Molecular Structure – Biodegradation Relationships of Long-Chain Polyesters

Abstract

Long-chain aliphatic polyesters are known to have properties akin to industrially relevant materials like HDPE or LDPE. Their ester bonds enable chain cleavage at mild conditions to facilitate, e.g., recycling, whereas their hydrocarbon chains dominate the material properties and result in a hydrophobic material. Polyesters derived from linear monomers exhibit a high crystallinity and melting point. So far, linear, long-chain aliphatic polyesters are mostly known to be exceptionally stable under aqueous conditions and their (bio-)degradation is a little studied subject. Nonetheless, there is a growing interest in the development of biodegradable polyethylene-like materials, particularly considering the widespread use of PE in applications such as mulch films , which often remain in the environment post-use. Even if polymers are not intentionally introduced into the environment, mismanagement of waste often leads to release of plastics into, e.g., marine ecosystems. A low density of ester bonds as predetermined breaking points in these polymers could facilitate hydrolytic degradation and thus provide a universal backstop to plastic accumulation in the environment. In the first part of this work, an in-depth examination and quantification of the parameters influencing the degradation or deconstruction of long-chain aliphatic polyesters is carried out (chapter 3). Therefore, a sensitive method for detecting even small degradation rates, specially tailored for long-chain polyesters, needs to be established beyond methods used so far like SEC or mass loss measurements. In a first step, the conditions required to degrade the well-established PE-like polyester PE 18,18 is explored, which has been known for its exceptional stability. Subsequently, the influence of a change in the microstructure of long-chain polyesters on their degradability is investigated. Especially the differences in degradability of long-long and short-long polyesters are determined and the influence of their exact monomer composition. In a last step, the crystallinity of PE-like polyesters is tailored by copolymerization of a linear long-chain diacid and diol with a branched, long-chain monomer derived from dimer fatty acids. The influence of the reduced crystallinity on the degradability is studied. All investigated polyesters should retain a PE-like crystal structure and hydrophobic character dominated by aliphatic chains to ensure comparability between the materials and enable the isolation of individual parameters’ contribution. Base and solutions of naturally occurring enzymes are used as hydrolysis media. These experiments should provide guidelines for the design of circular materials with regard to their (enzymatic) recycling as well as their hydrolytic stability or non-persistency. These guidelines are applied in the following chapter 4 for the development of rubbers on polyester basis. While rubbers offer a unique combination of stability and flexibility, their hydrocarbon nature and chemical crosslinking impedes mechanical and chemical recycling and significantly retards biodegradation. Fully amorphous, unsaturated polyesters consisting of saturated and unsaturated monomers are prepared and their sulfur vulcanization to elastic rubbers is investigated. The ester bonds are utilized for solvolysis reactions to enable deconstruction of these rubbers and the chemical recycling of the saturated monomers. Further, the hydrolytic stability of these materials and their biodegradability is analyzed using 13C labelled monomers. In the last part of this work (chapter 5), long-chain aliphatic polyanhydrides are introduced into a PE-like material to investigate its properties and probe for their potential to accelerate its’ hydrolytic degradation. Polyanhydrides are accessible via a simple one-pot polycondensation and are characterized by a labile anhydride bond already hydrolyzing with atmospheric moisture to the respective diacids. Acids released in a polyester matrix upon exposure to a trigger like water can even enable the hydrolysis of otherwise inert polyesters. Blends of polyesters and polyanhydrides as well as polyester-anhydride copolymers are characterized and their hydrolytic degradation is determined under environmentally relevant conditions.