Late Transition Metal and Aluminum Complexes for the Polymerization of Ethene and Acrylates

Polyethene, polyacrylates and polymethyl acrylates are versatile materials that find wide variety of applications in several areas. Therefore, polymerization of ethene, acrylates and methacrylates has achieved a lot attention during past years. Numbers of metal catalysts have been introduced in order to control the polymerization and to produce tailored polymer structures. Herein an overview on the possible polymerization pathways for ethene, acrylates and methacrylates is presented. In this thesis iron(II) and cobalt(II) complexes bearing tri- and tetradentate nitrogen ligands were synthesized and studied in the polymerization of tertbutyl acrylate (tBA) and methyl methacrylate (MMA). Complexes are activated with methylaluminoxane (MAO) before they form active combinations for polymerization reactions. The effect of reaction conditions, i.e. monomer concentration, reaction time, temperature, MAO to metal ratio, on activity and polymer properties were investigated. The described polymerization system enables mild reaction conditions, the possibility to tailor molar mass of the produced polymers and provides good control over the polymerization. Moreover, the polymerization of MMA in the presence of iron(II) complex with tetradentate nitrogen ligands under conditions of atom transfer radical polymerization (ATRP) was studied. Several manganese(II) complexes were studied in the ethene polymerization with combinatorial methods and new active catalysts were found. These complexes were also studied in acrylate and methacrylate polymerizations after MAO activation and converted into the corresponding alkyl (methyl or benzyl) derivatives. Combinatorial methods were introduced to discover aluminum alkyl complexes for the polymerization of acrylates and methacrylates. Various combinations of aluminum alkyls and ligands, including phosphines, salicylaldimines and nitrogen donor ligands, were prepared in situ and utilized to initiate the polymerization of tBA. Phosphine ligands were found to be the most active and the polymerization MMA was studied with these active combinations. In addition, a plausible polymerization mechanism for MMA based on ESI-MS, 1H and 13C NMR is proposed.

Asymmetric Diels-Alder Reactions of Chiral Acrylates of Cholic Acid Derivatives

New steroid-based chiral auxiliaries 6, 9, and 12 have been synthesized from readily available cholic acid. These new chiral auxiliaries place the reactive and the shielding sites in a 1,5 relationship to each other. Diels-Alder reaction of cyclopentadiene with corresponding acrylate esters (7, 10, and 13) have been examined. Acrylates 7 and 10 yielded cycloadducts with 29-88% diastereomeric excess with excellent endo selectivity in the presence of an excess of Lewis acids such as AlCl3, BF3.OEt(2), FeCl3, SnCl4, TiCl4, and ZnCl2. Treatment of acrylate 7 with cyclopentadiene in the presence of BF3.OEt(2) at -80 degrees C gave the endo adduct (>99%) with 88% de. Lewis acid catalyzed and uncatalyzed reactions of acrylates 7 and 10 with cyclopentadiene yielded cycloadducts with opposite stereochemistry. The chiral auxiliary was recovered in a nondestructive manner only via iodolactonization. Acrylate ester of alcohol 12 did not show any selectivity in either catalyzed and uncatalyzed reactions with cyclopentadiene. The presence of a flat aromatic surface at C-7 of the steroid was found to be essential to effect high diastereoselection.

Determination of the reactivity ratios for the oxidative copolymerizations of indene with methyl, ethyl and butyl acrylates

Full Paper: The copolyperoxides of various compositions of indene with methyl acrylate, ethyl acrylate and butyl acrylate have been synthesized by the free-radical-initiated oxidative copolymerization. The compositions of copolyperoxide obtained from H-1 and C-13 NMR spectra have been used to determine the reactivity ratios of the monomers. The copolyperoxides contain a greater proportion of the indene units in random placement. The NMR studies have shown irregularities in the copolyperoxide chain due to the cleavage reactions of the propagating peroxide radical. The thermal analysis by differential scanning calorimetry suggests alternating peroxide units in the copolyperoxide chain. From the activation energy for the thermal degradation, it was inferred that degradation occurs via the dissociation of the peroxide (O-O) bonds of the copolyperoxide chain. The flexibility of the polyperoxides in terms of glass transition temperature (T-g) has also been examined.

