Synthesis of well‐defined poly(dimethylsiloxane) telechelics having nitrobenzoxadiazole fluorescent chain‐ends via thiol‐ene coupling

Well‐defined PDMS telechelics having nitrobenzoxadiazole (NBD) fluorescent probes covalently attached at both chain‐ends were prepared in two steps and a series of fractionation procedures starting from commercially available divinyl‐terminated PDMS having a broad molar mass dispersity. First, thiol‐ene coupling between 6‐mercapto‐1‐hexanol and vinyl chain‐ends allowed the formation of dihydroxy‐terminated PDMS telechelics through the formation of a thioether linkage. The resulting material was then sequentially fractionated using dichloromethane/methanol mixtures to afford several well‐defined dihydroxy‐terminated PDMS fractions having sharp distributions of molar masses (M_n = 99.5–158 kDa and ? < 1.2). The NBD fluorescent probes were then attached at both chain‐ends by N,N′‐dicyclohexylcarbodiimide/4‐(dimethylamino)pyridine esterification coupling between the hydroxyl groups and 6‐(7‐nitrobenzofurazan‐4‐ylamino)hexanoic acid. The resulting fluorescent PDMS telechelics were characterized by SEC, ¹H NMR, UV–visible, and fluorescence spectroscopies. These materials are suitable probes to investigate the dynamics of polymer chains in bulk or at interfaces by the fringe pattern fluorescent recovery after photobleaching technique. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Topological polymer chemistry by electrostatic self‐assembly

Recent developments in topological polymer chemistry are outlined. First, nonlinear polymer topologies are systematically classified on the basis of topological considerations of constitutional isomerism in a series of alkanes (C_nH_2n+2), monocycloalkanes (C_nH_2n), and polycycloalkanes (C_nH_2n?2, C_nH_2n?4, etc.). Various pairs of topological isomers are identified in randomly coiled, flexible polymer molecules with cyclic and branched structures. An electro‐ static self‐assembly and covalent fixation strategy has subsequently been developed for the efficient synthesis of a variety of topologically unique polymers, including monocyclic and polycyclic polymers, topological isomers, and topological block copolymers. In this process, new telechelics with moderately strained cyclic onium salt groups carrying multifunctional carboxylate counteranions have been designed as key polymeric precursors. Further extensions of topological polymer chem‐ istry have been achieved by the use of cyclic telechelics (kyklo‐telechelics) and cyclic macromonomers, obtainable also by means of the electrostatic self‐assembly and covalent fixation process. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2905–2917, 2003     

Synthesis of low polydispersity polybutadiene and polyethylene stars by convergent living anionic polymerization

Star‐shaped polybutadiene stars were synthesized by a convergent coupling of polybutadienyllithium with 4‐(chlorodimethylsilyl)styrene (CDMSS). CDMSS was added slowly and continuously to the living anionic chains until a stoichiometric equivalent was reached. Gel permeation chromatography‐multi‐angle laser light scattering (GPC‐MALLS) was used to determine the molecular weights and molecular weight distribution of the polybutadiene polymers. The number of arms incorporated into the star depended on the molecular weight of the initial chains and the rate of addition of the CDMSS. Low molecular weight polybutadiene arms (M_n = 640 g/mol) resulted in polybutadiene star polymers with an average of 12.6 arms, while higher molecular weight polybutadiene arms (M_n = 16,000 g/mol) resulted in polybutadiene star polymers with an average of 5.3 arms. The polybutadiene star polymers exhibited high 1,4‐polybutadiene microstructure (88.3–93.1%), and narrow molecular weight distributions (M_w/M_n = 1.11–1.20). Polybutadiene stars were subsequently hydrogenated by two methods, heterogeneous catalysis (catalytic hydrogenation using Pd/CaCO₃) or reaction with p‐toluenesulfonhydrazide (TSH), to transform the polybutadiene stars into polyethylene stars. The hydrogenation of the polybutadiene stars was found to be close to quantitative by ¹H NMR and FTIR spectroscopy. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 828–836, 2006     

