Date of Award
Doctor of Philosophy in Chemistry
Organic catalysis for the Ring Opening Polymerization (ROP) of cyclic monomers is a rapidly emerging field of study that gained interest in 2005 with the advent of dual H-bonding catalysts. Synthesizing catalysts that produce fast reaction rates with superior reaction control over molecular weight (Mn) and molecular weight distributions (Mw/Mn) are of great interest for material applications. Current organic catalysts do not have the capabilities to satisfy these requirements, limiting the feasibility to pursue commercial scale applications.
Analysis of polymerizations is done using a number of techniques. Nuclear Magnetic Resonance (NMR) is a power spectroscopy technique used to evaluate reaction progression for polymerization reactions. Through reaction conversions, the kinetics of each catalyst can be measured and compared with one another. Through NMR titration experiments, binding studies were used to compare and in some cases quantify the interactions between monomer and alcohol/chain end with the catalyst and cocatalysts respectively.
Gel Permeation Chromatography (GPC) is another technique used for the analysis of polymers, which allows for the determination of the polymer molecular weight (Mn) and molecular weight distribution (Mw/Mn). The catalyst chosen to perform the ROP of monomer has a large impact on the control over the Mn and Mw/Mn. This method allows for the determination of polymer Mn and Mw/Mn, which translate to reaction control.
Organic catalysis for the Ring Opening Polymerization (ROP) of cyclic monomers is a rapidly emerging field of study that gained interest in 2005 with the advent of dual H-bonding catalysts. Synthesizing catalysts that produce fast reaction rates with superior reaction control over Mn and Mw/Mn are of great interest for material applications. Current organic catalysts do not have the capabilities to satisfy both requirements limiting the feasibility to pursue commercial scale applications. First, a review of H-bonding organic catalysts and their relative reactivity will be discussed.
The polymerization of cyclic esters by H-bonding (thio)urea has greatly increased since the first iterations of catalyst scaffolds. The incorporation of multi-armed H-bond donating species saw drastic increases in reaction rate. The incorporation of an oxygen (urea) in substitution of a sulfur (thiourea) saw an increase for all H-bond donors tested. These reactions also remained well controlled.
These catalysts have been shown to be tolerant of solvent free polymerizations. The adoption of solvent free reactions is greatly valued by the commercial industry. Solvent free conditions allowed for the polymerization of several copolymers that were not possible through reactions within solvent.
H-bonding (thio)urea catalysts used for the ROP of caprolactone were subjected to elevated temperatures (22-110°C). 1-O and 2-O produced linear Eyring plots out to 110°C (highest temperature evaluated). All other catalysts deviated from linearity at 80°C, due to decomposition of the H-bonding species. A switch to polar solvent alleviated decomposition for some H-bond donors while other remained curved. A mechanistic reasoning will be discussed.
The introduction of a chiral architecture into the catalyst scaffold made kinetic resolution of racemic lactide possible. This chiral scaffold was responsible for an increase in isotacticity (Pm) of the resulting polymer. Multi armed chiral H-bond donors saw increase reaction rates but only small increase in Pm value versus mono-armed H-bond donors. A decrease in reaction temperature produced enhanced the Pm values.
A new class of bifunctional, quinone derived catalyst was developed for the ringopening polymerization (ROP) of lactone monomers. Similar in architecture to other bifunctional catalysts, the quinone catalyst can activate monomer and alcohol/chain simultaneously. Attempts at ROP of both δ-valerolactone and L-lactide were unsuccessful. A mechanistic explanation is discussed.
H-bonding urea or thiourea catalyst paired with a base cocatalyst have been employed for organocatalytic ring-opening polymerization (ROP) of aliphatic lactones (TOSUO, 4-MCL, 3,5-MCL and 6-MCL). Random copolymers with low dispersities were synthesized. A series of copolymers of CL and 3,5-MCL were produced and evaluated using TGA and DSC. Variation of the substituent along with its position on the monomer resulted in a different reaction rates. The relative rates of ROP for functionalized ε–caprolactone (4-MCL, 3,5-MCL, 6-MCL, and TOSUO) by H-bonding organic catalysts have been evaluated and a mechanistic reasoning discussed. H-bonding organic catalysts saw increased reaction rates and control for all monomers versus both metal and enzymatic catalysts.
Fastnacht, Kurt, "THE DEVELOPMENT AND MECHANISTIC STUDY OF DUAL H-BONDING ORGANIC CATALYSTS FOR THE ROP OF CYCLIC ESTERS" (2018). Open Access Dissertations. Paper 784.