Thermal and Sonochemical Degradation Kinetics of Poly(n-butyl methacrylate-co-alkyl acrylates): Variation of Chain Strength and Stability with Copolymer Composition

The thermal degradation of poly(n-butyl methacrylate-co-alkyl acrylate) was compared with ultrasonic degradation. For this purpose, different compositions of poly (n-butyl methacrylate-co-methyl acrylate) (PBMAMA) and a particular composition of poly(n-butyl methacrylate-co-ethyl acrylate) (PBMAEA) and poly(n-butyl methacrylate-co-butyl acrylate) (PBMABA) were synthesized and characterized. The thermal degradation of polymers shows that the poly(alkyl acrylates) degrade in a single stage by random chain scission and poly(n-butyl methacrylate) degrades in two stages. The number of stages of thermal degradation of copolymers was same as the majority component of the copolymer. The activation energy corresponding to random chain scission increased and then decreased with an increase of n-butyl methacrylate fraction in copolymer. The effect of methyl acrylate content, alkyl acrylate substituent, and solvents on the ultrasonic degradation of these copolymers was investigated. A continuous distribution kinetics model was used to determine the degradation rate coefficients. The degradation rate coefficient of PBMAMA varied nonlinearly with n-butyl methacrylate content. The degradation of poly (n-butyl methacrylate-co-alkyl acrylate) followed the order: PBMAMA < PBMAEA < PBMABA. The variation in the degradation rate constant with composition of the copolymer was discussed in relation to the competing effects of the stretching of the polymer in solution and the electron displacement in the main chain. (C) 2012 Society of Plastics Engineers

Designing Temperature-Responsive Biocompatible Copolymers and Hydrogels Based on 2-Hydroxyethyl(meth)acrylates

Free-radical copolymerization of 2-hydroxyethyl methacrylate with 2-hydroxyethyl acrylate can be successively utilized for the synthesis of water-soluble polymers and hydrogels with excellent physicochemical properties, thus showing promise for pharmaceutical and biomedical applications. In the work presented it has been demonstrated that water-soluble copolymers based on 2-hydroxyethyl methacrylate and 2-hydroxyethyl acrylate exhibit lower critical solution temperature in aqueous solutions, whereas the corresponding high molecular weight homopolymers do not have this unique property. The temperature-induced transitions observed upon heating the aqueous solutions of these copolymers proceed via liquid−liquid phase separation. The hydrogels were also synthesized by copolymerizing 2-hydroxyethyl methacrylate and 2-hydroxyethyl acrylate in the absence of a bifunctional cross-linker. The cross-linking of these copolymers during copolymerization is believed to be due to the presence of bifunctional admixtures or transesterification reactions. Transparency, swelling behavior, mechanical properties, and porosity of the hydrogels are dependent upon the monomer ratio in the copolymers. Hydrogel samples containing more 2-hydroxyethyl methacrylate are less transparent, have lower swelling capacity, higher elastic moduli, and pores of smaller size. The assessment of the biocompatibility of the copolymers using the slug mucosal irritation test revealed that they are also less irritant than poly(acrylic acid).

Hydrotelluration of acetylenic esters: structural characterization of stereoisomers of methyl/ethyl beta-(aryltelluro)acrylates

Synthesis and complete characterization of some ester functionalized vinylic tellurides bearing an aryl ligand with varying steric and electronic effects bound to tellurium is described. Hydrotelluration of methyl propiolate using Ar2Te2/NaBH4 in methanol results in a mixture of stereoisomers of methyl β-(aryltelluro)acrylates, ArTeCH[double bond, length as m-dash]CHCOOMe (Ar = 4-MeOC6H4, 1A; 1-C10H7, 2A; 2,4,6-Me3C6H2, 3A; C5H5FeC5H4, 4A; 4-Me2NC6H4, 5A; and 2-C4H3S, 6A). The same reaction in ethanol provides isomeric mixtures of the ethyl esters ArTeCH[double bond, length as m-dash]CHCOOEt (1B–6B). However, in the reactions between methyl propiolate and Ar2Te2 (Ar = 2,4,6-Me3C6H2, 4-Me2NC6H4) in isopropanol or t-butanol, no exchange of alkyl groups between the parent ester and the solvent is observed, instead detelluration of the Ar2Te2 to Ar2Te is a competing reaction along with almost exclusive formation of the (Z)-isomers (3Aa, 5Aa). The geometry of the separated stereoisomers is established in solution, with the help of 1H, 13C and 125Te NMR spectrometry. Of particular interest is the observation that 125Te chemical shifts {deshielded in (Z) compared to (E); Δδ = 106–136 ppm} and the geminal heteronuclear coupling constants {2J(1H–125Te) values for (E) are more than seven times that of the corresponding (Z) isomer} can be used to distinguish between liquid isomers. Structural characterization in the solid state by single-crystal X-ray diffraction for the 2Ba, 3Aa, 3Ba, 5Aa, 8 (Z)-isomers as well as for both stereoisomers of 4-Me2NC6H4TeCH[double bond, length as m-dash]CHCOOEt (5Ba and 5Bb) is also presented. The carbonyl O atom of the ester group is invariably involved, at least in the solid state, in a secondary bonding interaction with the Te(II) atom. While an intermolecular Te⋯O interaction gives rise to one-dimensional supramolecular arrays in the crystal lattice of 5Bb with (E) configuration, it is realized intramolecularly in the case of the (Z)-isomers due to the cis position of the chalcogen atoms.