Engineering with metallo-supramolecular polymers: linear coordination polymers and networks

Here we demonstrate the synthesis of telechelics with different spacer units and different numbers of metal-complexing units, like α-methoxy-ω-(2,2′:6′,2″-terpyrid-4′-yl)-poly(ethylenoxide)₇₈ ( 1 ), bis(2,2′:6′,2″-terpyrid-4′-yl) di(ethylene glycol) ( 2 ), bis(2,2′:6′,2″-terpyrid-4′-yl)-poly(ethylene oxide)₁₈₀ ( 3 ) and tris[(2,2′:6′,2″-terpyrid-4′-yl)-oligo (ethylenoxy-)_3.33]glycerin ( 4 ) utilizing 4-chloro-2,2′:6′,2″-terpyridine. The complexation behaviour of a variety of metal-salts towards the telechelics was studied and different supramolecular architectures were investigated, such as symmetric polymeric complexes and linear coordination polymers. Furthermore, attempts have been undertaken to prepare metallo-supramolecular cross-linked systems.     

Etude de copolymères ABS (acrylonitrile–butadiène–styrène): Solvatation préférentielle du polybutadiène par les promoteurs

ABS resins formed by copolymerization of styrene and acrylonitrile (AN) in presence of polybutadiene, consist of a mixture of SAN graft copolymer on polybutadiene (PBut) and of ungrafted SAN copolymer (styrene-co-acrylonitrile). The kinetic study was completed by showing a preferential solvation of polybutadiene by the initiator. This solvation effect was studied as a function of the concentration ratio SAN/PBut and in relation with the type of initiator. The adsorption of initiator appeared to be maximum when its solubility parameter (σ) is close to that of polybutadiene. As a function of the polybutadiene characteristics, this selective adsorption can be given by where I₁ is the quantity of initiator in the polybutadiene medium, I is the total amount of peroxide, [PBut] is the concentration of polybutadiene, and M?_n its molecular weight. It has been shown furthermore that the preferential solvation of polybutadiene by the benzoyl peroxide can be increased by addition of SAN or acrylonitrile. The consequences of this solvation effect on the characteristics of the grafting reaction, more precisely on the molecular weight of grafted and ungrafted SAN and on the rate of polymerization, were examined.     

Synthesis, characterization and catalytic oxidation of tetrahydrofuran with 16-membered pentaazabis(macrocyclic) copper(II) complexes; {[Cu([16]aneN5)]2R}4+ (R = aromatic nitrogen–nitrogen linkers)

New square-planar bis(macrocyclic)dicopper(II) complexes containing phenylene bridges between 16-membered pentaaza macrocyclic subunits have been synthesized via in-situ one pot template condensation reaction (IOPTCR) of aromatic nitrogen-nitrogen linker (R = 1,4-phenylenediamine; benzidine; 4,4′-diaminodiphenylmethane; 4,4′-diaminodiphenylether; 4,4′-diaminodiphenylsulfone), formaldehyde, bis(1,3-diaminopropane)copper(II) perchlorate and 1,3-dibromopropane in a 1:4:2:2 molar ratio results in the formation of new series of binuclear copper(II) complexes; 1-phenyl- (1); 1,1′-phenyl- (2); 1,1′-diphenylmethan- (3); 1,1′-diphenylether- (4); 1,1′-diphenylsulfone- (5) bis(1,3,7,11,15-pentaazacyclohexadecane)copper(II)), {[Cu([16]aneN₅)]₂R}(ClO₄)₄″. The formation of the macrocyclic framework and the mode of bonding of the complexes have been confirmed by data obtained from elemental analyses, UV-visible, FT-IR, ¹H-NMR, electronic spectral studies, conductivity and magnetic susceptibility measurements. These bis(macrocyclic) complexes catalyzed efficiently the selective oxidation of tetrahydrofuran into tetrahydrofuran-2-one and a small amount of tetrahydrofuran-2-ol and 4-hydroxybutyraldehyde using dil. H₂O₂ as the oxidant.     

