Polymers are built on a universal architecture: they are composed of basic building blocks called monomers. Polymer synthesis involves linking monomers together to form long chains. Imagine threading glass beads onto a string and creating chains of different length (and weight).

Polymerization processes with limits

An important industrial process for producing polymers is free radical polymerisation (FRP). Each year the chemical industry uses FRP to produce 200 million tonnes of polymers of various types, such as polyacrylic, polyvinyl chloride (PVC) and polystyrene.

Although this production method has many advantages, it also has its limitations. FRP produces an uncontrollable mixture of countless polymers of different lengths; in other words, its dispersity is high. Dispersity is a measure of how uniform or non-uniform the length of the polymer chains in a material is. Material’s properties are determined to a large extent by this dispersity.

In the case of everyday polymers, polymers with both low and high dispersity are required. In fact, for many high-tech applications including pharmaceuticals or 3D printing, high dispersity can even be an advantage.

Polymers with new properties

However, if chemists want to produce polymer materials with very specific properties, they must first and foremost be able to adjust the dispersity as desired. This lets them produce a wide range of polymer materials that either contain uniform polymer species, i.e. have a low dispersity, or are highly dispersed with a great number of polymers of different lengths. Until now, this has hardly been possible.

A group of researchers led by Athina Anastasaki, Professor of Polymer Materials at the Department of Materials Science, has now developed a method of controlling radical polymerisation, thus enabling researchers to systematically and completely control the dispersity of polymer materials. The results of their research were recently published in the journal Chem.

In the past, in order to be able to control the radical polymerisation process at least to some extent, chemists would use a single catalyst. While this ensures that the resulting polymer chains become uniformly long, it doesn’t allow the overall dispersity to be controlled as desired.

Two catalysts do the trick

Now the ETH researchers simultaneously employ two catalysts with different effects — one is highly active, the other only slightly active. This enabled them to adjust the dispersity precisely as a function of the ratio in which they mixed the two catalysts. If the more active catalyst was more abundant, more uniform polymers were produced, which meant the resulting material had low dispersity. If, however, the less active catalyst was more abundant, a large number of different polymer molecules were formed.

This work means Anastasaki and her team have created a basis for the development of new polymer materials. In addition, their process is also scalable; it works not only in the laboratory, but also when applied to larger quantities of substances. Another advantage of this method is that even polymers with high dispersity can continue growing once the polymerisation process itself is complete — something that was previously considered impossible.

The high efficiency and scalability of the approach have already attracted interest from industry. Polymers produced with the new process could be put to use in medicine, vaccines, cosmetics or 3D printing.

Story Source:

Materials provided by ETH Zurich. Note: Content may be edited for style and length.

Scientists from the Boston University (BU) Alzheimer’s Disease and Chronic Traumatic Encephalopathy (CTE) Centers, the University of California, San Francisco (UCSF), and San Francisco VA Healthcare System teamed up to analyze the records of 13,323 individuals age 40 and older (average age 62) who participate in the internet-based Brain Health Registry. Of those, 725 or 5 percent of participants reported exposure to previous repetitive head impacts through contact sports, abuse or military service. In addition to repetitive head impact history, the scientists also examined the effects of having a TBI with and without loss of consciousness.

Along with self-report questionnaires of repetitive head impact and TBI history, participants completed measures of depressive symptoms and computerized cognitive tests. The findings, published in the journal Neurology, reveal that participants with a history of both repetitive head impacts and TBI reported greater depression symptoms than those who did not have such history. In addition, when repetitive head impacts and TBI were examined separately, a history of repetitive head impacts had the strongest effect on later-life symptoms of depression. The findings were independent of age, sex, racial identity and education level.

“The findings underscore that repetitive hits to the head, such as those from contact sport participation or physical abuse, might be associated with later-life symptoms of depression. It should be made clear that this association is likely to be dependent on the dose or duration of repetitive head impacts and this information was not available for this study,” said Michael Alosco, PhD, associate professor of neurology at BU School of Medicine (BUSM) and co-director of the BU Alzheimer’s Disease Center Clinical Core.

