A radical approach to understanding polymers

Polymerization is used to make a whole range of materials but understanding exactly what happens during synthesis when it involves free radicals is difficult. Now, New Zealand chemists have uncovered important clues by following the rates of reaction and the termination steps involved.

Radical polymerization is commonly used to make half the world’s polymers and now Greg Russell and his colleagues at the University of Canterbury have investigated the kinetics of the process that shuts off polymerisation, the termination rate coefficient, for some very common reactions. They have revealed that the diffusion behaviour of short polymer strands, oligomers, in the reaction system is the critical factor.

“The majority of chemists simply try to bring about reactions by mixing different chemicals together under different conditions,” Russell explains. “However it is also important, especially for those who make chemical products on a large scale, to have precise quantitative descriptions of the speeds at which reactions occur. Chemical kinetics is the field of work that develops such descriptions. It is therefore an area where chemistry and mathematics intersect.

Greg Russell
Greg Russell

He adds that the most difficult problem in radical polymerization is the termination step that stops the polymer growing longer. Diffusion of the growing polymers is affected by their size, concentration, temperature and so on, which makes termination a complicated process to study, one that even after 60 years of intensive investigation is still not fully understood.

Russell, who is on sabbatical at the University of Goettingen, Germany, worked with Philipp Vana there and have found that although highly specialized techniques for measuring termination rate coefficients under precisely controlled conditions have been employed the results they found on attempting to replicate earlier studies were inconsistent. “I have taken this information and attempted to see whether it is consistent with systems where many different termination reactions occur at once, as is the case in commercial processes,” Russell says. “For the monomer styrene [used to make polystyrene] I find there is consistency, but for methyl methacrylate [used for polymethylmethacryalate, PMMA] there is not.”

In trying to explain this result, he eliminated most of the conventional views, and came to the conclusion that the answer lies with the oligomers in each system, which seem to have slightly different diffusional behaviour.

LINKS

Macromol. Chem. Phys. 2010
http://dx.doi.org/10:1002/macp.200900668

Greg Russell
http://www.chem.canterbury.ac.nz/people/russell.shtml

Philipp Vana
http://www.fpm.chemie.uni-goettingen.de/pvana.htm

Catalytic troublemaker

Porous solid catalysts are a mainstay of the modern chemical industry, allowing reactions that would otherwise take an age to progress to be run much, much faster. One group of such catalysts are the zeolites and particularly important among them is one known as ZSM-5, an aluminosilicate material with an MFI structure. However, despite its attractions, ZSM-5 can behave badly because its chemical building blocks do not join together perfectly. This leads to chemical starting materials on which the catalyst is to act often becoming stuck before they can get into the reactive pores and be converted into product. Now, Dutch scientist Marianne Kox has discovered the nature of the miniscule deviations that can make ZSM-5 such a troublemaker.

Catalytic ZSM-5 isn't always on its best behaviour (Credit: Nature Materials/Weckhuysen et al)

Catalysts are essential to the production of a vast array of pharmaceutical drugs, agrochemicals, fuels and countless other chemical products that are made from simple starting materials. Kox and colleague Lukasz Karwacki, together with researchers at the Max Planck Institute for Coal Research in Mülheim an der Ruhr, Germany, ExxonMobil Chemical Europe Inc, Machelen, Belgium, the Centre for Nanoporous Materials, at the University of Manchester, UK, UOP LLC, a Honeywell Company, in Des Plaines, Illinois, USA, and Nicholas Copernicus University, Torun, Poland, have used a raft of spectroscopic techniques, on the micro scale to analyse the structure of zeolite ZSM-5 and have obtained spatial and time-resolved data on the three-dimensional interior of these porous materials. The data reveal the deviations from one porous unit to the next that can lead to reduced efficiency, catalytic poisoning, and unwanted chemical by-products.

