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

Moon river?

The media was recently drenched with the idea that water had been found on the Moon, offering speculation as to our nearest neighbour offering an oasis-like site for a lunar base from which we could launch missions to Mars and beyond. The truth, if it is ever confirmed, is a little more subtle.

Is moisture on the Moon, simply wishing on a star? (Photo by David Bradley)
Is moisture on the Moon, simply wishing on a star? (Photo by David Bradley)

The Apollo missions of the 1970s had always hinted at the presence of water on the Moon, although its presence in samples brought back to earth was thought to be nothing more than contamination. In 1998, scientists announced that the Lunar Prospector spacecraft had detected 300 million tonnes of water on the moon and hinted that there may be as much as 6 billion tonnes. In July, an analysis of tiny beads of volcanic glass collected by two Apollo missions revealed water trapped inside, suggesting that the Moon’s water had not been entirely vaporized by the violent events that led to its formation. The discovery had implications for the volcanic origin of possible water reservoirs at the Moon’s poles.

However, new evidence released at the end of September based on data from India’s Chandrayaan-1 probe and the Deep Impact and Cassini missions suggests that there may well be some degree of hydration up there. Researchers in India and the US used data from NASA’s Moon Mineralogy Mapper, the M3, aboard the Chandrayyan-1 satellite, which was launched into orbit around the moon in October 2008 to reveal the presence of water on the moon. Chandrayaan’s mission ceased in August 2009.

M3 uses reflectance spectrometry to determine the content of minerals in the thin layer of upper soil on the surface of the moon. The data revealed the presence of chemical bonds between hydrogen and oxygen atoms, like those found between the oxygen atom and its attendant hydrogen atoms in H2O.

However, the next generation of lunar astronauts are not likely to sip from moon springs or splash their silvery boots in lunar puddles because revelations of chemical bonds between hydrogen and oxygen atoms is indicative of water molecules but is even more indicative of hydroxyl ions (OH). It could be that good, old-fashioned H2O forms only when the solar wind doth blow and brings with it hydrogen atoms that can combine with the hydroxyl radicals forming “H+OH” (H2O). It may be that less than a litre of actual water is present per tonne of rock spread across the surface to a depth of a few centimetres and present as water of hydration of the minerals from which the rock is composed.

The rocks and soils that comprise the lunar surface contain about 45 percent oxygen, mostly in the form of silicate minerals. The constant deluge of hydrogen atoms from the solar wind could readily pull oxygen and hydroxyl from the soil and form water molecules on the fly, especially given the hydrogen ions are moving at one third the speed of light when they hit.

Taylor and other M3 team members believe their findings will be of particular significance as mankind continues to plan for a return to the moon. The maps created by M3 could provide mission planners with locations prime for extraction of needed water from the lunar soil.

Following the lunar announcement, Jim Bell, President of The Planetary Society, said: “The possible presence of minor amounts of hydrated material on the Moon is intriguing, though the findings still need to be confirmed by other methods and other investigators. Chandrayaan is another great example of the power and value of international collaboration in space exploration, and The Planetary Society congratulates the entire Chandrayaan, Deep Impact and Cassini teams.”

Researchers still hope to find liquid water at the bottom of the deepest, darkest lunar craters at depths that never see sunlight nor feel the solar wind. Such, hopefully, icy depths are akin to the cold places on the planet Mars where evidence of water ice has been found.

LINKS
Science, 2009, in press
Chandrayaan-1 site

Ozone eaters

Cutting emissions from petrochemical complexes and coal-fired plants may be the best way to reduce polluting ground-level ozone. At least, that is the case for Houston, Texas, one of the most polluted cities in the USA.

Sunlight catalyzes urban ozone formation from volatile organic compounds and nitrogen oxides emitted from cars, petrochemical refineries, and coal-fired power stations. Houston suffers some of the worst ozone levels in the US partly blamed on its large population, it is the fourth most populous city in the country, but perhaps more significant are its surrounding industries. Houston, after all carries out almost half of the nation’s petroleum refining.

Ozone levels (Credit: PNAS)

Ozone levels (Credit: PNAS)

Renyi Zhang and his colleagues are experts in using state-of-art laboratory instrumentation to investigate the processes involved in hydrocarbon oxidation reactions initiated by OH and other radical species. Chemical ionization mass spectrometry (CIMS) and in situ Fourier Transform Infrared Spectroscopy (FTIR) provide them with the raw data which they then feed to high-level ab initio molecular orbital calculations to provide a supercomputer model of the structures and energetics of the organic radicals. Our objectives are to quantitatively understand the photochemical oxidation mechanisms of atmospheric volatile organic compounds (VOCs) and their role in tropospheric ozone formation, Zhang explains.

They have now used a computer model to simulate how surface ozone levels changed with time and distance during a period of stagnant air in Houston in September, 1993. During the daytime, ozone levels significantly exceeded Federal air-quality standards; the highest ozone levels occurred around the city’s Southeastern petrochemical complexes. At night, continuous industrial nitrous oxide emissions removed almost all of the ozone from the atmosphere, forming an urban ozone hole.

Renyi Zhang

Renyi Zhang

Zhang’s results provide the link between industrial emissions and the rapid production of and strong diurnal variations in ozone levels. He and his team suggest that limiting industrial emissions of organic compounds and nitrogen oxides is absolutely necessary to address the problem of ozone pollution.

Further reading

Proc Natl Acad Sci USA, 2004, 101, 6346-6350
http://dx.doi.org/10.1073/pnas.0401484101

Renyi Zhang
http://atmo.tamu.edu/profile/RZhang

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