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

1/2-BPS states in M theory and defects in the dual CFTs

We study supersymmetric branes in AdS_7 x S^4 and AdS_4 x S^7. We show that in the former case the membranes should be viewed as M5 branes with fluxes and we identify two types of such fivebranes (they are analogous to giant gravitons and to dual giants). In AdS_4 x S^7 we find both M5 branes with fluxes and freestanding stacks of membranes. We also go beyond probe approximation and construct regular supergravity solutions describing geometries produced by the branes. The metrics are completely specified by one function which satisfies either Laplace or Toda equation and we give a complete classification of boundary conditions leading to smooth geometries. The brane configurations discussed in this paper are dual to various defects in three- and six-dimensional conformal field theories.Comment: 82 pages, 12 figures, added ref

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

Triclocarban: Commercial Antibacterial and Highly Effective H-Bond Donating Catalyst for Ring-Opening Polymerization

The antibacterial compound, triclocarban (TCC), is shown to be a highly effective H-bond donating catalyst for ring-opening polymerization (ROP) when applied with an H-bond accepting base cocatalyst. These ROPs exhibit the characteristics of “living” polymerizations. TCC is shown to possess the high activity characteristic of urea (vs thiourea) H-bond donors. The urea class of H-bond donors is shown to remain highly active in H-bonding solvents, a trait that is not displayed by the corresponding thiourea H-bond donors. Two H-bond donating ureas that are electronically similar to TCC are evaluated for their efficacy in ROP, and a mechanism of action is proposed. This “off-the-shelf” H-bond donor is among the most active and most controlled organocatalysts for the ROP of lactones

Triclocarban: Commercial Antibacterial and Highly Effective H‑Bond Donating Catalyst for Ring-Opening Polymerization

The antibacterial compound, triclocarban (TCC), is shown to be a highly effective H-bond donating catalyst for ring-opening polymerization (ROP) when applied with an H-bond accepting base cocatalyst. These ROPs exhibit the characteristics of “living” polymerizations. TCC is shown to possess the high activity characteristic of urea (vs thiourea) H-bond donors. The urea class of H-bond donors is shown to remain highly active in H-bonding solvents, a trait that is not displayed by the corresponding thiourea H-bond donors. Two H-bond donating ureas that are electronically similar to TCC are evaluated for their efficacy in ROP, and a mechanism of action is proposed. This “off-the-shelf” H-bond donor is among the most active and most controlled organocatalysts for the ROP of lactones

[Pb2F2](SeO4): a heavier analogue of grandreefite, the first layered fluoride selenate

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.Co-precipitation of PbF 2 and PbSeO 4 in weakly acidic media results in the formation of [Pb 2 F 2 ](SeO 4 ), the selenate analogue of the naturally occurring mineral grandreefite, [Pb 2 F 2 ](SO 4 ). The new compound is monoclinic, C2/c, a = 14.0784(2) Å, b = 4.6267(1) Å, c = 8.8628(1) Å, β = 108.98(1)°, V = 545.93(1) Å 3 . Its structure has been refined from powder data to R B = 1.55%. From thermal studies, it is established that the compound is stable in air up to about 300 °C, after which it gradually converts into a single phase with composition [Pb 2 O](SeO 4 ), space group C2/m, and lattice parameters a = 14.0332(1) Å, b = 5.7532(1) Å, c = 7.2113(1) Å, β = 115.07(1)°, V = 527.37(1) Å 3 . It is the selenate analogue of lanarkite, [Pb 2 O](SO 4 ), and phoenicochroite, [Pb 2 O](CrO 4 ), and its crystal structure was refined to R B = 1.21%. The formation of a single decomposition product upon heating in air suggests that this happens by a thermal hydrolysis mechanism, i.e., Pb 2 F 2 SeO 4 + H 2 O (vapor) → Pb 2 OSeO 4 + 2HF↑

[Pb2F2](SeO4): a heavier analogue of grandreefite, the first layered fluoride selenate

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.Co-precipitation of PbF 2 and PbSeO 4 in weakly acidic media results in the formation of [Pb 2 F 2 ](SeO 4 ), the selenate analogue of the naturally occurring mineral grandreefite, [Pb 2 F 2 ](SO 4 ). The new compound is monoclinic, C2/c, a = 14.0784(2) Å, b = 4.6267(1) Å, c = 8.8628(1) Å, β = 108.98(1)°, V = 545.93(1) Å 3 . Its structure has been refined from powder data to R B = 1.55%. From thermal studies, it is established that the compound is stable in air up to about 300 °C, after which it gradually converts into a single phase with composition [Pb 2 O](SeO 4 ), space group C2/m, and lattice parameters a = 14.0332(1) Å, b = 5.7532(1) Å, c = 7.2113(1) Å, β = 115.07(1)°, V = 527.37(1) Å 3 . It is the selenate analogue of lanarkite, [Pb 2 O](SO 4 ), and phoenicochroite, [Pb 2 O](CrO 4 ), and its crystal structure was refined to R B = 1.21%. The formation of a single decomposition product upon heating in air suggests that this happens by a thermal hydrolysis mechanism, i.e., Pb 2 F 2 SeO 4 + H 2 O (vapor) → Pb 2 OSeO 4 + 2HF↑