Chain-growth Propagation Research Articles

Synthesis of Polydiynes via an Unexpected Dimerization/Polymerization Sequence of C3 Propargylic Electrophiles.

Here, we describe the unexpected discovery of a Cu-catalyzed condensation polymerization reaction of propargylic electrophiles (CPPE) that transforms simple C3 building blocks into polydiynes of C6 repeating units. This reaction was achieved by a simple system composed of a copper acetylide initiator and an electron-rich phosphine ligand. Alkyne polymers (up to 33.8 kg/mol) were produced in good yields and exclusive regioselectivity with high functional group compatibility. Hydrogenation of the product afforded a new polyolefin-type backbone, while base-mediated isomerization led to a new type of dienyne-based electron-deficient conjugated polymer. Mechanistic studies revealed a new α-α selective Cu-catalyzed dimerization pathway of the C3 unit, followed by in situ organocopper-mediated chain-growth propagation. These insights not only provide an important understanding of the Cu-catalyzed CPPE of C3, C4, and C6 monomers in general but also lead to a significantly improved synthesis of polydiynes from simpler starting materials with handles for the incorporation of an α-end functional group.

Sequence-Controlled Synthesis of Advanced Clickable Synthetic Oligonucleotides.

Through thiol-ene photopolymerization of presynthesized oligomers, advanced clickable nucleic acids (CNA-2G) are synthesized with sequence-controlled repeating units. As examples, poly(thymine-adenine) (polyTA) CNA-2G and poly(thymine-thymine-cytosine) CNA-2G are synthesized by polymerizing thiol-ene heterofunctional dimers with pendant thymine-adenine nucleobases and trimer with pendant thymine-thymine-cytosine nucleobases. Based on size exclusion chromatography (SEC) analysis, polyTA and polyTTC have number average molecular weights of 2000 and 1800, respectively, which contain 7-8 pendant nucleobases. Based on the different behavior of the CNA-2G monomers and CNA-2G oligomers with two or more pendant nucleobases in photopolymerization, an unusual thiol-ene chain-growth propagation mechanism is observed for the former and a common thiol-ene step-growth propagation mechanism for the latter. The uncommon thiol-ene chain-growth propagation is hypothesized to rely on a six-membered ring mediated intramolecular hydrogen atom transfer process.

Activation Energies for Chain Growth Propagation and Termination in Fischer–Tropsch Synthesis on Iron Catalyst as a Function of Catalyst Particle Size

The influence of the catalyst particle size in determining Fischer–Tropsch synthesis (FTS) performance for nano-structured iron catalysts was investigated. The catalysts were prepared by a microemulsion method and to achieve a series of catalysts with different iron particle size, the water-to-surfactant molar ratio (W/S) in the microemulsion system varied from 4 to 12. The results demonstrate that by decreasing the levels of active phase of the iron catalyst, the termination rates for chain growth are increased compared to the propagation rates. In addition, the activation energy for chain propagation is lower than for chain termination, and this difference (Et – Ep) for the hydrocarbon product distributions which is characterised by α1, is lower than the hydrocarbon product distribution which is characterised by α2 The results indicate the H2 concentration on the catalyst surface is decreased by increasing the catalyst particle size. Thus, the dependence of α (α1, and/or α2) on H2 partial pressures is increased by decreasing of catalyst particle size and the dependence of α2 on H2 partial pressures is weaker than for α1.

A strategy for sequence control in vinyl polymers via iterative controlled radical cyclization.

There is a growing interest in sequence-controlled polymers toward advanced functional materials. However, control of side-chain order for vinyl polymers has been lacking feasibility in the field of polymer synthesis because of the inherent feature of chain-growth propagation. Here we show a general and versatile strategy to control sequence in vinyl polymers through iterative radical cyclization with orthogonally cleavable and renewable bonds. The proposed methodology employs a repetitive and iterative intramolecular cyclization via a radical intermediate in a one-time template with a radical-generating site at one end and an alkene end at the other, each of which is connected to a linker via independently cleavable and renewable bonds. The unique design specifically allowed control of radical addition reaction although inherent chain-growth intermediate (radical species) was used, as well as the iterative cycle and functionalization for resultant side chains, to lead to sequence-controlled vinyl polymers (or oligomers).

Stereoengineering of Poly(1,3-methylenecyclohexane) via Two-State Living Coordination Polymerization of 1,6-Heptadiene

External control over the rate of dynamic methyl group exchange between configurationally stable active species and configurationally unstable dormant species with respect to chain-growth propagation within a highly stereoselective and regiospecific living coordination polymerization of 1,6-heptadiene has been used to generate a spectrum of different physical forms of poly(1,3-methylenecyclohexane) (PMCH) in which the stereochemical microstructure has been systematically varied between two limiting forms. The application of this strategy to manipulate the bulk properties of PMCH and the solid-state microphase behavior of well-defined PMCH-b-poly(1-hexene) block copolymers is further demonstrated.

Dinuclear Bis-Propagators for the Stereoselective Living Coordinative Chain Transfer Polymerization of Propene

Modulation of steric interactions remote from the active sites within a series of dinuclear bis-propagators derived from racemic 2-4 was used to attenuate the rate of reversible chain transfer between active transition-metal centers and excess equivalents of inactive main-group-metal alkyl species relative to chain growth propagation, as a strategy for achieving the stereoselective living coordinative chain transfer polymerization of propene to provide isotactic stereoblock polypropene. Under identical conditions, the corresponding mononuclear propagator derived from racemic 1 produced only atactic polypropene.

