Cocatalyst Binding Effects in Organocatalytic Ring-Opening Polymerization of L-Lactide

Thiourea/alkylamine cocatalysts have previously been shown to be effective systems for the ring-opening polymerization (ROP) of lactide, but an experimental explanation for the varied activity and selectivity of these structurally similar alkylamine cocatalysts combined with thiourea is elusive. In this work, several alkylamine bases are shown to be weakly associated with a thiourea cocatalyst in solution, and the nature of cocatalyst interactions vary with the identity of the alkylamine. Kinetic analyses of the organocatalytic ROP reactions reveal noninhibitory behavior in [alkylamine] and a new mode of activity for thiourea. Reactivity patterns are proposed based on computed cocatalyst geometries, and a new cocatalyst pair for the ROP of lactide is disclosed

Investigations into Ring-Opening Polymerization of Functionalized ϵ-Caprolactones

Polymers are defined as large molecules constructed from small, repeating units (monomers) which make up everyday items, such as: plastic bags and bottles, waterproof coatings, pipes, etc. H-bonding urea (U) or thiourea (TU) catalysts paired with base cocatalysts used for organocatalytic ring-opening polymerization (ROP) of cyclic esters have been shown to exhibit fast rates of polymerization and result in polymers with controlled molecular weights (Mn) and low polydispersity indexes (PDI). Additionally, organocatalytic ring-opening polymerization (ROP) techniques have been shown to be tolerant of the incorporation of functional groups into the monomer feed, which is vital for the catalytic synthesis of biologically active polymers. H-bonding catalysts have seen considerable gains in the field of ROP recently, delivering fast reaction rates to rival that seen in metal-based catalysts but with enhanced control of Mn, PDI and functional group incorporation. Variation of the substituent position on the monomer allows for optimization of the material properties, such as: solubility, polarity, thermal, and mechanical. This research investigated the kinetics of H-bonding organic catalysts for the ROP of the functionalized monomers of interest (5-MeCL, 6-MeCL, and TOSUO). Until this point, these new monomers have only been polymerized to approximately 7% conversion after 8 days with a commonly used guanidine base organocatalyst for ROP. Using the dual H-bonding catalysts, whose electronics can be modified, along with different commercially available bases, provides a more tunable approach for the ROP of functionalized lactones

Controlled Organocatalytic Ring-Opening Polymerization of ε-Thionocaprolactone

For the first time, the controlled ring-opening polymerization (ROP) of ε-thionocaprolactone (tnCL) is conducted. The organocatalytic ROP of tnCL occurs without carbonyl scrambling, leading to homopoly(ε-thionocaprolactone) (PtnCL). The ROP by base catalysts alone is proposed to proceed via a nucleophilic mechanism, while the addition of an H-bond donating thiourea (TU) is shown to provide excellent reaction control. The increased reaction control provided by the TU occurs in the virtual absence of binding between tnCL and TU, and a mechanistic account for this observation is discussed. The monomer ring strain is measured and found to be similar to δ-valerolactone (VL). Copolymers with VL are synthesized, and the resulting analysis of the copolymer materials properties provides the only known physical characterizations of poly(thio(no)ester-co-ester)s

Topical Developments in High-Field Dynamic Nuclear Polarization

We report our recent efforts directed at improving high-field dynamic nuclear polarization (DNP) experiments. We investigated a series of thiourea nitroxide radicals and the associated DNP enhancements ranging from ε=25 to 82, which demonstrate the impact of molecular structure on performance. We directly polarized low-gamma nuclei, including [superscript 13]C, [superscript 2]H, and [superscript 17]O, by the cross effect mechanism using trityl radicals as a polarization agent. We discuss a variety of sample preparation techniques for DNP with emphasis on the benefits of methods that do not use a glass-forming cryoprotecting matrix. Lastly, we describe a corrugated waveguide for use in a 700 MHz/460 GHz DNP system that improves microwave delivery and increases enhancements up to 50%.National Institutes of Health (U.S.) (Grant EB002804)National Institutes of Health (U.S.) (Grant EB003151)National Institutes of Health (U.S.) (Grant EB002026)National Institutes of Health (U.S.) (Grant EB001960)National Institutes of Health (U.S.) (Grant EB001035)National Institutes of Health (U.S.) (Grant EB001965)National Institutes of Health (U.S.) (Grant EB004866)National Institute of General Medical Sciences (U.S.) (Grant GM095843)Natural Sciences and Engineering Research Council of Canada (Postdoctoral Fellowship