Conductivity of I2‐Doped High trans‐1,4‐Polybutadiene

High trans‐l,4‐polybutadiene ( ?96% (trans)) was prepared by lanthanum naphthenate catalytic system. The conductivity of obtained polybutadiene doped with iodine reaches about ?100 s/cm, which is 2 orders of magnitude higher than the value reported.^4,5 During the I₂‐doping, the conjugated sequence was formed through double bond shifting reaction. According to the relationship between conductivity and temperature, conducting mechanism of doped high trans‐l,4‐polybutadiene is fit on variable range hoping (VRH) model.     

Kinetics of cerium(IV) oxidation of macrocyclic complexes of chromium(III) and fate of chromium(IV) intermediates

Kinetics and mechanism of the cerium(IV) oxidation of Cr(III) complexes of a series of macrocyclic (or pseudomacrocyclic) ligands with [14]-membered intraligand ring-sizes have now been investigated at I = 1.0 M (LiClO₄) Temp. 30°C. The complexes of the formulation Cr(macrocycle)(X)(H₂O) where X = CHCl₂ and H₂O, n = 0 or 1 undergo oxidation to Cr(VI) with the formation of chromium(IV) intermediates. The observed kinetic parameters for the Ce(IV) oxidation of Cr(III) macrocyclic complexes have been discussed in terms of changes brought about by the macrocyclic ligands on the Cr(III)—Cr(IV) redox potentials and in specific rates for Cr(IV)—Cr(V) conversion. On the basis of this study, it has been suggested that the trapping of Cr(IV) is easier when a macrocyclic ligand having a symmetrical intra-ligand ring size and unsaturation in the cyclic structure is coordinated equatorially. Cyclic voltammetric studies indicate the formation of Cr(IV) transient in the case of electrochemical oxidation of trans-Cr(Me₄[14]tetraene)(H₂O).     

Macrocyclic Ring Closure of OH-assisted Prins Reaction. A new and efficient synthesis of (R,S)-muscone

A new strategy for the synthesis of muscone (1) using the OH-assisted Prins reaction for macrocyclic ring closure has been developed. The monoacetal 4 of (Z,E)-4,8-dodecadienedial (3) , easily obtainable from (Z,E,E)-1,5,9-cyclododecatriene (2) , is treated with methallylmagnesium chloride, and the resulting C₁₆-precursor 5 is subjected to acid-catalyzed cyclization in dilute (?1%) solutions. This results in formation of the bicyclic dihydropyran derivatives 6 which directly yield muscone (1) on heating with a noble metal catalyst saturated with hydrogen. The five-step pathway proceeds with readily available starting materials in conventional steps and excellent overall yield (～40%). This new principle of macrocyclic ring formation has also been used successfully for the preparation of 3-methylcyclotridecanone (34) and should be generally applicable for other suitable ring systems.     

Alkyl chloride-boron trichloride initiated polymerizations of isobutylene: Detailed analysis of the initial propagation steps

Living oligomerizations of isobutylene initiated either by cumyl chloride, the [1:1]-adduct of cumyl chloride to isobutylene ( P ), the [1:2]-adduct of cumyl chloride to isobutylene ( P2 ), diisobutylene hydrochloride ( P1 '), or triisobutylene hydrochloride ( P2 ') have been studied in presence of BCl₃ and benzyltriethylammonium tetrachloroborate. In contrast to common opinion, the gross propagation rates of the various telechelics depended significantly on the degree of polymerization with k_rel = 0.07, 1.8, 2.3, 1.3, 1.1, 1.0, 1.0 for Ph-C(CH₃)₂-[CH₂-C(CH₃)₂]_n-Cl with n = 1, 2, 3, 4, 5, 6, 7, respectively. These results are compared with the initiation efficiencies of (CH₃)₃C-(CH₂-C(CH₃)₂]_n-Cl (K_rel = 0.3 and 0.8 for n = 1 and 2, respectively). The consequences for the synthesis of telechelics with very narrow molecular weight distribution are discussed.     

