(62d) NMP Homopolymers and Two-Stages Copolymers Synthesis in Microtube Reactors: Improvement of the Macromolecular Architecture Control

Microfluidic devices are now widely used in scientific fields such as biology, chemistry and chemical engineering. These devices revealed to be powerful tools to study highly exothermic, fast and diffusion-controlled chemical reactions. However, these microsystems only begin to be used within the framework of free radical and living free radical polymerizations. For the later, Atom Transfer Radical Polymerization (ATRP) in solution, Reversible Addition-Fragmentation Transfer (RAFT), Nitroxide-Mediated Polymerization (NMP) in miniemulsion and anion ring opening polymerization have been reported up to now. Here, we look at the NMP in solution of acrylic and styrenic monomers having very different behaviors in terms of their reactivity and exothermicity. We are also interested in the livingness of the polymer by studying the copolymerization of the homopolymer synthesized with a comonomer. Moreover, we look at the influence of the reactor (macro or micro scale reactor) on the control of the macromolecular architecture.

The first block is carried out either continuously in a stainless steel tube microreactor of 0.9 mm ID at 140°C in toluene with a TIPNO alcoxyamine as initiator or in sealed tubes (i.e. in batch process). The copolymerization is also carried out in a similar microreactor or in sealed tubes. However, for the continuous process, the viscous first block solution is mixed with the pure comonomer through a multi or bilamination micromixer (either an interdigital multilamination micromixer or a basic T-junction) prior to enter in the second microtube reactor. Several operating conditions like different reaction times, initial alcoxyamine concentrations and the presence of a rate accelerating agent were investigated.

Concerning the first homopolymer block, for the less reactive and exothermic monomer (styrenic) and for low molecular weights targeted, no influence was observed between the batch and the microtube reactor. The number-average molecular weight (Mn) increases linearly with monomer conversion and the polydispersity index (PDI) ranges from 1.1 to 1.4 for batch and continuous reactions, indicating that the controlled nature of the polymerization mechanism is maintained. However, for high molecular weights targeted, the microtube reactor allows to get lower PDIs than the batch reactor. For a highly reactive and exothermic monomer (acrylate), whatever the molecular weight targeted, the microtube reactor returned the lowest PDIs whereas he polydispersity indexes obtained in the batch reactor are higher than 1.5 giving evidences of the lost in the polymerization control. The presence of a rate accelerative agent like acetic anhydride allows to increase the monomer conversion by at least 20% without affecting significantly the controlled nature of the polymerization. Then, the use of an accelerative agent enables the synthesis of a copolymer by a continuous two-stages process in which the residual first monomer is not removed from the reactive medium.

Concerning the block copolymer, it was found that the interdigital multlamination micromixer allows on one hand a higher incorporation rate of the second comonomer onto the first block (higher overall monomer conversion) and on the second hand a lower overall PDI compared to the T-junction and the batch process.

In conclusion, it was demonstrated that the use of microfluidic devices (microtube reactor and micromixer) can significantly improve the control over the polymerization as well as the macromolecular architecture.