Polymerizations in Continuous Flow: Recent Advances in the Synthesis of Diverse Polymeric Materials

Abstract

The number of reports using continuous flow technology in tubular reactors to perform precision polymerizations has grown enormously in recent years. Flow polymerizations allow highly efficient preparation of polymers exhibiting well-defined molecular characteristics, and has been applied to a slew of monomers and various polymerization mechanisms, including anionic, cationic, radical, and ring-opening. Polymerization conducted in continuous flow offers several distinct advantages, including improved efficiency, reproducibility, and enhanced safety for exothermic polymerizations using highly toxic components, high pressures, and high temperatures. The further development of this technology is thus of relevance for many industrial polymerization processes. While much progress has been demonstrated in recent years, opportunities remain for increasing the compositional and architectural complexity of polymeric materials synthesized in a continuous fashion. Extending the reactor processing principles that have heretofore been focused on optimizing homopolymerization to include multisegment block copolymers, particularly from monomers that propagate via incompatible mechanisms, represents a major challenge and coveted target for continuous flow polymerization. Likewise, the spatial and temporal control of reactivity afforded by flow chemistry has and will continue to enable the production of complex polymeric architectures. This Viewpoint offers a brief background of continuous flow polymerization focused primarily on tubular (micro)reactors and includes selected examples that are relevant to these specific developments.

Advances in the Synthesis of π‐Conjugated Polymers by Photopolymerization

“…Producing soluble polymers with high molecular weight remains a challenge for both oxidative and reductive photopolymerization. The inherent high optical density of CPs adds a technical challenge to using light for their polymerization, likely necessitating the use of flow chemistry [55–57] . Flow chemistry has been shown to offer greater control of photopolymerizations by offering shorter path lengths and more homogenous irradiation compared to batch reactions [58] .…”

Progress and Perspectives Beyond Traditional RAFT Polymerization

“…The translation of precision polymer synthesis into continuous flow can provide an array of processing benefits, including improved heat transfer, enhanced mixing, and online adjustment capabilities. [ 109 ] In particular, photoRAFT is well suited to a continuous flow regime due to the short optical path lengths in photoflow reactors, resulting in negligible intensity gradient compared with conventional batch photopolymerization systems. [ 110 ] Leveraging this advantage, Junkers and Wenn conducted a series of RAFT photopolymerizations of butyl acrylate using various photocatalysts under UV irradiation.…”

Polymerizations by RAFT: Developments of the Technique and Its Application in the Synthesis of Tailored (Co)polymers

“…Side reactions and bad reproducibility of the products may result from insufficient control over the temperature gradient within the reactor and insufficient temperature control. [ 115,116 ] Therefore scaling up of polymerization is limited for exothermic polymerization reactions as the heat flow may become insufficient in bulk or solution polymerizations, unless heat exchange devices are incorporated into the reactor. In suspension or emulsion polymerizations this is not so much of an issue, as the “reactors” are in fact small droplets or micelles of typically hydrophobic monomers in a water matrix, where viscosity of the whole system is not largely affected by the molecular weight of the formed polymers.…”

“…This allows processes to run faster (for example, exothermic reactions at higher temperatures) without loss of control and it results in higher conversion rates under safe operation conditions. [ 115,116 ] Parameters such as the geometry of the flow reactor, the flow rate and viscosity of the reaction medium are some of the parameters to be considered, as in very good controlled polymerization reactions the distribution of residence time within the reactor should be kept narrow. [ 117 ]…”