A potassium-promoted Mo carbide catalyst system for hydrocarbon synthesis

Potassium-promoted Mo carbide catalysts prepared by temperature-programmed carburization with H2–C3H8 have been evaluated for Fischer–Tropsch synthesis (FTS). Temperature-programmed carburization of MoO3–Al2O3 with a mixture of H2–C3H8 was a two-stage conversion involving the transformation of Mo oxide to the final carbide phase via an intermediate oxycarbide phase. Compensation effect and isokinetic relationship were observed for both oxycarbide and carbide phase formation suggesting that a similar topotactic mechanism was involved in the solid phase transformation. CO seemed to adsorb more strongly than H2 as implicated by its higher heat of desorption and site concentration. Both acid and basic centres were detected on the Mo carbide surface. The K-promoter increased site concentration for both weak and strong basic sites. The relationship between FT activity and K loading paralleled the trend in CO2 heat of desorption with K addition. FT activity attained a maximum at about 3%K loading. CO hydrogenation over the Mo carbide catalyst was most suitably described by an enolic intermediate mechanism. A generalized kinetic model was derived for predicting the reaction rate for individual hydrocarbons as a function of chain growth propagation and olefin-to-paraffin ratio.

LaCo 1− xCu xO 3− δ perovskite catalysts for higher alcohol synthesis

Perovskite-type materials LaCoO 3, LaCo 1− x Cu x O 3− δ , and Cu 2O/LaCoO 3 were synthesized by the mechano-synthesis process known as reactive grinding. Their characterization was performed by BET, X-ray diffraction, SEM, O 2-TPD, and H 2-TPR. The partial cobalt substitution by copper in perovskite lattice gives rise to a distorted structure of LaCoO 3 and influences the thermal stability and redox properties of perovskites. Temperature-programmed reduction (TPR) analysis showed a complete reduction of Co 3+, Cu 2+ to metallic state in the temperature range of 310–580 °C. A lower cobalt reduction temperature observed for LaCo 1− x Cu x O 3− δ in comparison to LaCoO 3 produces a finely dispersed bimetal on a La 2O 3 support. Quantitative TPR data showed that cobalt ions in the grain boundaries of the ground perovskites are directly reduced to metals at a relatively low temperature (310–450 °C). The existence of strong cobalt–copper interaction in perovskites could enhance the metallic dispersion of cobalt and prevent copper sintering. The reduced forms of LaCo 1− x Cu x O 3− δ catalysts were tested as alcohol synthesis catalysts in a fixed bed flow reactor system at 275 °C and space velocity of 4000 h −1 (H 2/CO = 2/1). A mixture of C 1–C 7 alcohols with the chain growth propagation factors of 0.34–0.42 was produced. The alcohol productivity is from 36.5 to 49.6 mg/g cat/h and selectivity towards higher alcohols took values in the range of 40–49.5%. The preliminary catalytic data indicated that copper located outside of the perovskite lattice is solely leading to the production of methanol and methane whereas its location in the octahedral position of the perovskite precursor framework is necessary for higher alcohol synthesis. Therefore, a uniform distribution of the metallic cobalt–copper atoms in the prereduced catalysts is crucial for the conversion of carbon monoxide and hydrogen into higher alcohols.

Electro-optical and Morphological Properties of Bragg Transmission Gratings Written In Holographic Polymer Dispersed Liquid Crystals by Thiol-ene Photopolymerization

AbstractHolographic Bragg transmission gratings formed via the anisotropic phase separation of nanosized LC droplets (H-PDLC) were fabricated using thiol-ene photopolymerization. Using coherent UV laser light and a single prism, electrically switchable transmission gratings in the Bragg regime were written. The performance of the thiol-ene-based gratings were compared with those of multifunctional acrylate-based gratings written under similar conditions. Optical and electro-optical measurements suggest that thiol-ene polymers offer promise as hosts for improved H-PDLC performance. Interesting differences in the diffraction efficiencies for s– and p-probe beams are noted for the two matrices. Morphology studies by TEM and SEM exhibit striking differences in droplet shape and uniformity. These differences are speculated to be due to differences in polymer MW growth due to the step-growth propagation mechanism for thiol-enes as compared to the chain-growth propagation mechanism in multifunctional acrylates. The response times of the thiol-ene gratings were ten times slower than those of acrylates.

Hydrocarbon selectivity model for the slurry phase Fischer–Tropsch synthesis on precipitated iron catalysts

A new hydrocarbon product distribution model for Fischer–Tropsch-synthesis (FTS) reaction has been developed. The model is based on the assumption that there are two types of active sites on the catalyst surface: type-1, where growth of hydrocarbon intermediates occurs, and type-2, where reversible readsorption of 1-olefins occurs. The readsorbed 1-olefins form alkyl intermediates, C n H 2 n+1 , on the surface, which in turn can participate in several reactions: chain growth propagation, hydrogenation to n-paraffins, and dehydrogenation to 2-olefins. Chain growth is considered on the basis of methylene monomer (CH 2) insertion mechanism. Steady-state mass balances for adsorbed intermediates C 1–C 50 and equations for the product formation rates were derived. The model has been tested using results from experiments conducted over long periods of time under industrially relevant conditions in a slurry reactor. The model predictions provide information on hydrocarbon product distribution, 2-olefin content and total olefin content as a function of carbon number. The curved line for the distribution of paraffins and olefins can be fitted well with the proposed hydrocarbon selectivity model, over the entire range (C 1–C 50) of carbon numbers. The kinetic model proposed in this paper can be used for interpretation of experimental data, comparison of performance of different catalysts, and reactor modelling and simulation studies.