New chain transfer agents for reversible addition-fragmentation chain transfer (RAFT) polymerisation in aqueous solution

New chain transfer agents for free radical polymerisation via reversible addition-fragmentation chain transfer (RAFT) were synthesised that are particularly suited for aqueous solution polymerisation. The new compounds bear dithioester and trithiocarbonate moieties as well as permanently ionic groups to confer solubility in water. Their stability against hydrolysis was studied, and compared with the one of a frequently employed water-soluble RAFT agent, using UV-Vis-spectroscopy and ¹H-NMR measurements. An improved resistance to hydrolysis was found for the new RAFT agents compared to the reference one, providing good stabilities in the pH range between 1 and 8, and up to temperatures of 70 °C.     

Nanographenes as active components of single-molecule electronics and how a scanning tunneling microscope puts them to work

Single-molecule electronics, that is, realizing novel electronic functionalities from single (or very few) molecules, holds promise for application in various technologies, including signal processing and sensing. Nanographenes, which are extended polycyclic aromatic hydrocarbons (PAHs), are highly attractive subjects for studies of single-molecule electronics because the electronic properties of their flat conjugated systems can be varied dramatically through synthetic modification of their sizes and topologies. Single nanographenes provide high tunneling currents when adsorbed flat onto conducting substrates, such as graphite. Because of their chemical inertness, nanographenes interact only weakly with these substrates, thereby preventing the need for special epitaxial structure matching. Instead, self-assembly at the interface between a conducting solid, such as the basal plane of graphite, and a nanographene solution generally leads to highly ordered monolayers. Scanning tunneling spectroscopy (STS) allows the current-voltage characteristics to be measured through a single molecule positioned between two electrodes; the key to the success of STS is the ability to position the scanning tunneling microscopy (STM) tip freely with respect to the molecule in all dimensions, that is, both parallel and perpendicular to the surface. In this Account, we report the properties of nanographenes having sizes ranging from 0.7 to 3.1 nm and exhibiting various symmetry, periphery, and substitution types. The size of the aromatic system and the nature of its perimeter are two essential features affecting its HOMO-LUMO gap and charge carrier mobility in the condensed phase. Moreover, the extended pi area of larger substituted PAHs improves the degree of self-ordering, another key requirement for high-performance electronic devices. Self-assembly at the interface between an organic solution and the basal plane of graphite allows deposition of single molecules within the well-defined environment of a molecular monolayer. We have used STM and STS to investigate both the structures and electronic properties of these single molecules in situ. Indeed, we have observed key electronic functions, rectification and current control through single molecules, within a prototypical chemical field-effect transistor at ambient temperature. The combination of nanographenes and STM/STS, with the PAHs self-assembled in oriented molecular mono- or bilayers at the interface between an organic solution and the basal plane of graphite and contacted by the STM tip, is a simple, reliable, and versatile system for developing the fundamental concepts of molecular electronics. Our future targets include fast reversible molecular switches and complex molecular electronic devices coupled together from several single-molecule systems.     

Computational studies of the Brookhart's type catalysts for ethylene polymerisation. Part 2: ethylene insertion and chain transfer mechanisms

This report describes complete density functional theory studies performed to elucidate ethylene polymerisation mechanisms of the Brookhart-type catalysts N,N′-(2,6-dimethylphenyl)ethylenediimine nickel (II) and N-(2,6-dimethylphenyl)pyridine-2-carboxaldiimine nickel (II). Both catalysts showed conformations that blocked the active site and thus hindered ethylene coordination. These conformations were related to the rotational capacity of the ancillary aryl group attached to the ligands, which generates agostic interactions between nickel and hydrogen atoms. It was calculated that these conformations were 8.3 and 10.7 kcal/mol (respectively for each catalyst) more stable than non-blocking cationic conformations. From a design perspective, catalytic systems would need to include bulky constituents to avoid these types of blocking conformation. The two possible mechanisms already proposed for the chain initiation step evoke approaches for ethylene inside and out of the equatorial plane of the catalyst. Herein, the in-plane mechanism was found to be the most favourable. However, for chain propagation, the out-plane approach for ethylene proved to be the most feasible, due to nickel agostic interactions with the growing alkyl chain. The energy barriers calculated for the propagation step were 9.3 and 10.6 kcal/mol for the two catalysts, respectively. These findings indicate the ethylenediimine catalyst is slightly more active than the pyridine catalyst. Three possible chain transfer mechanisms were also considered for the two catalysts: β-hydride transfer, β-hydride transfer to monomer and an association mechanism; the last two being the mostly likely mechanisms in thermodynamics and kinetic terms. Chain transfer processes for the pyridine-2-carboxaldiimine based catalyst were found to be more favourable than those of the ethylenediimine based catalyst. This finding is in agreement with experimental results which indicate that the former catalyst gives rise to lower molecular weight polymers.     

