A new class of plastics

It has long been the dream of chemists to act as creator and, with a mere lift of the finger, create well-determined molecules with even more clearly defined functions. This is what Christophe Detrembleur and his team at the Center for Education and Research on Macromolecules of the University of Liege have been attempting to do.  Their objective is to create polymers with really innovative functions. They have taken a stride forward in this respect: they have perfected a method that makes it possible to control the reactivity of growing polyolefins and therefore to prepare new copolymers with brand-new properties. In addition, the process works in mild experimental conditions, at 40°C and at 10-50 bars. This is the opposite of the conventional industrial process which is not controlled and which generally takes place at much higher temperatures (several hundred degrees) and at high levels of pressure (1000 bars or more). This technology has application implications for many fields of activity such as the biomedical and energy industries and the environment. 

PE, EVA, PVC (1)... These are yet more strange sets of initials that have invaded our daily lives: films used for food-packaging, pipes, glues, adhesives…, applications of this technology affect all areas whether related to food, the medical industry, the automobile industry, agriculture, or the energy industry. The techniques of industrial polymerization that makes it possible to produce these plastics have become increasingly popular in the second half of the 20th century to the point where, today, mankind is in continuous contact with one or other product that is the result of polymerization. 

"Polyethylene (PE)is one of the most common industrially-produced polymers (plastics of the polyolefins family). Its chemical inertia, its transparency and its excellent mechanical properties make it an exceptional plastic. However, it is very apolar, which complicates the work when you want to mix it with other more polar polymers. When mixing them, we are looking for a synergy of properties, we combine the properties of the two polymers into one material", explains Christophe Detrembleur director of research at the FNRS in the Center for Education and Research on Macromolecules (CERM) of the University of Liege, whose last study has just been published in Nature Chemistry(2) (and which is the subject on the cover of the march 2014 volume).For example, combining a hard material with a soft material to create an elastomer, a kind of rubber which can be used as a shock-absorber. However, in most cases, an intimate mixture of the two products is an essential condition for obtaining this synergy which is often difficult with polyethylene. This can be obtained particularly by adding copolymers of ethylene and polar vinyl monomers (which are composed of strings of units of ethylene and units of polar monomers).

Apart from this application, these functional copolymers can be found in many products in our daily lives. An important example is EVA (vinyl ethyl acetate) which is found in plastic films for food-products, in smartphones, films for greenhouses and many adhesives and glues.  

Controlling the growth

"These ethylene-based copolymers are mainly produced by the conventional technique known as radical polymerization which produces very interesting materials but the process is a bit chaotic", he continues. In fact, ethylene copolymers are currently industrially obtained in conditions which do not allow for precise control of the stringing of monomer units and therefore the final properties of the material. 

The polymerization process takes place in three steps: initiation, propagation and termination. The first phase, initiation, consists of managing an active species, a radical (from which the technique gets its name), which will initiate the growth of chains, therefore the addition of monomers (in the case of VEA, ethylene and vinyl acetate) to each other, and enable the polymer chain to grow at a macromolecular level; this is the propagation phase. The last phase which is called termination, fixes the chains into their final configuration.

"All this is done in a chaotic and random manner all along the polymerization process in such a way that, at the end of the reaction, you have chains of different sizes which can also have different compositions and therefore different properties from each other. Here in the laboratory, we are trying to find a way of controlling the growth of these chains".

For the team, the aim is to succeed in initiating all these chains at the same time and controlling their growth to obtain chains of uniform length and functionality with the monomers becoming incorporated in a clearly-defined manner according to the experimental conditions. 

How can this be achieved? By limiting the polymerization process to the two first phases, initiating the chains and their growth while avoiding irreversible terminations: when all the units of monomers are consumed, the macromolecules formed remain active. 

Functional copolymers

« The most important example in our article is the EVA copolymers which concern a very vast range of products such as glues, film-packaging, latexes, etc. This is a huge industry involving enormous volumes of material."

Vinyl acetate is a liquid monomer while ethylene is gaseous; mixing them requires working under pressure. Christophe Detrembleur’s team uses a control agent, a cobalt-based complex organometallic which makes it possible to control this polymerization. First, the latter is placed in a reactor with the first monomer, vinyl acetate and then, the reactor is placed under ethylene pressure.

The researchers have shown that by modulating the working pressure, it is possible to control the quantity of each of the monomers in the copolymer and therefore its properties.  For example, at 10 bars of ethylene pressure, the copolymer is composed of around 15% ethylene and 85% vinyl acetate. If the pressure increases to 50 bars, each monomer is present to a degree of around 50%.

"This is particularly important because we are completely changing the polarity of the polymer and therefore its functionality. For example, its vitreous transition temperature (the temperature at which it « melts »). We have shown that we can easily change the level of ethylene in the copolymer simply by changing the pressure during the polymerization. In fact, we are following the development of the molar mass according to the function of the conversion: the more we incorporate monomers, the more the chain will grow and therefore its molar mass increases. It is one of the parameters we are following to show that we are controlling the chains".

