Homogeneous polymerization has been performed by controlled radical polymerization (ATRP, NMRP) in microreactors to reach controlled linear polymers. The microscale has lead to an improved control over the polymerization in the case of highly reactive monomers thanks to a better heat removal [1]. Polymerization of block copolymers on that scale has provided interesting results in terms of polydispersity because of a better mixing [2]. However the potential of these devices may still be higher and reaching more complex controlled architectures is a new promising challenge. In this study, a particular focus is drawn on highly branched structures, with an expected end use of drug carriers.

A one-step synthesis of branched polymers has been carried out in batch reactors but the control over the polymerization is limited when high degree of branching is achieved. In fact, the reactivity of the polymer increases with conversion because the number of active sites increases (one per branch) in the chains. The higher heat removal capacity of microreactors has shown a decrease in polydispersity of the obtained branched polymers. The next step has been to make a linear shell grown from the highly branched core. This core is a very compact structure which active centres are not easy to get to. A second advantage of microstructured devices has thus been developed: the better mixing of a highly viscous branched polymer solution with a liquid comonomer. The control over the core-shell architecture has been particularly improved thanks to a micromixer.

[1] Rosenfeld C., C. Serra, C. Brochon and G. Hadziioannou, High temperature nitroxide-mediated radical polymerization in a continuous microtube reactor: towards a better control of the polymerization reaction, Chem. Eng. Sci., 62 (2007) 5245-5250.

[2] Rosenfeld C., C. Serra, C. Brochon and G. Hadziioannou, Influence of micromixer characteristics on polydispersity index of block copolymers synthesized in continuous flow microreactors, Lab. Chip., 8 (2008) 1682-1687.