Abstract
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.
Highlights
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)
When the TBD catalyzed ROP of lactones was disclosed in 2006,23 it was the perfect storm of a successful catalyst
Urea H-bond donors in combination with base cocatalysts have been shown to be highly effective for the ROP of lactones
Summary
The catalysts in this chapter conduct polymerization via non-nucleophilic, H-bond mediated pathways. H-bonding organocatalysts for ring-opening polymerization (ROP) are highly controlled systems for the synthesis of macromolecules.[1–3] This class of catalyst is constituted by one of a host of H-bond donating moieties (most commonly a thiourea or urea) and a base cocatalyst, which effect ROP of lactones and carbonates by simultaneous activation of monomer by (thio)urea and of initiating/propagating alcohol by base.[1,4]. Reaction rates and control are highly dependent on the H-bond donators selectivity for binding monomer versus polymer.[1] (Thio)ureas have been shown to be superior in both rate and selectivity over other organocatalytic species for ROP.[2,3] These species by themselves do not possess the ability to effect polymer transformations and must be paired with a base cocatalyst for alcohol/chain end activation.[4]. We report on the (co)polymerizations of TOSUO and methyl functionalized ε-caprolactone monomers (4-MCL, 3,5-MCL and 6-MCL) using H-bond donating (thio)ureas and base cocatalysts
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