Jack Van Schijndel

Oil, gas, and coal are hardly renewable on the scale of product lifetime, and excessive use of these fossil-based sources leads to global environmental concern. So it is of utmost importance to look for renewable sources as well as to recycling products towards their original building blocks, i.e., material circulation. Most important is the awareness that after product usage, waste can be seen as a resource. Selective depolymerization of redesigned new polymeric materials can be a significant driver towards the development of renewable and sustainable polymer materials. Chemical recycling, or even more precise, molecular recycling of materials reshape product life cycles. This thesis describes the design and synthesis of molecularly recyclable aromatic polyesters that demonstrate under certain conditions selective and reversible hydrolytic depolymerization.

The polyesters studied typically contain aliphatic ester groups because, for electronic reasons, derivatives of aliphatic carboxylic acids are more electrophilic than the analogous derivatives of aromatic carboxylic acids. Therefore, polyesters derived from aromatic carboxylic acids, e.g., polyethylene terephthalate (PET), are much less electrophilic and, therefore, far less sensitive to hydrolysis than aliphatic polyesters, e.g., polylactic acid (PLA). However, aromatic polyesters are of high technical relevance because of their excellent material properties. Hence, a combination of both is evident and, by taking nature as a guide, could be found in the building blocks of lignin.

The first part of the thesis describes how these building blocks (monomers) are synthesized in a more sustainable way than previously reported. A greener version of the Knoevenagel reaction, which is one of the most important methodologies for carbon-carbon double-bound formation in synthetic organic chemistry, has been developed. The direct use of piperidine, pyridine, or other harmful amines is avoided, and the use of a solvent can be limited or even excluded. The developed method with ammonium bicarbonate as a catalyst provides high yields of the products in this carbon-carbon formation reaction.

Understanding the fundamental steps of the reaction is necessary to gain more insight into the role of different reactants and to be able to control the reaction. Ammonia plays a role as a precursor for this Green Knoevenagel reaction. A closed catalytic cycle demonstrates how starting reactants and ammonia form “in situ” catalytic intermediates, that catalyze the carbon-carbon coupling. Besides, it is demonstrated that the in situ formed intermediates play a second catalytic role in the consecutive decarboxylation reaction resulting after hydrogenation in the building blocks, as mentioned above.

A procedure has been developed to synthesize various highly crystalline homopolyesters successfully (e.g., poly(phloretic acid), poly(dihydroferulic acid), and poly(dihydrosinapinic acid)) that could be depolymerized with minimal loss of material after separations. Because these PET analogs are composed of only one type of monomer, this cycle of polymerizing and depolymerizing can be applied several times, which is a significant advantage compared to PET. Because economic factors primarily motivate the recycling of plastics, molecular recycling has proven to be attractive, as long as the depolymerization conditions include catalysts that can be easily separated after use or subsequently used with the recovered building blocks.

Finally, it has been shown that a second decarboxylation of the formerly used building blocks is possible in a quantitative way, and so preparing biobased styrene alternatives. With these styrene alternatives, free radical polymerization can be applied to produce polymer materials based on these styrene alternatives. The result of the work described in the thesis demonstrates that it is possible to synthesize the building blocks sustainably and to characterize the obtained molecularly recyclable aromatic polyesters. These polymers can be returned to their building blocks under relatively favorable conditions and can be repolymerized over and over again with minimal material loss.