A systematic investigation of the ring size effects on the free radical ring-opening polymerization (rROP) of cyclic ketene acetal (CKA) using both experimental and theoretical approach

Radical ring-opening polymerization (rROP) reaction of cyclic ketene acetals (CKA) is an interesting route to biodegradable polymers. Contrary to their tremendous potential, fundamental understanding of their reaction kinetics and thermodynamics is still limited. We present experimental and theoretical investigations for rROP reactions of CKA to systematically elucidate the effects of monomer ring sizes on the homopolymerization. We aim to provide insights on the structural-reactivity relationship of CKA by studying the thermodynamics and kinetics of the forward ring-opening propagation reactions and key side reactions, namely ring-retained propagation and radical back-biting reaction leading to branching. Experimental results show that for the CKA with smaller ring sizes, significant amount of ring-retained side products are formed when up to 90% of the monomers are converted. However, for the larger ring sizes (7 and 8 membered), almost complete ring-opening polymerization with <1% of ring-retained products are formed. Density functional theory (DFT) calculations show that kinetic effects from the collision frequency dominate in differentiating between ring-opening propagation, ring-retained propagation, and backbiting. The results corroborate well with experiments and reports in the literature. Our systematic study from the first principle and experimental validation provide insights into CKA rROP to apply radical polymerization to generate biodegradable polymers.     

Synthesis,Structural Aspects,Antimicrobial Activity,and Ion Transport Investigations of Four New [1 + 1] Condensed 12‐Membered Cyclophane Amides

Two new diamines ( a , b ) and their four [1 + 1] condensed macrocyclic peptides ( c–f ) were synthesized via dilution method affording the expected dilactams in reasonably high‐yield yields. The compounds were characterized by elemental analyses, mass, FT‐IR, ¹H, and ¹³C NMR spectral data. Mass spectra reveal their [1 + 1] cyclic condensation. The macrocycles having two peptide units in the ring would constitute three configurational isomers: cisoid–cisoid, transoid–transoid, and cisoid–transoid. Their ¹H and ¹³C NMR show one signal for each chemically equivalent SCH₂ and CONH and could be assigned symmetrical structures lacking any configurational isomerism. The ion transport activity of the compounds ( c ) and ( d ) on Na⁺ and K⁺ ions were evaluated by CH₂Cl₂ liquid–liquid membrane method using a U tube system. Slightly higher affinity for potassium than sodium was also observed, which is undoubtedly related to the matching size of the cavity between K⁺ and cyclic peptides. The antimicrobial and antifungal activities of four macrocyclic amides (c , d , e , and f) were illuminated using disk diffusion method in dimethyl sulfoxide as well as the minimal inhibitory concentration dilution method, against several bacteria and yeast cultures. Comparing all methods, in most cases, the ( e ) and ( f ) compound seems to show slightly higher biological activity than the ( c ) and ( d ) macrocycles. This may be because of the presence of extra chlorine atoms on the exterior part of the molecules.     

Divalent metal complexes (Ca, Sr, Ba, Yb) of the unsymmetrical 3-(2'-thienyl)-5-(trifluoromethyl)pyrazolate ligand

The divalent complexes [M(ttfpz)(2)(thf)(4)] (ttfpz = 3-(2'-thienyl)-5-(trifluoromethyl)pyrazolate; M = Yb, 1, Ca, 2, Sr, 3, Ba, 4; thf = tetrahydrofuran) and [M(ttfpz)(2)(dme)(n)] (M = Ca, 5, Sr, 6, Yb, 7, n = 2; M = Ba, 8, n = 3; dme = 1,2-dimethoxyethane) have been prepared by redox transmetallation/protolysis reactions employing the free metals, Hg(C(6)F(5))(2) and ttfpzH in donor solvents and their structures determined. The monomeric structures exhibit η(2)-bound pyrazolate ligands with eight-coordinate metal atoms for complexes 1-7 and a ten-coordinate metal for 8. The pyrazolate ligands in the thf-complexes 1-4 as well as dme-derivatives 5 and 6 are in a transoid configuration, whilst in complex 7 the ttfpz ligands exhibit a cisoid relationship. In 8 the ligands have an intermediate role in between cisoid and transoid.     