Controlling the architecture

"At the end of the polymerization process, the chemistry developed makes it possible to functionalize the extremity of the chain, which is more interesting and which is the object of the publication –to succeed in creating polymers in blocks or sequences (you do not get a mixture of A and B polymers, but you have A and B which are strongly linked). Only controlled polymerization techniques make it possible to prepare some”. 

In fact, apart from the control of the molar mass, the researchers are succeeding in controlling the architecture thanks to a very simple process. By beginning, for example, with copolymerization at 50 bars and then changing the pressure to 10 bars after a few hours of reaction: at 50 bars, the chains formed will have a 50/50 composition of ethylene and vinyl acetate and then during the second phase (at 10 bars), these chains that were initially formed will get longer by incorporating 15% ethylene and 85% vinyl acetate. At the end of the process, we obtain a copolymer A (containing 50% ethylene) which is linked to a copolymer B (15% ethylene). We now form a bi-sequential AB copolymer. 

From this point on the possibilities are infinite: we can terminate the AB copolymer by an important chemical function for the targeted application, we can couple two Abs with each other and thus form a trisequential symmetrical copolymer ABA: in a few seconds, the molar mass of the copolymer is doubled, its mechanical propertied evolve and with these, the possible applications.

"From the moment you can control the reactivity of the growth of your chains, you can more or less do what you want, add the functionality required for the desired property, » he adds. “Sometimes you need a very long polymer or a very short one, with a clearly-defined functionality at the extremity of the chain to allow it to grip on to a surface for example. For some applications, you may want polymers in the shape of a tree and, by using this technique, you have that possibility". 

Mild but slow conditions

Another particularity of this system, which rather surprised the researchers, was that it works in relatively mild experimental conditions: at 40°C and at 10-50 bars. In contrast with the conventional industrial process which is not controlled and is generally carried out at high temperatures (several hundred degrees) and high pressures (1000 bars or more).

"The more you increase the temperature and pressure, the more chaotic the process. The more moderate the temperatures, the more you can minimize or avoid the secondary reactions". When we prepare sequenced copolymers, the interest of the process is that we work in "one-pot": this sequenced copolymer AB, is polymerized at 50 bars and then 10 bars, everything is done in the same reactor without isolating anything between the two phases. We leave a few hours for the reaction to continue (6-7) at 50 bars, we consume one part of the vinyl acetate and then we change the pressure and the second block forms in a few hours. We do not use all the vinyl acetate during the first phase so that we can use it not only as a monomer but also as a solvent for the AB copolymer which forms during the second phase, explains Christophe Detrembleur.

The only small drawback to these advantageous conditions is that the process for controlled radical polymerization is slower than the conventional system. 

Better than nature?

The polyethylenes are the most commonly-produced polymers in the world. Consequently, succeeding in controlling ethylene-based copolymerizations in mild conditions represents a kind of Holy Grail for the polymer researchers who envisage an incredibly varied range of possible applications in the future. 

To this end, the team at CERM is working on the ethylene homopolymer, a polymerization of only ethylene.  "We would like to have access to block copolymers where at least one sequence (a block) is pure polyethylene. We are working on it, we are succeeding in creating polyethylene, but we do not yet know if it is controlled. Once the control conditions are established, we should be able to have the possibility of forming block copolymers for which a block of polyethylene would be connected to a polar polymer: thus you considerably increase the range of applications. Our technique does not in any way attempt to supplant polyethylenes and their copolymers that are prepared by conventional means, we want to give access to brand new products with a high added value making it possible to increase the field of applications for polyolefins (for the biomedical industry and batteries…). Developing products in water: today, the majority of latex is produced industrially by conventional radical polymerization, the objective is to adapt our new procedure to the production of poleofin-based latex, but this is still in the future".

In summary, the particularity of this discovery is to easily modulate the reactivity of the system without hanging the control agent and doing this only by using experimental criteria, while controlling the polymerization of monomers that have opposite reactivities. "It is quite unique. It is also one of the reasons this article was accepted in Nature Chemistry, because no one as yet succeeded in finely-controlling ethylene in radical polymerization. This opens the way for polymers that are impossible to prepare by the conventional method, and for particular applications", says the delighted researcher. Therefore at CERM, researchers are studying variants of copolymers as vectors of medicines. 

Nature is also made up of a succession of biological macromolecules where each monomer unit is located in a precise place. "If you reverse one of these, it no longer results in the same thing. In synthesized plastics, the creation of monomer units, the structure of the polymer and its functionality are also extremely important because they govern the final properties of the material. But nature does it better than us: I can’t decide the position of each monomer but I can control the polymer and the quantity of monomers incorporated. On the other hand, succeeding in controlling different ABCDE monomers and being able to assemble them one by one is still beyond us. Some teams are working on this, but it is a very laborious task…The ultimate Holy Grail is to be able to control these combinations", concludes Christophe Detrembleur.

(1) PE: polyethylene, a plastic constructed by the stringing of ethylene units
EVA: vinyl ethylene-acetate
PVC: polyvinyl chloride
(2) Nature Chemistry 2014,6:179-187, www.nature.com/nchem/journal/v6/n3/pdf/nchem.1850.pdf