Asymmetric copolymers: neither block nor random – ASYMCOPO

We are using two techniques for polymer synthesis: semicontinuous polymerization with carefully controlled addition of monomers to prepare polymers with a linear gradient, and high throughput automated synthesis to prepare a library of asymmetric block and multiblock copolymers.

These polymers are then characterized using a variety of techniques. We are primarily interested in their ability to self-organize, either in the solid phase or in solution. We investigate their microphase separation in bulk using differential scanning calorimetry, and their self-assembly behavior in solution using a range of scattering-based techniques: dynamic light scattering and small angle neutron or X-ray scattering. We have also investigated the relationship between composition and molecular weight using two-dimensional chromatography (SEC-LCCC).

To date, we have prepared a library of gradient copolymers based on several different acrylate monomers, with composition profiles that are very close to linear. In parallel, we have developed techniques to allow the preparation of block copolymers in high throughput conditions. We have also measured the reactivity ratios of a wide range of monomers, which determine how they copolymerize.

With a library of polymers in place, we are now at the stage of characterizing their behavior. We hope that our results will lay the groundwork for a new way of classifying polymers based on their degree of asymmetry. These polymers are expected to show applications ranging from shock- and noise-absorption to improved interfacial stabilizers.

Our preliminary results on reactivity ratio measurement have appeared in two peer-reviewed articles to date:
1. Sykes, K. J., Harrisson, S. and Keddie, D. J., Phosphorus-Containing Gradient (Block) Copolymers via RAFT Polymerization and Postpolymerization Modification. Macromol. Chem. Phys., 2016, 217: 2310–2320. dx.doi.org/10.1002/macp.201600087
2. RAFT Copolymerization of Vinyl Acetate and N-Vinylcaprolactam: Kinetics, Control, Copolymer Composition, and Thermoresponsive Self-Assembly. L. Etchenausia, A. Malho Rodrigues, S. Harrisson, E. D. Lejeune, M. Save, Macromolecules 2016 49, 6799-6809. dx.doi.org/10.1021/acs.macromol.6b01451

Two conference presentations are also planned for 2017.

The goal of the ASYMCOPO project is to develop new materials that mimic the useful shock- and noise-absorbing and interfacial stabilizer properties of gradient copolymers, but which do not reauire complex synthetic procedures for their preparation.
Gradient copolymers, whose composition varies continuously as a function of chain length, are an intriguing class of materials with many potential applications, including as shock- and noise-absorbers and as interfacial stabilizers. However, the preparation of a polymer with a pre-defined gradient of composition requires a semi-continuous process with careful control over monomer addition and constant feedback in order to regulate the composition at each stage of the polymerization.
It has recently been noted that the very idea of a gradient copolymer is something of a contradiction in terms: while the average composition of all the chains of a polymer may vary smoothly from one end of the chain to the other, each individual chain is composed of discrete monomer units, and at any point in the chain the composition must take one of only two possible values. Thus polymers which, on average, exhibit a gradient composition contain a broad distribution of structures. Furthermore, it is impossible to reconstruct the composition gradient of the entire polymer from a single polymer chain.
It is thus natural to ask, what is the defining structural feature of a gradient copolymer? And can these properties be mimicked using structures that are easier to prepare, using well-understood batch polymerization processes?
In the ASYMCOPO project we aim to answer this question by introducing the concept of asymmetric copolymers: polymers that are intermediate in structure between block and random copolymers. We have developed a framework for the classification of this class of polymers, centered on the concept of an ‘moment of asymmetry,’ which we will use to guide our research in first synthesizing, then exploring the properties of this new class of materials. In particular, we seek to define structures consisting of 2, 3, or 4 random copolymer segments which mimic the useful properties of gradient copolymers, but are substantially easier to synthesize.
This project requires the preparation of an extensive library of copolymers of different structures, and hence necessitates the involvement of a laboratory with expertise in high-throughput polymer synthesis and characterization. The Institute for Organic and Macromolecular Chemistry, in Jena, Germany, is the Europe’s leading research group in this domain. In parallel, synthesis of well-defined gradient copolymers requires careful control of polymerization conditions, employing the expertise of the IMRCP laboratory of Toulouse, France, in polymerization kinetics, polymer synthesis and characterization.
This Franco-German International Collaborative Research Project is submitted to the ANR generic program within the societal challenge “Stimulate Industrial Renewal” and the “Materials and Processes” axis. The thematic priority of the project is ‘Multifunctional Multiscale Materials.’ The proposed research is fundamental (Technology Readiness Level 1) and knowledge-based.