Ten reasons why rigid contact lenses are obsolete

There have only been minor improvements in rigid lens material developments since silicone acrylates and fluoro-silicone acrylates were introduced over a quarter of a century ago. Although there have been enhancements in mechanical lathing technology in the rigid lens field - primarily as a result of developments in computer-controlled systems - rigid lenses are still manufactured using labour-intensive lathing processes, which is why the lens unit cost remains much higher than for disposable soft lenses.

Optimization of rheological properties of photopolymerizable alumina suspensions for ceramic microstereolithography

Microstereolithography (MSL) is a rapid prototyping technique to fabricate complex three-dimensional (3D) structure in the microdomain involving different materials such as polymers and ceramics. The present effort is to fabricate microdimensional ceramics by the MSL system from a non-aqueous colloidal slurry of alumina. This slurry predominantly consists of two phases i.e. sub-micrometer solid alumina particles and non-aqueous reactive difunctional and trifunctional acrylates with inert diluent. The first part of the work involves the study of the stability and viscosity of the slurry using different concentrations of trioctyl phosphine oxide (TOPO) as a dispersant. Based on the optimization, the highest achievable solid loadings of alumina has been determined for this particular colloidal suspension. The second part of the study highlights the fabrication of several micro-dimensional alumina structures by the MSL system. (C) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

The application of trimethylsilyl diazomethane to the synthesis of optically active pyrazolines and related studies

The 1,3-dipolar cycloadditions of trimethylsilyl diazomethane with camphorsultam-derived acrylates are reported as a means for the efficient synthesis of optically active pyrazolines. Trimethylsilyl diazomethane is a safe, commercially available diazoalkane which provides Δ¹-pyrazolines 1n good yield and diastereoselectivity when camphorsultam-derived acrylates are used as the reaction dipolarophiles . These initial cycloadducts are subsequently converted to stable, characterizable Δ²-pyrazolines upon desilylation.

A manifold of reactions that can be applied to these Δ²-pyrazolines has been developed which includes pyrazoline reduction, N-N bond reduction, addition to the pyrazoline C=N by mild carbon nucleophiles, and both solvolytic and reductive chiral auxiliary removal. Additionally, it has been demonstrated that the pyrazoline reduction products can take part in peptide coupling reactions that allow for the pyrazolidines to serve as proline-like molecules. The development of this methodology is a general solution to the problem of highly substituted, functionalized pyrazoline synthesis. Importantly, the pyrazolines thus provided have been demonstrated to be amenable to reactions that add to their value as synthetic intermediates.

Selectivity, activity, and stability of ruthenium-carbene based olefin metathesis initiators

The olefin metathesis reaction has found many applications in polymer synthesis and more recently in organic synthesis. The use of single component late metal olefin metathesis catalysts has expanded the scope of the reaction to many new applications and has allowed for detailed study of the catalytic species.

The metathesis of terminal olefins of different steric bulk, different geometry as well as electronically different para-substituted styrenes was studied with the ruthenium based metathesis initiators, trans-(PCy₃)₂Cl₂Ru=CHR, of different carbene substituents. Increasing olefin bulk was found to slow the rate of reaction and trans internal olefins were found to be slower to react than cis internal olefins. The kinetic product of a11 reactions was found to be the alkylidene, rather than the methylidene, suggesting the intermediacy of a 2,4-metallacycle. The observed effects were used to explain the mechanism of ring opening cross metathesis and acyclic diene metathesis polymerization. No linear electronic effects were observed.

