Towards greener synthesis of sustainable terpene-based monomers for renewable polymers

Project background (identification of the problem and its importance and relevance to sustainability)

Demand for bio-sourced and ideally bio-degradable plastics and polymers is increasing as the governments and the society are making a continuous shift towards a more sustainable future. Broadly, to produce such materials, the four crucial aspects and their environmental impact need to be considered: (i) the source of the feedstock, (ii) the synthetic route towards building blocks, (iii) the conditions for polymer manufacture, and (iv) the usability and performance of newly created materials.

The ideal feedstock for building blocks of polymeric materials should be of natural origin (plants), inexpensive and non-food based to avoid the competition with food market. The synthetic route to monomers benefits of being environmentally benign, with mild or recoverable solvents and reagents, and maximized yield and atom economy. Finally, the green polymers obtained should allow for comparable performance and properties with traditional, synthetic polymers, with the simultaneous advantage of being bio-compostable or recyclable/degradable using currently industrially implemented technologies.

Terpenes and terpenoids have been recognized as promising natural starting materials for monomer synthesis due to their low cost, by-product nature and intrinsic functionality. Herein, we present the application of simple, green oxidative cleavage chemistry to four selected cyclic, olefinic monoterpenes to synthesise a range of biomass-based monomeric units compatible with the production of sustainable polyamides, polyesters, polycarbonates, and polyurethanes.

Proposed solution and methodology

In this study, our primary aim is to synthesise a range of novel, sustainable monomers from four selected mono and bicyclic olefinic terpenes: a-pinene, 3-carene, a-terpineol and terpinene-4-ol. Once the functionalized monomeric terpenes are obtained, various polymeric materials with tuneable chemical and physical properties might be synthesised. Phase 1 of proposed project aims to convert the chosen alkene terpenoids into 1,6-diols, via oxidative cleavage of oxidized synthons, which have the potential to directly react with natural diacids or the dimethyl esters of natural diacids and produce various polyesters via polycondensation polymerization.

Another range of polyesters is going to be accessed via direct oxidative cleavage of olefinic terpenes into difunctional monomeric units (hydroxy acids). Polyamides are to be acquired by transforming 1,6-dicarbonyl intermediates, via reductive amination, into diamines, and subsequent polymerization with a diverse range of natural dicarboxylic acids.

Phase 2 of proposed project aims to utilize the synthesised diols and diamines to obtain polyesters with amine moiety incorporated in the structure, and polyurethanes. Polyurethanes can be acquired by reacting terpene-based 1,6-diamines with carbonyldiimidazole, and subsequent reaction of acquired dicarbamides with terpenoic 1,6-diols. Polyesters with embedded amine moiety are of particular interest due to possessing two functional groups capable of breaking down by bacteria in the environment.

Terpenoid-based acrylate esters can be acquired by esterification of diols with acrylic acid. The subsequent addition polymerization of aliphatic acrylates with diamines affords the plethora of potentially biodegradable esters.