Honokiol and Magnolol as Renewable Resources for the Synthesis of Polymers Towards Biomedical and Engineering Applications

Abstract

This dissertation has focused on the design, synthesis, and characterization of novel polymers derived from the renewable resources honokiol and magnolol. These natural products – isolated from magnolia officinalis – are highly functional and, thus, amenable to a broad range of synthetic organic transformations. The transformations explored in this work have yielded diverse monomers and, subsequently, polymer types which have included polycarbonates, thermosets, and olefin-based polymers that were studied for potential engineering or biomedical materials. A guiding theme in the development of polymers from honokiol and magnolol has been to control and vary the final polymer properties through the monomer design and polymerization chemistry. This work has used scalable syntheses, well-known chemistries, and, as the first examples of polymers synthesized from these natural products, it has laid the foundation to further explore material applications and new polymers based on honokiol and magnolol. Poly(honokiol carbonate) (PHC) was synthesized in one step from honokiol using step-growth polycondensation techniques. Synthetic conditions were screened to yield polymers of varying molecular weight and the resulting polymers were studied in their thermomechanical and biological properties. PHC shares comparable thermomechanical properties with established engineering materials, both renewable- and petroleum-based, with which it was compared. Additionally, PHC serves as a good substrate for cell growth over an extended period of time. In an effort to take advantage of both the phenolic and alkenyl functionalities of magnolol, thermoset syntheses via thiol-ene chemistry were performed directly from magnolol and with a library of magnolol-based monomers. The thermomechanical and hydrolytic degradation properties were controlled via monomer design in order to leverage beneficial biological effects, namely radical scavenging, in future biomedical applications. Degradation products and model compound studies showed antioxidant behavior similar to what has been observed for the natural product, magnolol. A similar approach in controlling polymer properties through monomer design was realized in the synthesis of olefin-based polymers from magnolol through acyclic diene metathesis (ADMET) chemistry. Magnolol is an alkene-containing natural product and, as such, avoids the necessity to install alkene moieties that other renewable resources often require before metathesis chemistry. The rigid polymers displayed a wide range of glass transition temperatures (Tvg) up to 180 °C after the initial polymerization. Additionally, several strategies were employed for post-polymerization modification and further tuning of the thermomechanical and physical properties.