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

Coupled equilibria in H-bond donating ring-opening polymerization: The effective catalyst-determined shift of a polymerization equilibrium

In the classic view of catalysis, a catalyst cannot alter the thermodynamically-determined endpoint of a reversible reaction. This conclusion is predicated on the assumption that the catalyst does not perturb the energy of product or reactant or does so to an equal extent. In the H-bond mediated ring-opening polymerization (ROP) of lactone monomers, the strength of the interactions of thiourea with product and reactant are not equal, and the magnitudes of these interactions are of similar energy to the free energy of reaction. The total monomer concentration at equilibrium in the thiourea/base cocatalyzed ROP of lactones is shown to be a function of the initial concentration of thiourea. Because the binding of thiourea to monomer and the polymerization reaction itself are both reversible, the application of varying amounts of thiourea catalyst directly alters the total amount of monomer in the reaction solution at equilibrium, which can be recovered at the end of the reaction

Organocatalytic ring-opening polymerization of thionolactones: Anything O can do, S can do better

The H-bond mediated organocatalytic ring-opening polymerizations (ROPs) of four new thionolactone monomers are discussed. The kinetic and thermodynamic behavior of the ROPs is considered in the context of the parent lactone monomers. Organocatalysts facilitate the retention of the S/O substitution as well as the synthesis of copolymers. The thionoester moieties in the polymer backbone serve as a chemical handle for a facile crosslinking reaction, and the porosity of the resulting crosslinked polymer can be tuned by altering the thioester density in the (co)polymer. The crosslinked polymers are shown to be degradable in water, and an Au3+ recovery application is demonstrated

Octahedral Group IV Bis(phenolate) Catalysts for 1‑Hexene Homopolymerization and Ethylene/1-Hexene Copolymerization

Octahedral group IV bis­(phenolate) catalysts are highly active catalysts for the isospecific polymerization of 1-hexene and the copolymerization of ethylene with 1-hexene. These catalysts are active for the production of high molecular weight copolymers even at 130 °C. The copolymerization parameters for these complexes were determined; all of the bis­(phenolate) complexes tested incorporate 1-hexene with high efficiency to give random copolymers. The complexes prepared from the more sterically demanding ligands showed higher molecular weights but similar comonomer incorporations to those prepared from the less sterically demanding ligands

Propylene polymerization with cyclopentadienyltitanium(IV) hydroxylaminato complexes

Complexes of the type Cp*TiX2(ONR′R′′) (Cp* = η5-C5Me5; X = Me, Cl; R = R′ = Et, TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl); X = R′ = Me, R′′ = tBu) were synthesized by several routes. Upon activation with [Ph3C]+[B(C6F5)4]− and AliBu3, these complexes generate highly active catalysts for propylene polymerization, although significant catalyst deactivation was observed. Activation with B(C6F5)3/AliBu3 or methylaluminoxane (MAO) resulted in reduced polymerization activity, the latter leading to increased catalyst lifetime. Model studies showed that the interaction of AlMe3 with Cp*Ti(Me)2(ONEt2) led to the formation of Cp*Ti(Me)2(η2-O(AlMe3)NEt2). The X-ray crystal structure confirmed that the hydroxylaminato ligand remained η2 bound to the titanium center with the AlMe3 bound to the complex through the oxygen atom of the hydroxylaminato ligand. Exchange reactions with organic ethers revealed the metalloether to be a comparable donor to PhOMe. Cp*Ti(Me)2(ONtBu(Me)) was revealed to be a weaker donor than Cp*Ti(Me)2(ONEt2); Cp*Ti(Me)2(TEMPO) did not bind to AlMe3. AliBu3 bound more weakly to Cp*Ti(Me)2(ONEt2) than AlMe3. Reaction of Cp*Ti(Me)2(ONEt2) and Cp*Ti(Me)2(ONtBu(Me)) with B(C6F5)3 resulted in clean formation of the zwitterionic contact ion pairs [Cp*Ti(Me)(η2-ONEt2)]+[MeB(C6F5)3]− and [Cp*Ti(Me)(η2-ONtBu(Me))]+[MeB(C6F5)3]−, respectively, whereas reaction of Cp*Ti(Me)2(η2-ONtBu(Me)) with [Ph3C]+[B(C6F5)4]− resulted in the clean formation of the solvent-separated ion pair [Cp*Ti(Me)(η2-ONtBu(Me))]+[B(C6F5)4]−. Reaction of Cp*Ti(Me)2(η1-TEMPO) with B(C6F5)3 results in the elimination of methane to result in the formation of the contact ion paired [Cp*Ti(η2-TEMPO)]+[MeB(C6F5)3]−, in which one of the TEMPO methyl groups has undergone C−H activation, resulting in a η2-bound TEMPO ligand, confirmed by 1H, gROESY, and 1H−15N HMBC NMR. Addition of AliBu3 or AlMe3 to the cationic [Cp*Ti(Me)(ONtBu(Me))]+[B(C6F5)4]− resulted in decomposition of the cations

Bisurea and Bisthiourea H-Bonding Organocatalysts for Ring-Opening Polymerization: Cues for the Catalyst Design

A series of conformationally flexible bis(thio)urea H-bond donors plus base cocatalyst were applied to the ring-opening polymerization (ROP) of lactones. The rate of the ROP displays a strong dependence on the length and identity of the tether, where a circa five methylene-unit long tether exhibits the fastest ROP. Any constriction to conformational freedom is deleterious to catalysis. For the ROP of δ-valerolactone (VL) and ϵ-caprolactone (CL), the bisurea H-bond donors are more effective, but for lactide, the bisthioureas are more active catalysts. The ROP reactions are rapid and controlled across a wide range of reaction conditions, including solvent-free conditions, exhibiting excellent weight control from low Mn to high polymers. The active mechanism is highly dependent on the identity of the base cocatalyst, and a mechanistic rationale for the observations is discussed. Implications for the design of future generation catalysts are discussed