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.