There was a dose-response-like pattern between head trauma and depression symptoms. Specifically, participants without any history of either TBI or repetitive head impacts had the fewest symptoms. While depression symptoms increased when a history of TBI alone was present, depression symptoms were highest for the groups who had a history of both repetitive head impacts and TBI. Indeed, the group that had a history of repetitive head impacts and TBI with loss of consciousness reported the most depressive symptoms.

A similar cumulative effect was seen among those exposed to repetitive head impacts and TBI on tests of memory, learning, processing speed, and reaction time. Participants with a history of repetitive head impacts or TBI had worse performance on some of the tests compared to those without any head trauma history, and those with both a history of repetitive head impacts and TBI with loss of consciousness had worse performance on almost all of these computerized cognitive tests.

“These findings add to the growing knowledge about the long-term neurological consequences of brain trauma,” said Robert Stern, PhD, professor of neurology, neurosurgery and anatomy & neurobiology at BUSM and director of clinical research at the BU CTE Center. “It should be noted that not all people with a history of repetitive hits to the head will develop later-life problems with cognitive functioning and depression. However, results from this study provide further evidence that exposure to repetitive head impacts, such as through the routine play of tackle football, plays an important role in the development in these later-life cognitive and emotional problems,” added Stern, one of the senior authors of the study.

A major limitation of the study is that the researchers did not have access to measurements or estimates of the degree of repetitive impact exposure nor TBI frequency. In October, BU researchers reported a dose-response relationship between the number of years of exposure to tackle football (regardless of the number of concussions) and the presence and severity of the degenerative brain disease chronic traumatic encephalopathy (CTE). In a sample of 266 deceased football players, each year of exposure to tackle football was associated with 30 percent increased odds of having CTE and 17 percent increased odds of having severe CTE. It is unknown if any subjects in this study have CTE or any other neurodegenerative disease.

The research team plans to extend their work through continued collaboration between BU and UCSF investigators utilizing data from the Brain Health Registry. “We are excited to partner with BU on this important study that used the Brain Health Registry to increase our understanding on the long-term effects of repetitive head impacts and TBI,” said Michael Weiner, MD, PI of the Brain Health Registry and professor-in-residence in radiology and biomedical imaging, medicine, psychiatry and neurology at UCSF. “The Brain Health Registry is a novel and exciting resource for both the scientific community and the general public. It allows for large-scale recruitment, screening and study of dementia, and more than 60,000 individuals across the world are enrolled. It offers a way for the general public to track their thinking, memory, mood, and behavior over time, and also serves as a readiness registry for future research and clinical trials of prevention and treatment.”

The research was funded by the National Institutes of Health, grant numbers U01NS093334; K23AG046377; K23NS102399; P30AG013846.

Researchers from the University of Cincinnati found toxic levels of pollution in two central reservoirs in Tikal, an ancient Maya city that dates back to the third century B.C. in what is now northern Guatemala.

UC’s findings suggest droughts in the ninth century likely contributed to the depopulation and eventual abandonment of the city.

“The conversion of Tikal’s central reservoirs from life-sustaining to sickness-inducing places would have both practically and symbolically helped to bring about the abandonment of this magnificent city,” the study concluded.

A geochemical analysis found that two reservoirs nearest the city palace and temple contained toxic levels of mercury that UC researchers traced back to a pigment the Maya used to adorn buildings, clayware and other goods. During rainstorms, mercury in the pigment leached into the reservoirs where it settled in layers of sediment over the years.

But the former inhabitants of this city, made famous by its towering stone temples and architecture, had ample potable water from nearby reservoirs that remained uncontaminated, UC researchers found.

The study was published in the Nature journal Scientific Reports.

UC’s diverse team was composed of anthropologists, geographers, botanists, biologists and chemists. They examined layers of sediment dating back to the ninth century when Tikal was a flourishing city.

Previously, UC researchers found that the soils around Tikal during the ninth century were extremely fertile and traced the source to frequent volcanic eruptions that enriched the soil of the Yucatan Peninsula.

