Date of Award
Doctor of Philosophy in Chemistry
The use of non-biodegradable polymeric materials has been a growing threat to the environment. Attention has been greatly focused on monomers derived from sustainable feedstocks for the development of biodegradable and environmentally friendly polyesters. Precisely tailoring such polymers with complex architecture and similar functionality to macromolecules used commercially has been a daunting challenge. Ring-opening polymerizations (ROP) of cyclic monomers is a good approach towards making well defined, structurally complex, and degradable macromolecules. In this regard developing effective and efficient catalyst systems to perform fast and controlled polymerizations to construct various polymer architectures is vital. Lately, great advances in ROP with respect to catalyst design in obtaining efficient transformations, and monomer designs for the development of degradable polymers are constantly being reported. Here in we discuss the development of H-bonding organic cocatalyst systems while catalyzing ROPs of cyclic lactones in a fast and controlled manner.
The complicated mechanistic interplay of (thio)urea/base H bonding organo-catalysts in ring-opening polymerization (ROP) of cyclic lactones has been broadly discussed in this work. Understanding these systems led to the design and development of a wide range of ureas and thioureas such as bis-(thio)ureas and tri-ureas that showed enhancement of rate for ROPs without compromising the control of the reaction. These new catalysts showed superior activity compared to the conventional mono-(thio)urea compounds such as cyclohexyl thiourea and base, which was the foundation of H-bonding thiourea/base catalyst systems. These systems showed comparable rates to metal catalysts commonly used for the ROP of cyclic lactones. A comprehensive study to understand the catalyst activity has been performed. NMR studies and computational studies were used to show the modes of activity of these H-bonding catalyst systems and to understand the structure-activity relationship. These studies also revealed the activity of these catalysts not only in non-polar solvents but also in polar aprotic solvents such as acetone.
Commercially available urea known as triclocarban was used in the presence of a base to catalyze ROP of cyclic lactones. These systems showed enhanced rates and good control over the polymerization. Studies carried out using NMR titrations to understand the mode of monomer activation by urea suggested the presence of a hyperactive imidate species that exist in the transition state. Solvent-free ROPs of cyclic lactones conducted in the presence of urea/base cocatalyst systems showed fast rates and good control. These systems allowed one-pot copolymer synthesis which were inaccessible in solvent.
A Hammett-style relationship of thiourea and urea mediated ROP of valerolactone was performed to understand the activity of these systems which can undergo two types of mechanisms. This study also shows how substituent groups in the thio(urea) affects the rates of ROP. Studies carried out in polar and non-polar solvents suggested that the solvent used plays a dominant role in determining the mechanisms - neutral H-bonding and (thio)imidate mechanism - during ROP in the presence of (thio)urea and base systems. Evidence reveal that in polar solvents such as acetone the catalysts perform ROP via (thio)imidate type mechanism while in non-polar solvents the neutral H-bonding path is prevalent.
Pothupitiya, Jinal U., "ORGANOCATALYTIC RING-OPENING POLYMERIZATION OF CYCLIC LACTONES: DEVELOPMENT OF NEW CATALYSTS GUIDED BY MECHANISM" (2019). Open Access Dissertations. Paper 866.