CO₂

CHEMISTS at the University of Bath, UK, have developed a process to create polycarbonate plastic from sugars and CO₂, which is more sustainable and safer than conventional methods.

Polycarbonate plastics are used in a wide variety of products, including drinks bottles, spectacle lenses, and scratch-resistant coatings for mobile phones, CDs and DVDs. However, the conventional process to make them uses phosgene, a highly toxic gas, and bisphenol-A (BPA), a substance under investigation for its hormone-mimicking properties that is widely banned for use in baby bottles and food packaging. The new technique, developed by Antoine Buchard and his colleagues at Bath’s Centre for Sustainable Chemical Technologies (CSCT), requires neither of these chemicals, and uses low pressures and room temperature.

The researchers use a variety of sugars, including thymidine and deoxyribose, found in DNA, and mannose, found in various plants, algae and yeast, and in waste streams like coffee grounds.

In the first part of the process, the researchers create a derivative of the sugar with an alcohol function, by reacting it with acetone and methanol. They then synthesise a cyclic monomer from the sugar derivative and CO₂. Buchard explained that the alcohol function of the sugar derivative reacts with CO₂ via the action of a base called DBU (1,8-Diazabicyclo[5.4.0]undec-7-ene), at ambient temperature and pressure. Adding tosyl chloride then creates the cyclic monomer. It is this step that replaces the conventional step using phosgene.

The cyclic monomer can then be polymerised in one of two ways, either in solution at room temperature or by melting at relatively low temperatures – for example 140˚C for the mannose monomer and 70˚C for the deoxyribose monomer. The polymerisation process is catalysed either using a tin-based metal complex or an organocatalytic system with a strong base and an alcohol.

The sugar-based polycarbonate is strong, transparent and scratch-resistant, like conventional polycarbonate, but has the added advantage of being biodegradable in the environment. Enzymes such as lipase and cellulase from soil bacteria break the plastic back down into harmless sugars and CO₂. The different sugars produce polymers of different thermal and physical properties, such as glass transition temperatures and degradation rates. The different sugars can be modified with different chemical functions to change the properties of the final polymer. The polycarbonate made from thymidine is also biocompatible, and Buchard believes it could one day be suitable for use as tissue scaffolds.

The team has also investigated the use of glucose and xylose from lignocellulosic biomass. Most of the sugars tested by the team are available at fairly large scales, and though the market is currently small for many of them, Buchard told The Chemical Engineer that there is the potential for mass production.

“For deoxyribose and thymidine sugars, these are rarer sugars, produced by organisms, which would be in the first instance confined to niche markets. However, some biotechnologies exist and are being developed to mass produce them, if needed,” he added.

Buchard told The Chemical Engineer that the next step for the research will be improving and scaling up the process and fully investigating the properties of the polymers, including mechanical toughness, impact resistance, durability, degradability and recyclability. The researchers are also looking to engage with industries producing sugar-containing waste streams, and with plastics end users to develop polymers suitable for commercialisation.

“We have started to expand our range of sugar-based monomers to other types of polymers than polycarbonates (such as urethane, esters, olefins), aiming for thermoplastics but also elastomers, and we are also exploring study the copolymerisation of our monomers. The beauty of sugars is that they are a very diverse and versatile feedstock, so the possibilities are endless,” said Buchard.

Polymer Chemistry doi.org/b8z8

Polymer Chemistry doi.org/b8z9

Macromolecules doi.org/f88bxq