Emulsion polymerisation of vinyl acetate in relation to the chemical structure of polyvinyl alcohol

The effects of content and sequence distribution of residual acetyl groups in the polymer chain, and carbonyl content of polyvinyl alcohol, on the emulsion polymerisation of vinyl acetate in the presence of polyvinyl alcohol were investigated. Properties of modified polyvinyl alcohol grades as a protective colloid were also studied. Results show that the viscosity, particle size, stability and the freeze-thaw stability of the emulsion are determined largely by the fine chemical structure of the polyvinyl alcohol used in the polymerisation.     

Emulsion polymerisation of vinyl chloride in relation to the chemical structure of the emulsifier

The influence of the chemical structure of the emulsifier on the rate of polymerisation, the yield and the properties of the polymer in the emulsion polymerisation of vinyl chloride was studied. The highest molecular weights were obtained with the sodium salts of the sulphuric acid esters of fatty alcohols with a chain length of 10 and 18 C atoms. In the group of aliphatic sulphonates, sulphates and salts of fatty acids there was no great difference. In general the effect of emulsifiers increases with the chain length. Unsaturated emulsifiers decrease the rate of polymerisation. Non-ionic emulsifiers are less active than ionic ones.     

Polyaniline/porous carbon electrodes by chemical polymerisation: Effect of carbon surface chemistry

Polyaniline/porous carbon composite electrodes were prepared by chemical polymerisation and characterized in terms of porosity and performance as electrochemical capacitors.To obtain the composite electrodes two methods were used. The first method consisted of mixing, directly, the activated carbon with chemically polymerised polyaniline. The second one consisted of mixing the activated carbon with aniline and subsequent chemical polymerisation. Additionally, the second process was carried out with the porous carbon previously thermally treated in N₂ up to 900 °C in order to remove surface oxygen groups.Changes in porosity with the polyaniline addition were analysed. It has been proved that the method used strongly affects the porous structure. Dealing with the electrochemical performance, polyaniline and carbon mechanically mixed seem to work independently, being the composite behaviour a combination of the corresponding performance of both materials separately. The composites prepared by the second method (polymerisation over carbon) reveal the key role of surface chemistry in polyaniline coating. Aniline reacts with the oxygen complexes and their positive effect in capacitance is not observed.The second method (polymerisation over carbon) using a thermally treated carbon seems to be the best one since a more porous (or thinner) polyaniline film is produced.     

High temperature non-aqueous dispersion polymerisation of aromatic main chain liquid crystal polymers using organo-clay stabilisation

A high-temperature non-aqueous dispersion polymerisation (NAD) route to poly(4-oxybenzoate-co-4-phenylene isophthalate) liquid crystal polymers (LCP) is described. Stable dispersions were obtained in the presence of organo-clay stabilisers. The use of organo-clay stabilisers is discussed and the advantages of the NAD route are contrasted with traditional melt polycondensation routes.     

Electroless Co-P for diffusion barrier in Pb-free soldering

Lead-free solders with high tin content and high melting temperature limit the reliability of electroless nickel/immersion gold finish on copper as diffusion barrier. Autocatalytic cobalt-phosphorus is proposed as a barrier metallization for copper in lead-free soldering. Autocatalytic deposition of cobalt is carried out in hypophosphite containing electrolytes at 95 °C and pH 8. Autocatalytic Co-P/Au finish with about 4 wt% P content strongly limits interdiffusion and intermetallic compounds formation with respect to the Ni-P/Au finish with Sn-Pb and Sn-Ag-Cu solder alloys. Contact angle of Sn-Pb solder alloy with Ni-P/Au and Co-P/Au layers is comparable, while in the case of Sn-Ag-Cu alloy contact angle is much lower for Co-P/Au than for Ni-P/Au layers. Mechanical strength of lead-free joints for Ni-P/Au and Co-P/Au finishes is evaluated with shear test on BGA coupons, obtaining higher joint strength values for autocatalytic cobalt.     

废旧电子产品迎来新的商机

【《亚洲塑料橡胶》2003第9期文】现在,在许多产品设计中都整体考虑到如何利用塑料,以使现今的高性能电器产品更结实、更耐用,延长它们的使用寿命。但当它们使用终了后,面临的是怎么样的问题呢?     

Fillers for electrical/electronics

Although it consumes only about 5–7% of plastics, the electrical and electronics market sector exercises large demands on additives, which will certainly grow in volume and value. Covering both consumer and industrial products, E and E involves housings and enclosures for all types of equipment and an increasing volume of moulded connectors and circuitry. John Murphy reports.     