Integrated genomic approaches implicate osteoglycin (Ogn) in the regulation of left ventricular mass

Left ventricular mass (LVM) and cardiac gene expression are complex traits regulated by factors both intrinsic and extrinsic to the heart. To dissect the major determinants of LVM, we combined expression quantitative trait locus1 and quantitative trait transcript (QTT) analyses of the cardiac transcriptome in the rat. Using these methods and in vitro functional assays, we identified osteoglycin (Ogn) as a major candidate regulator of rat LVM, with increased Ogn protein expression associated with elevated LVM. We also applied genome-wide QTT analysis to the human heart and observed that, out of 22,000 transcripts, OGN transcript abundance had the highest correlation with LVM. We further confirmed a role for Ogn in the in vivo regulation of LVM in Ogn knockout mice. Taken together, these data implicate Ogn as a key regulator of LVM in rats, mice and humans, and suggest that Ogn modifies the hypertrophic response to extrinsic factors such as hypertension and aortic stenosi

A Poly(thioester) by Organocatalytic Ring-Opening Polymerization

Organocatalysts typically used for the ring-opening polymerization (ROP) of cyclic ester monomers are applied to a thiolactone, ε-thiocaprolactone (tCL). In the absence of an H-bond donor, a nucleophilic polymerization mechanism is proposed. Despite the decreased ability of thioesters and thiols (versus esters and alcohols) to H-bond, H-bonding organocatalysts—a thiourea in combination with an H-bond accepting base—are also effective for the ROP of tCL. The increased nucleophilicity of thiols (versus alcohols) is implicated in the increased Mw/Mn of the poly(thiocaprolactone) versus poly(caprolactone), but deleterious transesterification is suppressed in the presence of a thiourea. The thioester monomer, tCL, is shown to be thermodynamically similar to ε-caprolactam but kinetically similar to ε-caprolactone

Rate Accelerated Organocatalytic Ring-Opening Polymerization of l-Lactide via the Application of a Bis(thiourea) H-bond Donating Cocatalyst

A cocatalyst system consisting of an alkylamine base and a bis(thiourea) featuring a linear alkane tether is shown to dramatically increase the rate of ring-opening polymerization (ROP) of l-lactide versus previously disclosed monothiourea H-bond donors. Rate acceleration occurs regardless of the identity of the alkylamine cocatalyst, and the ROP remains controlled yielding poly(lactide) with narrow molecular weight distributions, predictable molecular weights and high selectivity for monomer. This H-bond mediated ROP of l-lactide constitutes a rare, clear example of rate acceleration with bis(thiourea) H-bond donors versus monothioureas, and the bis(thiourea) is shown to remain highly active for ROP at fractional percent catalyst loadings. Activation at a single monomer ester by both thiourea moieties is implicated as the source of rate acceleration

Cooperative Hydrogen-bond Pairing in Organocatalytic Ring-Opening Polymerization

Thiourea (TU)/amine base co-catalysts are commonly employed for well-controlled, highly active ‘living’ organocatalytic ring-opening polymerizations (ROPs) of cyclic esters and carbonates. In this work, several of the most active co-catalyst pairs are shown by 1H-NMR binding studies to be highly associated in solution, dominating all other known non-covalent catalyst/reagent interactions during ROP. One strongly-binding catalyst pair behaves kinetically as a unimolecular catalyst species. The high selectivity and activity exhibited by these ROP organocatalysts is attributed to the strong binding between the two co-catalysts, and the predictive utility of these binding parameters is applied for the discovery of a new, highly active co-catalyst pair

Synthesizing Stilbene by Olefin Metathesis Reaction Using Guided Inquiry to Compare and Contrast Wittig and Metathesis Methodologies

In this experiment, students are asked to conduct a catalytic cross-metathesis experiment and compare this reaction to the Wittig reaction within the confines of green chemistry. Students synthesize stilbene from styrene using Grubbs second-generation catalyst. Products can be minimally characterized by IR spectroscopy and melting point, but using 1H NMR spectroscopy is preferred. Students find that the Wittig reaction is selective for cis-stilbene while the metathesis reaction produces \u3e98% trans-stilbene. Students determine the cis/trans selectivity, turnover number, and maximum turnover frequency of the reaction. The experiment is conducted alongside the synthesis of stilbene using Wittig chemistry from a published procedure