Catalytic ZSM-5 (Credit: Nature Materials/Weckhuysen et al)

Kox is working as part of the Vici project run by Bert Weckhuysen, Professor of Inorganic Chemistry and Catalysis at Utrecht University in The Netherlands. Details of the research were published in Nature Materials. The team developed a new approach that correlates confocal fluorescence microscopy with focused ion beam–electron back-scatter diffraction, transmission electron microscopy lamelling and diffraction, atomic force microscopy and X-ray photoelectron spectroscopy to study a wide range of coffin-shaped zeolite crystals of differing shapes, sizes, structures, and chemical compositions.

The powerful combination of techniques demonstrates “a unified view on the morphology-dependent MFI-type [zeolite] intergrowth structures and provides evidence for the presence and nature of internal and outer-surface barriers for molecular diffusion,” the team say. “It has been found that internal-surface barriers originate not only from a 90° mismatch in structure and pore alignment but also from small angle differences of 0.5 to 2 degrees for particular crystal morphologies. Furthermore, outer-surface barriers seem to be composed of a silicalite outer crust with a thickness varying from 10 to 200 nanometres.”

LINKS

Nature Mater, 2009, 8, 959-965
http://dx.doi.org/10.1038/nmat2530

Bert Weckhuysen

Pores for thought

A solid, but sponge-like material has been synthesised by chemists in Singapore. The silica-type material has the most complicated pore structure ever reported and is discussed in the latest issue of the new journal Nature Chemistry.

Mesoporous silica is a technologically important material, that extends the basic principle of porous silica to a specific range of pore size: 2 to 50 nanometres. Materials with pores smaller than this are microporous and anything bigger is macroporous. The meso materials in the middle have great potential in catalysis, chemical separation, gas storage, drug delivery and even imaging. All important for particular applications is the precise size and shape of the pores. Different shapes will be responsive to specific small molecules that might enter in different ways and have diverse effects on how the material interacts or changes those small molecules.

The mesoporous IBN-9 structure (Credit: Ying et al/Nature Chem.)

The mesoporous IBN-9 structure (Credit: Ying et al/Nature Chem.)

The size of the pores also endows any such porous material with a huge internal surface area in a given small volume that would if laid flat cover dozens of football pitches. It is this enormous surface to volume ratio – often around one thousand square metres per gram – that endows mesoporous materials with the ability to absorb, or more strictly adsorb, large volumes of small molecules.

Until now, such materials have been limited to a single network of pores, or at most two disconnected pore systems. This gives them enormous potential as sieves for separating molecules if a mixture of large and small molecules is strained through the material only those that fit the pores can be adsorbed. Such porous materials are essential to the process of catalytic cracking in the petroleum industry in which every molecule found in vehicle fuels has essentially passed through such a material.

Various views of the IBN-9 pores (Credit: Ying et al/Nature Chem.)

Various views of the IBN-9 pores (Credit: Ying et al/Nature Chem.)

Now, Jackie Ying, Yu Han, Leng Leng Chng, and Lan Zhao of the Institute of Bioengineering and Nanotechnology, in Singapore, and colleagues Daliang Zhang, Junliang Sun, and Xiaodong Zou of the Berzelii Center EXSELENT on Porous Materials, in Stockholm, Sweden, have created a mesoporous silica that has not two but three interwoven but disconnected pore systems. This added layer of complexity gives chemists an extra variable with which to work in creating pore sizes and shapes tailored to particular molecules and so particular types of chemical separation or catalytic process.

To make their new triply porous material, known as IBN-9, Ying and colleagues designed a new templating agent around which the complex structure formed. The template compound, a surfactant made from N,N-dimethyl-L-phenylalanine in two steps, has large head-groups and long hydrocarbon tails ending with an amine group. The template then acts as a support around which silica can grow and crystallise, dissolving the template leaves behind the mesoporous silica.

By fine-tuning the ratio of molecular size to head-group and the degree of water repellence, or hydrophobicity, of the tail, the researchers can design the pores of the new materials. They suggest that the presence of both long and short channels in fibres of the material could lead to separation or controlled-release applications that offer different diffusion rates in different directions.

Further reading

Nature Chem., 2009, 1, in press
http://dx.doi.org/10.1038/nchem.166