Optical Rotatory Feature of (R or S)-1,1′-Binaphthalene-2,2′-diol (BINOL) in Various Solvents

IntroductionElieletal.1,2 haveeverreportedthatsolventtypeandconcentrationwouldaffecttheequilibriumofcis 3 hydrox ythiane S oxideconformationwhichwascausedbyintermolec ularandintramolecularhydrogenbondformedwiththeinter actionbetweenthesolute solventandthesolute soluteinthesolutionandhaveimportantinfluenceonitsspecificrotation .Uptonowitisnotveryclear,however,whetherhydrogenbondingaffectsspecificrotation (inmagnitudeandoccasion allyinsign)directlyorwhetheritdoessoindirectly (byaf fectingconform…     

Optical Rotatory Feature of （R or S）—1,1’—Binaphthalene—2,2’—diol（BINOL）in various Solvents

The changes of the specific rotation and sign of optically active BINOL have been studied in polar/non-polar solvents and at the different pH values of solvent.It is considered that these changes are determined by the equilibriurn studies between cisoid and transoid conformations of BINOL with the same configuration(R or S) which related to the change of the dihedral angle between two naphthalene ring planes of BINOL.     

Circular dichroism—LXVIII compounds from valerian—XII: On the chiroptical properties of valepotriates and related compounds

The CD of saturated iridoids containing a 2.9-dioxatricyclo[4.3.1.03,7] decan skeleton derived from valepotriates is determined by the acetal chromophore. For the less symmetric ring system of 1,6-anhydro hexapyranoses the sign of this CD is predictable. The vicinal effect for enones and diketones in this series is appreciable. A new rule for the correlation between the CD around 190–205 nm of enol ethers and their absolute conformation is given; the same reasoning is applicable to explain the known rules for cisoid and transoid vinyl cyclo-propanes.     

Polymerization of coordinated monomers. XX. Ab initio molecular orbital study on the binary molecular complex composed of methyl methacrylate and boron trichloride

The stereo and the electronic structures of methyl methacrylate and the binary molecular complex composed of methyl methacrylate and boron trichloride are determined by quantum chemical calculations based on ab initio molecular orbital methods with the STO-3G basis set. The Stable structures of methyl methacrylate in the free state are a transoid from and a cisoid form. The transoid form is slightly (0.5 kcal/mol) more stable than the cisoid form. The total energy change accompanying binary molecular complex formation is calculated to be ?43 kcal/mol. The most stable structure of the binary molecular complex composed of methyl methacrylate and boron trichloride is a twisted form in which the dihedral angle between the vinyl group and the ester group is 19.7°. A large amount of electron transfer from methyl methacrylate to boron trichloride is calculated to occur on binary molecular complex formation. Mulliken's population analysis shows the electron transfer to be the origin of the twist of the binary molecular complex. The energy level of the lowest unoccupied molecular orbital is substantially lowered by the coordination of boron trichloride.     

The anionic ring-opening polymerization and copolymerization of cyclic carbonates

Cyclic carbonates are eligible to ring-opening polymerization using a wide variety of initiators such as carbanionic or alcoholate species as well as initiators known to be effective for the ring-opening polymerization of lactones and for the group transfer polymerization of vinyl monomers. Depending on the catalyst, high molecular weight polymers may be obtained in high yields (kinetically controlled regime) or a ring-chain equilibrium is observed upon end-biting, back-biting and transesterification reactions (thermodynamically controlled regime). The polymerizability of the cyclic carbonates is strongly dependent on their structure. Five-membered cycles generally cannot be polymerized, whereas six-membered cycles can be polymerized and copolymerized in an ideal manner. The polymerizability of higher cyclics, in particular when containing aromatic ring systems, is highly dependent on the substitution pattern of the aromatics. Since the active species in the polymerization of aliphatic cyclic carbonates was disclosed to be of alcoholate type, a copolymerization with ϵ-caprolactone is easily achieved, the reactivity of the cyclic carbonate, however, being by far larger than that of the lactone. On the other hand, the copolymerization with pivalolactone exerts a different behaviour, since the active species of the growing pivalolactone chain after a few steps assumes the character of a carboxylate anion which is unable to promote the ring-opening polymerization of cyclic carbonates. Since carbanionic species may be used as initiators for the ring-opening polymerization of cyclic carbonates, polystyryl, polybutadienyl, and polyisoprenyl anions may be used as initiators to achieve the corresponding block copolymers. To obtain block copolymers with poly(methyl methacrylate) blocks a group transfer polymerization of the respective acrylate has to be performed, followed by the polymerization of the cyclic carbonate. The latter, however, rather proceeds by a metal- free anionic process than by a group transfer process. The ring-opening polymerization and copolymerization of cyclic carbonates allows the preparation of a broad variety of new polymers with remarkable properties.     