Macrocyclic Mechanism‐Based Inhibitor for Neuraminidases

A macrocyclic mechanism‐based inhibitor for neuraminidases (NAs) bearing a 2‐difluoromethylphenyl aglycone and a linker between the aglycone and C‐9 positions of sialic acid was synthesized and evaluated. The macrocyclic structure was designed to keep the aglycone moiety in the active site of the neuraminidase after cleavage of the glycoside bond. When Vibrio chorelae neuraminidase (VCNA) was treated with a similar acyclic derivative in the presence of detergent, the irreversible inhibition property was disabled. In contrast, this macrocyclic compound acted as an irreversible inhibitor for VCNA in the presence of detergent. Inhibition assay for various NAs using this macrocyclic compound revealed that the irreversible inhibition property depends on the k_cat of the neuraminidase treated. NAs having small k_cat values, such as Influenza viruses, Clostridium, Trypanosoma cruzi, and Human, were also inhibited irreversibly. However, Salmonella typhimurium NA, which has an extremely high k_cat, was not affected irreversibly by the inhibitor. Interestingly, in contrast to common k_cat inhibitors, the irreversibility of inhibition by this macrocyclic compound is inversely proportional to the k_cat of the target neuraminidase.     

Effects of tris(pentafluorophenyl)borane on the activation of a metal alkyl‐free Ni‐based catalyst in the polymerization of 1,3‐butadiene

The activation of a metal alkyl‐free Ni‐based catalyst with B(C₆F₅)₃ was investigated in the polymerization of 1,3‐butadiene. A catalyst of bis(1,5‐cyclooctadiene)nickel (Ni(COD)₂)/B(C₆F₅)₃ was found to have high catalytic activity and 1,4‐cis stereoregularity. The catalyst was also found to provide polybutadiene having a molecular weight (M_w) of up to 117,000, even in the absence of AlR₃ and MAO. Variations in the mol ratio of B(C₆F₅)₃ to Ni affected catalytic activity, 1,4‐cis stereoregularity, and the M_w of polybutadiene, while the molecular weight distribution (MWD) of polybutadiene showed little correlation with the mol ratio of B(C₆F₅)₃ to Ni. The use of other borane compounds such as B(C₆H₅)₃, BEt₃, and BF₃ etherate in place of B(C₆F₅)₃ clearly showed the two main functions of B(C₆F₅)₃ in the present catalyst. The high Lewis acidity of B(C₆F₅)₃ enabled it to activate catalytic complexes, thus inducing the polymerization. The steric bulkiness of B(C₆F₅)₃ suppressed chain transfer reactions, contributing to the production of polybutadiene with a high M_w. Kinetic studies showed that the catalyst had an induction period, possibly due to the time needed for the formation of catalytic complexes starting from Ni(COD)₂. A plot of ?ln (1?X), where X is the fractional conversion, as a function of time resulted in a linear relationship, showing that the present catalyst system followed first‐order kinetics with respect to monomer concentration. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1164–1173, 2004     

Cis-trans isomerization of polybutadiene double bonds during olefin metathesis

The cis-trans isomerization of polybutadiene double bonds during metathesis degradation using WCl₆/(CH₃)₄Sn catalyst system has been estimated kinetically along with productive metathesis. The isomerization was followed both for noncrosslinked and for crosslinked polybutadiene. Ninety-six percent cis-1, 4 units are found to isomerize into ca. 75% trans-1, 4 units. The rate of stereomutation is found to be different in the presence and absence of a low-molecular-weight olefin. The results are explained with the help of a stereo model originally proposed by Katz (Advances in Organometallic Chemistry, Academic, New York, 1977, Vol. 16.)     