“Archaeologists and anthropologists have been trying to figure out what happened to the Maya for 100 years,” said David Lentz, a UC professor of biological sciences and lead author of the study.

For the latest study, UC researchers sampled sediment at 10 reservoirs within the city and conducted an analysis on ancient DNA found in the stratified clay of four of them.

Sediment from the reservoirs nearest Tikal’s central temple and palace showed evidence of toxic algae called cyanobacteria. Consuming this water, particularly during droughts, would have made people sick even if the water were boiled, Lentz said.

“We found two types of blue-green algae that produce toxic chemicals. The bad thing about these is they’re resistant to boiling. It made water in these reservoirs toxic to drink,” Lentz said.

UC researchers said it is possible but unlikely the Maya used these reservoirs for drinking, cooking or irrigation.

“The water would have looked nasty. It would have tasted nasty,” said Kenneth Tankersley, an associate professor of anthropology in UC’s College of Arts and Sciences. “There would have been these big algae blooms. Nobody would have wanted to drink that water.”

But researchers found no evidence of the same pollutants in sediments from more distant reservoirs called Perdido and Corriental, which likely provided drinking water for city residents during the ninth century.

Today, Tikal is a national park and a UNESCO World Heritage site. Researchers believe a combination of economic, political and social factors prompted people to leave the city and its adjacent farms. But the climate no doubt played a role, too, Lentz said.

“They have a prolonged dry season. For part of the year, it’s rainy and wet. The rest of the year, it’s really dry with almost no rainfall. So they had a problem finding water,” Lentz said.

Co-author Trinity Hamilton, now an assistant professor of biology at the University of Minnesota, worked on the analysis of ancient DNA from algae that sank to the reservoir bottom and was buried by centuries of accumulated sediment.

“Typically, when we see a lot of cyanobacteria in freshwater, we think of harmful algal blooms that impact water quality,” Hamilton said.

Finding some reservoirs that were polluted and others that were not suggests the ancient Maya used them for different purposes, she said.

Reservoirs near the temple and palace likely would have been impressive landmarks, much like the reflecting pool at the National Mall is today.

“It would have been a magnificent sight to see these brightly painted buildings reflected off the surface of these reservoirs,” said co-author Nicholas Dunning, head of geography in UC’s College of Arts and Sciences.

“The Maya rulers conferred to themselves, among other things, the attribute of being able to control water. They had a special relationship to the rain gods,” Dunning said. “So the reservoir would have been a pretty potent symbol.”

UC’s Tankersley said one popular pigment used on plaster walls and in ceremonial burials was derived from cinnabar, a red-colored mineral composed of mercury sulfide that the Maya mined from a nearby volcanic feature known as the Todos Santos Formation.

A close examination of the reservoir sediment using a technique called energy dispersive X-ray fluorescence spectrometry found that mercury did not leach into the water from the underlying bedrock. Likewise, Tankersley said, UC ruled out another potential source of mercury — volcanic ash that fell across Central America during the frequent eruptions. The absence of mercury in other nearby reservoirs where ash would have fallen ruled out volcanoes as the culprit.

Instead, Tankersley said, people were to blame.

“That means the mercury has to be anthropogenic,” Tankersley said.

With its bright red color, cinnabar was commonly used as a paint or pigment across Central America at the time.

“Color was important in the ancient Maya world. They used it in their murals. They painted the plaster red. They used it in burials and combined it with iron oxide to get different shades,” Tankersley said.

“We were able to find a mineral fingerprint that showed beyond a reasonable doubt that the mercury in the water originated from cinnabar,” he said.

Tankersley said ancient Maya cities such as Tikal continue to captivate researchers because of the ingenuity, cooperation and sophistication required to thrive in this tropical land of extremes.

“When I look at the ancient Maya, I see a very sophisticated people with a very rich culture,” Tankersley said.

UC’s team is planning to return to the Yucatan Peninsula to pursue more answers about this remarkable period of human civilization.