Coil Flow Inversion as a Route To Control Polymerization in Microreactors Dambarudhar Parida, † Christophe A. Serra,* ,† Dhiraj K. Garg, ‡ Yannick Hoarau, ‡ Florence Bally, § Rene ́ Muller, † and Michel Bouquey † † Groupe d’Intensification et d’Inté gration des Proce ́ de ́ s Polyme ̀ res (G2IP), Institut de Chimie et Proce ́ de ́ s pour l’E ́ nergie, l’Environnement et la Sante ́ (ICPEES) - UMR 7515 CNRS, E ́ cole Europe ́ enne de Chimie, Polymè res et Mate ́ riaux (ECPM), Universite ́ de Strasbourg (UdS), Strasbourg, France ‡ Laboratoire des Sciences de l’Ingé nieur, de l’Informatique et de l’Imagerie (ICUBE), Universite ́ de Strasbourg (UdS), Strasbourg, France § Institut de Science des Mate ́ riaux de Mulhouse (IS2M), UMR CNRS 7361, Universite ́ de Haute Alsace, Mulhouse, France * S Supporting Information ABSTRACT: Linear and branched polymers of 2-(dimethylamino)ethyl methacrylate (PDMAEMA) were synthesized in flow by atom transfer radical polymerization (ATRP) and self-condensing vinyl copolymerization adapted to ATRP, respectively, in capillary type stainless steel coiled tube (CT) microreactors. Coil flow inversion (CFI) was introduced to achieve better mixing and narrower residence time distributions during polymerization. This strategy was adopted to improve control over macromolecular characteristics and polymer architecture. Polydispersity index (PDI), as an overall indicator of control over polymerization, was significantly lower for CFI in the case of linear PDMAEMA, 1.39 compared to 1.53 for CT. For branched polymers containing up to 10 mol % of inimer, a reduced PDI was also obtained for CFI microreactor. As for the branching efficiency, it was found to follow the following trend CFI > CT > batch reactor. 1. INTRODUCTION Application of microreaction technology in polymer synthesis dates back roughly to one decade 1 and has showed enormous potentials to produce polymers with well-defined characteristics. Microdevices derive these potentials from their high surface-to- volume ratio, small diffusion pathways, and large interfacial areas which give them the ability to overcome heat transfer and mixing limitations often encountered in their macroscale counterparts. Thus, microreactors and micromixers were found to be elements of choice when comes the need to increase the control of macromolecular characteristics. Ionic, free radical, controlled/ “living” radical, and more recently enzyme-catalyzed polymer- ization reactions were carried out successfully in microreactors, 2,3 the latter allowing an improved control over architecture and chemical composition. 4-8 Their high surface-to-volume ratio also allowed considering new operating windows like higher temperatures, which permitted for instance to carry out extremely exothermic reactions (ionic polymerizations) at much more convenient conditions (noncryogenic). 9-12 Controlled radical polymerizations like ATRP can benefit a lot from the special features of microdevices in terms of conversion, molecular weight, and architecture. In the first ever reported experiment of ATRP in a microreactor, Beers and co-workers demonstrated the possibility to synthesize polymer libraries just by changing the flow rate or reactants’ feed ratio. 13 They were also able to synthesize well controlled block copolymers of poly(ethylene oxide-2-hydroxypropyl methacrylate) by using a special design of a three-input-one-output chip reactor. 14 Evidences were also reported that microreactors can accelerate a slow polymerization reaction like ATRP. 15 Futhermore, microreactors were found to achieve higher branching structures compared to batch reactor. 16 Finally, by using a continuous-flow microfluidic system, Beers and co-workers were able to produce a gradient solution of two comonomers for the synthesis of statistical-copolymer-brush gradient on a silicon substrate initially layered with an ATRP initiator. 17 For most applications, ATRP catalyst need to be removed from the polymer solution. This downstream operation which is usually quite time- consuming may be eased and operated in flow within millireactors. One such example was reported by Zhu and co- workers, who used silica gel supported catalyst to pack a tubular millireactor (i.d. = 3.75 mm) for the ATRP synthesis of methyl methacrylate homo- and copolymers. 18,19 However, the observed broadening of molecular weight distribution suggested the trapping of molecules in the pores and the difficulty encountered by polymer chains to reach catalytic sites with increasing viscosity. To alleviate this problem, Cunningham and co-workers used a copper tube millireactor (i.d. = 1.65 mm) for Received: January 21, 2014 Revised: April 11, 2014 Published: May 6, 2014 Article pubs.acs.org/Macromolecules © 2014 American Chemical Society 3282 dx.doi.org/10.1021/ma5001628 | Macromolecules 2014, 47, 3282-3287