Kinetics and mechanism of a macrocyclic chromium(III) complex oxidation to chromium(IV) by hexacyanoferrate(III) in strongly alkaline media

Oxidation of the macrocyclic Cr(III) complex cis-[Cr(cycb)(OH)₂]⁺, where cycb=rac-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, by an excess of the hexacyanoferrate(III) in basic solution, slowly produces Cr(V) species. These species, detected using e.p.r. spectroscopy, are stable under ambient conditions for many hours, and the hyperfine structure of the e.p.r. spectrum is consistent with the interaction of the d-electron with four equivalent nitrogen nuclei. Electro-spray ionization mass spectrometry suggests a concomitant oxidation of the macrocyclic ligand, in which double bonds and double bonded oxygen atoms have been introduced. By comparison basic chromate(III) solutions are oxidized rapidly to chromate(VI) by hexacyanoferrate(III) without any detectable generation of stable Cr(V) intermediates.Kinetics of oxidation of the macrocyclic Cr(III) complex in alkaline solution has been studied under excess of the reductant. Rate determining formation of Cr(IV) by a second order process involving the Cr(III) and the Fe(III) reactants is seen. This reaction also involves a characteristic higher order than linear dependence on the hydroxide concentration. Reaction mechanisms for the processes, including oxidation of the coordinated macrocyclic ligand – under excess of the oxidant- are proposed.     

A model of the kinetics of macrocyclic BPA carbonate formation

Cyclic oligocarbonates are synthesized by two-phase interfacial reactions of aryl bischloroformates in the presence of an amine phase transfer catalyst and an alkali: nClOCOArOCOCl + 4nNaOH → (OArOCO)_n + 2nNaCl + nNa₂CO₃ + 2nH₂O. The organocarbonate product may be a macrocyclic oligomer or a linear chain polymer. A qualitative mechanism for this behavior has been proposed by Brunelle, Boden, and co-workers. Four steps are identifiable: activation of the aryl bischloroformate by the amine catalyst, hydrolysis of a portion of this intermediate at the aqueous/organic phase interface, oligomerization between activated and hydrolyzed moieties also at the interface, and chain terminating carbamate formation that leads to polymer. An important modification is made within the framework of the Brunelle and Boden mechanism. While the intramolecular cyclization reaction is formally second-order overall, it behaves as a first-order process. The kinetic constants for both this pseudo-first-order cyclization step and the corresponding second-order linearization step are simply related. It is speculated that the above relationship can be generalized for a whole class of pseudo-first- and second-order rate constants for similar macrocyclic reactions.     

Hydrogen bonding between carboxylic acids and amide-based macrocycles in their host–guest complexes

For 12- and 13-membered macrocycles in which two amide linkages are integrated in the macrocyclic ring systems, the formation of 1:1 host–guest complexes with acetic and benzoic acids has been confirmed by NMR titrations. The complex formation occurs with the formation constants of 8–27 M^? 1, under competition with the dimerisation of acid molecules. Benzoic acid tends to form more stable complexes than acetic acid. The binding force is due to a pair of hydrogen bonds, O_carboxyl–H…O = C_amide and C = O_carboxyl…H–N_amide, between the carboxyl group of a guest molecule and the amide group of a host molecule. The former bond is stronger than the latter, and defines the stability of the complexes. The formation of the pair of hydrogen bonds is accompanied by the conformational conversion of the amide group from the trans-form to the cis-form. The influence of such a conversion on the internal molecular motion is observed as a slight broadening of signal width.

For 12- and 13-membered macrocycles in which two amide linkages are integrated in the macrocyclic ring systems, the formation of 1:1 host–guest complexes with acetic and benzoic acids has been confirmed by NMR titrations. The binding force for the complex formation is due to a pair of hydrogen bonds, O_carboxyl–H…O = C_amide and C = O_carboxyl…H–N_amide. The former bond is stronger than the latter and dominates the hydrogen-bond formation. The formation of the pair of hydrogen bonds is accompanied by the conformational conversion of the amide group from the stable trans-form to the less stable cis-form.     