Organocatalyzed Step‐Growth Polymerization through Desymmetrization of Cyclic Anhydrides: Synthesis of Chiral Polyesters

The polymerization of prochiral bis‐anhydrides with diols catalyzed by a cinchona alkaloid was shown to provide chiral polyesters in good yields and with high levels of stereocontrol. The structures of the polyesters were determined by ¹H and ¹³C NMR analyses, whereas their size was estimated by both size‐exclusion chromatography (SEC) and MALDI‐TOF mass spectrometry, which indicated that moderate degrees of polymerization were attained through this step‐growth polymerization. The enantioselectivity of the process was evaluated by using chiral HPLC analysis of the bis‐lactones resulting from a controlled chemoselective degradation of the polyesters. The best stereocontrol was reached for oligomers formed from bis‐anhydride and diol monomers bearing rigid aromatic spacers between the reactive functional groups. In this case, average enantioselectivities were comparable to those observed during ring‐opening of simple anhydrides with similar alcohols. In contrast, the use of more flexible spacers between reactive entities generally led to lower levels of stereocontrol.     

Direct observation and study of the successive elemental steps of catalytic ring-opening metathesis polymerisation (ROMP) reactions: a retrospective review of a fruitful collaboration with John Osborn

The results of a collaboration (1984–1992) with John Osborn are summarised. A VT ¹H NMR study of the ROMP of norbornene and some of its derivatives, initiated by well-defined tungsten–carbene complexes, showed that it was generally possible to follow the formation of the propagating tungsten–carbene complexes and their interconversion with the intermediate transoid (but not the cisoid) tungstacyclobutane complexes. The stability of the transoid complexes varies considerably with the nature of the monomer and initiator. Kinetic data were derived in a number of cases.     

A stable and long-lived germaimine

Spectroscopic identification of the first reported stable germaimine is provided, together with an analysis of its bond structure; it exists in cisoid and transoid forms.     

Metal-induced B-H activation: addition of acetylene, propyne, or 3-methoxypropyne to Rh(Cp*), Ir(Cp*), Ru(p-cymene), and Os(p-cymene) half-sandwich complexes containing a chelating 1,2-dicarba-closo-dodecaborane-1,2-dichalcogenolato ligand

The addition reactions of the 16e half-sandwich complexes [M(eta5-Cp*)[E2C2(B10H10)]] (Cp*=pentamethylcyclopentadienyl: 1S: E=S, M=Rh; 2S: E=S; M=Ir; 2Se: E=Se, M=Ir) and [M(eta6-p-cymene)[S2C2(B10H10)]] (p-cymene=4-isopropyltoluene; 3S: M=Ru; 4S: M=Os), with acetylene, propyne, and 3-methoxypropyne lead to the 18e complexes 5-19 with a metal-boron bond in each case. The reactions start with an insertion of the alkyne into one of the metal-chalcogen bonds, followed by B-H activation, transfer of one hydrogen atom from the carborane via the metal to the terminal carbon of the alkyne, and concomitant ortho-metalation of the carborane. The E-eta2-CC and the C(1)B units are arranged either cisoid or transoid at the metal. X-ray structural analyses are reported for one of the starting 16e complexes (4S), the cisoid complex 12S (from 2S and HC[triple bond]C-CH3), and the transoid complexes 9S and 14S (from 1S and HC[triple bond]C-CH2OMe, and from 3S and HC[triple bond]CH, respectively). All new complexes 5-19 were characterized by NMR spectroscopy (1H, 11B, 13C, and 77Se and 103Rh NMR spectroscopy when appropriate).     

环氧乙烷环氧丙烷开环聚合反应动力学研究

环氧乙烷和环氧丙烷由于具有较高的环张力,因而容易发生开环聚合。本文综述了环氧乙烷合环氧丙烷开环聚合反应的动力学研究进展,考察了环氧乙烷和环氧丙烷开环聚合反应的机理,分别讨论了各类催化剂体系中环氧乙烷和环氧丙烷开环聚合的动力学常数、两者的竞聚率及开环聚合产物的分子量分布,并指出了开环聚合反应动力学研究对于环氧乙烷和环氧丙烷的开环聚合研究及工业应用的重要性。     

Precise Synthesis and Thin Film Self-Assembly of PLLA-b-PS Bottlebrush Block Copolymers

The ability of bottlebrush block copolymers (BBCPs) to self-assemble into ordered large periodic structures could greatly expand the scope of photonic and membrane technologies. In this paper, we describe a two-step synthesis of poly(l-lactide)-b-polystyrene (PLLA-b-PS) BBCPs and their rapid thin-film self-assembly. PLLA chains were grown from exo-5-norbornene-2-methanol via ring-opening polymerization (ROP) of l-lactide to produce norbornene-terminated PLLA. Norbonene-terminated PS was prepared using anionic polymerization followed by a termination reaction with exo-5-norbornene-2-carbonyl chloride. PLLA-b-PS BBCPs were prepared from these two norbornenyl macromonomers by a one-pot sequential ring opening metathesis polymerization (ROMP). PLLA-b-PS BBCPs thin-films exhibited cylindrical and lamellar morphologies depending on the relative block volume fractions, with domain sizes of 46–58 nm and periodicities of 70–102 nm. Additionally, nanoporous templates were produced by the selective etching of PLLA blocks from ordered structures. The findings described in this work provide further insight into the controlled synthesis of BBCPs leading to various possible morphologies for applications requiring large periodicities. Moreover, the rapid thin film patterning strategy demonstrated (>5 min) highlights the advantages of using PLLA-b-PS BBCP materials beyond their linear BCP analogues in terms of both dimensions achievable and reduced processing time.     

