COU RT ESY O F HA R L EY K L EIN
NEWS OF THE WEEK
UNTANGLING POLYMER FLOW
industry-ready tool that can calculate the dynamics of branched-polymer melts. They tested their model on LDPE, a popular material for plastic bags. The real advance that made this computational feat possible, says Daniel J. Read, a research team member MODELING: New algorithm closely at the University of Leeds, in England, is the “priority describes dynamics of polymer melt distribution” used in the calculation. That’s a “posh” term, he says, for describing how far polymer segments that are tangled up in the mixture will stretch before NEW MODEL that predicts how complex, going with the flow. To determine the distribution, he branched polymers flow will enable companies adds, “we work out how forces are transmitted from to develop new plastics more efficiently and to the outside of a hugely branched molecule to the inprocess materials more easily than ever before. side.” The feat is similar to predicting the outcome of a Until now, polymer scientists have been able to simtug-of-war game with a highly branched piece of rope. ulate only how simplistic, molten polymers move. The The researchers’ method “provides a major step new computational technique, developed by a team of toward finally unraveling the LDPE tangle,” says Ronresearchers in Europe, closely accounts for the flow of ald G. Larson, a chemical engineer at the University of real, tangled polymers, such as low-density polyethylMichigan, in a commentary associated with the team’s ene (LDPE) (Science, DOI: 10.1126/science.1207060). report. But whether it can predict LDPE properties such Combining a decades-old “tube model” that defines as diffusion and adhesion remains to be seen, he adds. how far polymer chains can move sideways during Even though there are always further questions to flow with a mathematical algorithm that predicts the answer, says team leader Tom C. B. McLeish of Durdistribution of polymer chain sizes and shapes in a ham University, in England, “it will be very exciting” given reaction mixture, the researchers designed an in the meantime to watch whether industry can use this predictive tool to speed up the To see simulations of polymer melts flowing development of new materials, such as MORE ONLINE through different barriers, go to cenm.ag/vid7. biopolymers.—LAUREN WOLF
A
CATALYST CALCULUS ORGANIC CHEMISTRY: Math teases out
ideal balance of catalyst qualities
have developed a method that zeroes in on the optimum properties of catalysts for a given reaction. The new method uncovers relationships between catalysts’ chemical properties that normally are difficult to detect (Science, DOI: 10.1126/science.1206997). Those hidden relationships, say the researchers who developed the approach, can be crucial determinants of catalyst efficiency. They note that industrial chemists, drawing on small sets of completed experiments, have often used math to predict optimum reaction temperatures and concentrations, thereby reducing the number of experiments they must try before they achieve their desired result. “We took that process and adapted it to look at classic physical organic parameters of catalysts,” including steric and electronic effects, says Matthew S. Sigman of the University of Utah. Sigman and graduate student Kaid C. Harper sought catalysts for a reaction that forms chiral alcohols. They tested a small, diverse library of ligands in the reaction, and on the basis of those results, developed an equa-
ADAPTED FROM SCIENCE
B
Y TAKING a mathematical approach, chemists
Enantioselectivity tion to predict optimal catalyst characteristics for that reaction. The equation helped them eliminate a poorly performing ligand class and develop alternatives with quinoline and proline moieties that gave better results. The equation indicated, and the researchers confirmed experimentally, that electron-rich ligands’ performance Electronic effects varied greatly with size, whereas electron-poor ligands of all sizes worked uniformly poorly. “It’s not something we would’ve predicted, but it’s extremely important for understanding catalysts in the future,” Sigman says. University of Pennsylvania organic chemist Marisa C. Kozlowski says researchers have noticed relationships like the ones Sigman observed, but identifying and optimizing catalyst systems with such relationships has been challenging. She praised Harper and Sigman’s method as an easily accessible way to address that issue. Chemists can recalculate the equation for other reactions of interest with a standard mathematical software package, Sigman notes.—CARMEN DRAHL
WWW.CEN-ONLINE.ORG
13
OCTOBER 3, 2011
Time-lapsed images of low-density polyethylene flowing through a 1.4-mm gap over the course of about one minute show close agreement between experiment (left) and model (right).
O CH3O N
N H
N O
Steric effects
This 3-D plot, based on data from a library of compounds (black dots), shows the effect of sterics and electronics on ligand performance; compounds at the red peak are the most effective.
O
