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".