In studying the different carbene ligands, a series of ester-carbene complexes was synthesized. These complexes were found to be highly active for the metathesis of olefinic substrates, including acrylates and trisubstituted olefins. In addition, the estercarbene moiety is thermodynamically high in energy. As a result, these complexes react to ring-open cyclohexene by metathesis to alleviate the thermodynamic strain of the ester-carbene ligand. However, ester-carbene complexes were found to be thermolytically unstable in solution.

Thermolytic decomposition pathways were studied for several ruthenium-carbene based olefin metathesis catalysts. Substituted carbenes were found to decompose through bimolecular pathways while the unsubstituted carbene (the methylidene) was found to decompose unimolecularly. The stability of several derivatives of the bis-phosphine ruthenium based catalysts was studied for its implications to ring-closing metathesis. The reasons for the activity and stability of the different ruthenium-based catalysts is discussed.

The difference in catalyst activity and initiation is discussed for the bis-phosphine based and mixed N-heterocyclic carbene/phosphine based ruthenium olefin metathesis catalysts. The mixed ligand catalysts initiate far slower than the bis-phosphine catalysts but are far more metathesis active. A scheme is proposed to explain the difference in reactivity between the two types of catalysts.

Enantioselective Total Synthesis of Diketopiperazinecontaining Natural Products: (–)-Lansai B, (+)-Nocardioazines A and B, and (–)-Acetylapoaranotin

Diketopiperazine (DKP) motif is found in a wide range of biologically active natural products. This work details our efforts toward two classes of DKP-containing natural products.

Class one features the pyrroloindoline structure, derived from tryptophans. Our group developed a highly enantioselective (3 + 2) formal cycloaddition between indoles and acrylates to provide pyrroloindoline products possessing three stereocenters. Utilizing this methodology, we accomplished asymmetric total synthesis of three natural products: (–)-lansai B, (+)-nocardioazines A and B. Total synthesis of (–)-lansai B was realized in six steps, and featured an amino acid dimerization strategy. The total synthesis of (+)-nocardioazine B was also successfully completed in ten steps. Challenges were met in approaching (+)-nocardioazine A, where a seemingly easy last-step epoxidization did not prove successful. After re-examining our synthetic strategy, an early-stage epoxidation strategy was pursued, which eventually yielded a nine-step total synthesis of (+)-nocardioazine A.

Class two is the epidithiodiketopiperazine (ETP) natural products, which possesses an additional episulfide bridge in the DKP core. With the goal of accessing ETPs with different peripheral structures for structure-activity relationship studies, a highly divergent route was successfully developed, which was showcased in the formal synthesis of (–)-emethallicin E and (–)-haematocin, and the first asymmetric synthesis of (–)-acetylapoaranotin.

Preparation and chracaterization of novel graft copolymers of LLDPE

A series of novel fluorine surfactants, a, b, c, d, e and their acrylates, A, 13, C, D and E, were synthesized via poly( ethylene oxide) ( PEG) ( 200, 600, 1000, 2000, 5000) and perfluorooctane poly (ethylene oxide) ether as the main starting materials. Their chemical structures were characterized by means of FTIR and H-1 NMR. The surface activity and surface tension( y) of surfactants a, b, c, d and e were evaluated by maximum bubble pressure method. Surfactants A, 13, C, D and E were adopted as the grafting monomers of linear low density polyethere( LLDPE), and grafting reaction was carried out by melt reactive extrusion procedure. Their surface properties were characterized with measuring contact angle and XPS. It was found that the hydrophilic property of the graft copolymers was better than the palin LLDPE. Thermal properties of graft copolymers were studied by DSC. It was found that their crystalline temperatures of graft copolymers were faster than that of the plain LLDPE.

Inkless microcontact printing on SAMs of Boc- and TBS-protected thiols.

We report a new inkless catalytic muCP technique that achieves accurate, fast, and complete pattern reproduction on SAMs of Boc- and TBS-protected thiols immobilized on gold using a polyurethane-acrylate stamp functionalized with covalently bound sulfonic acids. Pattern transfer is complete at room temperature just after one minute of contact and renders sub-200 nm size structures of chemically differentiated SAMs.