The mechanism of TBD-catalyzed ring-opening polymerization of cyclic esters

Triazabicyclodecene (TBD) has recently been shown to be an effective organocatalyst for the ring-opening polymerization (ROP) of cyclic esters. Using DFT methods, we have studied possible mechanisms of this reaction. Our studies explain not only the narrow polydispersity index (PDI) observed in the ROP of six-membered ring lactones, but also the surprising failure of the ROP for the more reactive butyrolactone.     

One-step synthesis of poly(alkyl methacrylate)-b-polyester block copolymers via a dual initiator route combining RAFT polymerization and ROP

One-step synthesis of well-defined poly(alkyl methacrylate)-b-polyester block copolymers was successfully performed at 30?°C using a dual initiator for reversible addition–fragmentation chain transfer (RAFT) polymerization and ring-opening polymerization (ROP). 4-Cyano-1-hydroxypent-4-yl dithiobenzoate, a hydroxyl-functionalized RAFT agent, was used as a dual initiator for the RAFT polymerization of methacrylic monomers and the ROP of lactones. Diphenyl phosphate (DPP) was used as a catalyst for ROP. The two polymerization reactions proceeded in a controlled manner without mutual interference. DPP was found to be a suitable catalyst for a dual initiator route combining RAFT polymerization and the ROP of lactones.     

Kinetics and mechanisms in anionic ring-opening polymerization

Recent advances in the anionic ring-opening polymerization (AROP), including covalent (pseudoanionic) polymerization, are reviewed. Thermodynamics, kinetics, and mechanisms of AROP are discussed, covering mostly polymerization of oxiranes, lactones and cyclic siloxanes as monomers. The following general problems of AROP are discussed: anionic polymerizability, thermodynamics - particularly of the monomers exhibiting low ring strain, chemistry of initiation, structures and reactivity of active species. New phenomena, particularly polymerization with reversibly aggregating species are analyzed in more detail. Chain transfer to polymer - the major side reaction - is analyzed quantitatively, by introducing the selectivity parameter β, expressed by the ratio k_p/k_tr. This parameter has been determined for the anionic and pseudoanionic polymerization of ϵ-caprolactone.     

Regioselective ring-opening anionic polymerization of β-lactones

New aspects of anionic polymerization of 4-membered lactones are presented, attention being paid to regioselectivity of ß-lactones ring-opening reactions. It has been demonstrated that supramolecular complexes of alkali metal alkoxides used as initiators enable control of lactones polymerization, and due to anion activation yield polymers with specific molecular architecture. Synthesis of the analogue of natural polymer poly(3-hydroxybutyrate) via anionic polymerization of ß-butyrolactone is discussed.     

Crystal and molecular structure of 2-acetyl-5-methyl-3-phenylfuran

The molecular structure of the title compound indicates that it possesses a transoid structure of the O=C-C-O group, and a dihedral angle of about 40° between the furan and phenyl rings. The same conformation is predominant also in CDCl₃ solution as indicated by europium chelate induced proton shifts. The corresponding 2,4-dinitrophenyhydrazone has, however, the cisoid isomer stabilized by an intramolecular hydrogen bond.     

Mechanism and diastereoselectivity of aziridine formation from sulfur ylides and imines: a computational study

A computational investigation of the title reaction involving semistabilized (R = Ph) and stabilized (R = CO2Me) sulfur ylides has been performed using DFT methods including a continuum model of solvent. Our results provide support for the generally accepted mechanism and are in very good agreement with observed cis/trans selectivities. This study shows that betaine formation is nonreversible, and that selectivity is thereby determined at the initial addition step, in the case of semistabilized ylides. Our analysis indicates moreover that addition TS structures are governed by the steric strain induced by the N-sulfonyl group, which favors the transoid approach in the case of syn betaine formation and the cisoid mode of addition in anti TSs. The observed low trans selectivity is accounted for by the favorable Coulombic interactions and stabilization by C-H...O hydrogen bonding allowed in the cisoid anti addition TS. In the case of stabilized ylides, the endothermicity of betaine formation combined with the high barrier to ring closure render the elimination step rate- and selectivity-determining. Accordingly, the low cis selectivity observed in stabilized ylide reactions is explained by the lower steric strain in the elimination step generated by the formation of the cis aziridine (as compared to the trans case).