Development of new initiators for polymerisation reactions

The development of simple, stable, and well defined catalyst systems for the initiation of ionic polymerisation is reviewed. The principles involved are illustrated by reference to the use of potassium t-butoxide in dimethyl sulphoxide for initiating anionic polymerisation of ethylene oxide and of the hexachloroantimonate salt of the triphenyl methyl cation for polymerisation of tetrahydrofuran. Other stable organic cations initiate polymerisation via charge transfer complexes and can give rise to cation-radicals as intermediates. It is suggested that related cation-radicals may play a role in lignin biosynthesis.     

Influence of moisture on the photochemically induced polymerisation of epoxy groups in different chemical environment

The influence of atmospheric humidity on the photochemically induced cationic polymerisation of epoxy groups in different chemical environments is shown. With increasing humidity, the polymerisation rate of glycidylethers decreases, whereas for epoxides with endocyclic epoxy groups the rate of polymerisation increases and the rate for epoxides with exocyclic epoxy groups is nearly not influenced. The reasons for the different behaviour are energetic differences for the reactions between the positively charged chain ends with water and the following proton transfer. Furthermore the reaction rate is influenced by the different stereochemistry of the attack of the next monomer.     

DFT study of hydrogenolysis as a chain transfer mechanism in olefin polymerisation catalysed by nickel-diimine-type catalysts

This report describes a computational DFT study, based on the hybrid density functional B3LYP method, designed to investigate the hydrogenolysis process that occurs in ethylene polymerisation catalysed by a nickel-bulk diimine catalyst (N,N′-bis(2,6-diisopropylphenyl)-2,3-butanediimine nickel(II) methyl cation [{(2,6-(ⁱPr)₂Ph)NC(Me)-C(Me)=N(2,6-(ⁱPr)₂Ph)}NiCH₃]⁺. Mechanisms of ethylene and hydrogen insertions into the growing polymer chain and ethylene insertion into the hydride complex formed after hydrogenolysis were investigated. The results obtained for these catalysts were then compared to those calculated for metallocene catalysts. Due to a higher difference in energy barriers between ethylene and hydrogen insertion, it may be concluded that controlling the molecular weight of the resultant polymer using nickel diimine catalysts is even more difficult than for the metallocene catalysts. Through statistical thermodynamics and kinetics calculations it was shown that the activity and molecular weights of polymers produced by nickel-diimine catalysts are dramatically decreased in the presence of hydrogen with respect to those of polymers generated by metallocene catalysts. These findings help to clarify the contradictory experimental results published in the scientific and technical literature regarding the effect of hydrogen on the catalytic activity and molecular weight of polymers produced using nickel diimine catalysts.     

Nanopore/electrode structures for single-molecule biosensing

Biological and solid-state nanopores have recently attracted much interest as ultrafast DNA fragment sizing and sequencing devices. Their potential however goes far beyond DNA sequencing. In particular, nanopores offer perspectives of single-molecule (bio)sensing at physiologically relevant concentrations, which is key for studying protein/protein or protein/DNA interactions. Integration of electrode structures into solid-state nanopore devices moreover enables control and fast switching of the pore properties, e.g. for active control of biopolymer transport through the nanopore. We present some of recent work in this area, namely the fabrication and characterization of nanopore/electrode architectures for single-(bio)molecule sensing. Specifically, we introduce a new technique to fabricate ultra-small metal nanopores with diameters smaller than 20 nm based on ion current feedback (ICF) controlled electrodeposition. It offers precise control of the pore conductance, is easily multiplexed, and can be extended to a wide range of different metals.     

Predicting chain conformation and entanglement of polymers from chemical structure

The characteristic ratio C_∞ and the entanglement molecular weight M_e are two key molecular parameters that control melt viscoelasticity, solid mechanical (brittle/ductile) behavior, and adhesion of polymers. We show that the characteristic ratio C_∞ and the entanglement molecular weight M_e can be predicted from chemical structure by group additivity with uncertainties usually less than ～ 7% for C_∞ and ～ 15% for M_e, comparable with the accuracies of experimental values.     

Polyester chain scission by selective chemical site attack

Tribenzylamine moieties copolymerized into poly(ethylene terephthalate) (PET) are selectively cleaved by mild hypochlorous acid oxidation to give dibenzylamines plus aldehyde and/or carboxyl groups. When either 3,3′-dicarboxytribenzylamine (DCTBA) or 4,4′-DCTBA were used, molecular weight was reduced, consistent with a mechanism where only two of the three carbon–nitrogen bonds which cleave can give polyester backbone scission. When copolymerized 3,5 DCTBA was cleaved, no chain scission resulted, but about one-third of the nitrogen content was lost, again consistent with the random statistical oxidative attack at each of the three carbon–nitrogen bonds. Borane ester, B[(CH₂)₁₀CO₂Me]₃, copolymerized into PET, lost molecular weight progressively after melt spinning, drawing, and boil off of the resulting fibers. Chain scission resulted from rapid oxidation of borane bonds to borate esters followed by hydrolytic cleavage to boric acid and/or intermediate boronic acids.