Characterization of Polymer-Supported Rare-Earth Metal Complexes and Their Catalytic Behavior in Polymerization of Conjugated Dienes

The polymerization and catalytic behavior of catalyst systems composed of polymer-supported rare-earth metals were investigated. The catalyst systems show high catalytic activity and stereospecificity for butadiene polymerization. The catalytic efficiency for SMC (styrene-2-(methylsul-finyl)ethyl methacrylate copolymer).NdCl_3-Al(i-C₄H₉)₃ system is twice or three times that of the NdCl₃.4DMSO system. The activity of the ternary system SAAC (styrene-acrylic acid copolymer).Nd-Ph₃CCl-Al(i-C₄H₉)₃ was up to 170 kg polybutadiene/(g Nd-h). The cis-1,4 content of polybutadiene was more than 98%. This system was also used for isoprene polymerization. The cis-1,4 content of the polyisoprene obtained was about 96%     

Spontaneous Ring‐Opening Polymerization of Macrocyclic Aromatic Thioether Ketones under Transient High‐Temperature Conditions

Summary: Spontaneous ring‐opening polymerization of macrocyclic aromatic thioether ketones [ 1,4‐SC₆H₄COC₆H₄ ]_n (n = 3 and 4), in which the thioether linkages are para to the ketone, occurs during rapid, transient heating to 480 °C, to afford a soluble, semi‐crystalline poly(thioether ketone) of high molar mass (η_inh > 1.0 dL · g⁻¹). Corresponding macrocyclic ether ketones, and a macrocyclic thioether ether ketone in which the thioether linkage is para to the ether rather than to the ketone, show no evidence of polymerization under analogous conditions.

The uncatalysed ring‐opening polymerization of macrocycle 1 , within the pores of an alumina microfiltration membrane, leads to formation of polymer 3 with the microstructure shown in the above scanning electron micrograph.     

Terminal Alkyne Coupling Reactions Through a Ring: Effect of Ring Size on Rate and Regioselectivity

Terminal alkyne coupling reactions promoted by rhodium(I) complexes of macrocyclic NHC-based pincer ligands—which feature dodecamethylene, tetradecamethylene or hexadecamethylene wingtip linkers viz. [Rh(CNC-n)(C₂H₄)][BAr^F₄] (n=12, 14, 16; Ar^F=3,5-(CF₃)₂C₆H₃)—have been investigated, using the bulky alkynes HC≡CtBu and HC≡CAr’ (Ar’=3,5-tBu₂C₆H₃) as substrates. These stoichiometric reactions proceed with formation of rhodium(III) alkynyl alkenyl derivatives and produce rhodium(I) complexes of conjugated 1,3-enynes by C−C bond reductive elimination through the annulus of the ancillary ligand. The intermediates are formed with orthogonal regioselectivity, with E-alkenyl complexes derived from HC≡CtBu and gem-alkenyl complexes derived from HC≡CAr’, and the reductive elimination step is appreciably affected by the ring size of the macrocycle. For the homocoupling of HC≡CtBu, E-tBuC≡CCH=CHtBu is produced via direct reductive elimination from the corresponding rhodium(III) alkynyl E-alkenyl derivatives with increasing efficacy as the ring is expanded. In contrast, direct reductive elimination of Ar'C≡CC(=CH₂)Ar’ is encumbered relative to head-to-head coupling of HC≡CAr’ and it is only with the largest macrocyclic ligand studied that the two processes are competitive. These results showcase how macrocyclic ligands can be used to interrogate the mechanism and tune the outcome of terminal alkyne coupling reactions, and are discussed with reference to catalytic reactions mediated by the acyclic homologue [Rh(CNC-Me)(C₂H₄)][BAr^F₄] and solvent effects.