A fully automated platform for photoinitiated RAFT polymerization

Abstract

Oxygen tolerant polymerizations including Photoinduced Electron/Energy Transfer-Reversible Addition-Fragmentation Chain-Transfer (PET-RAFT) polymerization allow for high-throughput synthesis of diverse polymer architectures on the benchtop in parallel. Recent developments have further increased throughput using liquid handling robotics to automate reagent handling and dispensing into well plates thus enabling the combinatorial synthesis of large polymer libraries. Although liquid handling robotics can enable automated polymer reagent dispensing in well plates, photoinitiation and reaction monitoring require automation to provide a platform that enables the reliable and robust synthesis of various polymer compositions in high-throughput where polymers with desired molecular weights and low dispersity are obtained. Here, we describe the development of a robotic platform to fully automate PETRAFT polymerizations and provide individual control of reactions performed in well plates. On our platform, reagents are automatically dispensed in well plates, photoinitiated in individual wells with a custom-designed lightbox until the polymerizations are complete, and monitored online in real-time by tracking fluorescence intensities on a fluorescence plate reader, with well plate transfers between instruments occurring via a robotic arm. We found that this platform enabled robust parallel polymer synthesis of both acrylate and acrylamide homopolymers and copolymers, with high monomer conversions and low dispersity. The successful polymerizations obtained on this platform make it an efficient tool for combinatorial polymer chemistry. In addition, with the inclusion of machine learning protocols to help navigate the polymer space towards specific properties of interest, this robotic platform can ultimately become a self-driving lab that can dispense, synthesize, and monitor large polymer libraries.

Fig. 1

(A) Manual PET-RAFT polymerizations require (l) reagent dispensing into individual wells of a 96-well plate manually, which is prone to pipetting, positional, or volumetric errors, and (2) use of an overhead lamp to initiate PET-RAFT reactions that restricts control over individual reactions throughout the plate. (B) Automated PET-RAFT polymerization allows for (1) simple custom polymer design by uploading a synthesis template consisting of reaction parameters such as monomer content and degree of polymerization, (2) automated dispensing of reagents using liquid handling robotics to eliminate any human error associated with manual pipetting steps while reducing time and improving efficiency, and (3) automated reaction monitoring using online fluorescence tracking and automated spatial and temporal control over individual reactions using a lightbox with 96 multiplexed LEDs.

Fig. 2

(A) The automated platform contains a liquid handling robot, our custom-designed lightbox for PET-RAFT initiation, a UV-vis spectro-photometer plate reader for online reaction monitoring, and a robotic arm to facilitate plate transfers between instruments. (B) 96-well plate containing PET-RAFT reaction mixtures placed on our lightbox. The fluorescence data of the reaction mixture at t = 0 is used to automatically control LED lighting profiles on the lightbox which resulted in wells only containing PET-RAFT reactions being lit for photoinitiation. (C) Fluorescence ratio vs. time plot of lightbox-initiated pDMA PET-RAFT polymerization in wells A1 and A12 of a single 96 well plate using warm-white LEDs. Shaded areas represent LED activation and polymer photoinitiation of their respective corresponding wells.

Fig. 3. Heatmap of light intensities at different well positions using (left) lightbox with all LEDs on (variance = 2.3%) and (right) an overhead lamp (variance = 29.2%).

Fig. 4

(A) A 96-well plate containing six different homopolymer reactions, each with eight replicates for a total of 56 used wells. Each well contained monomer, ZnTPP photocatalyst, CTA, and DMSO. (B) Fluorescence ratio vs. time plots of DMA, HEA, MA, MEA, and NAM polymerized on the lightbox (top) and lamp (bottom). Graphed lines represent the mean fluorescence ratio for all replicates, and shaded areas represent a 95% confidence interval. Correlations of fluorescence ratios and monomer conversions calculated by ¹H-NMR spectroscopy of all homopolymers polymerized using (C) the lightbox with constant LED lighting and (D) the lamp.

Fig. 5

(A) Fluorescence ratio vs. time plots of pHEA, pDMA, pMEA, pEA, pMA, and pNAM homopolymers synthesized on the lightbox with LED multiplexing. (B) Correlation of polymer fluorescence ratio and monomer conversions calculated using ¹H-NMR spectroscopy. Homopolymers were synthesized on the lightbox with feedback-controlled LED multiplexing.

Fig. 6

Lightbox lighting patterns of each 30 minute timepoint for a plate containing homopolymers and copolymers synthesized in parallel. Colored wells indicate that the corresponding LED for that well is on, where red = homopolymers, blue = copolymers with 50 : 50 monomer ratio, green = copolymers with 33 : 33 : 33 monomer ratios, and orange = copolymers with 25 : 25 : 25 : 25 monomer ratios. White wells indicate that the corresponding LED for that well is off, thus indicating feedback control of multiplexed lighting and photoinitiation.

Fig. 7

Fluorescence ratio vs. time graphs of (A) pHEA, pDMA, pMEA, pEA, pMA, and pNAM homopolymers, (B) copolymers with 50 : 50 monomer ratios, (C) copolymers with 33 : 33 : 33 monomer ratios, and (D) copolymers with 25 : 25 : 25 : 25 monomer ratios synthesized on the automated platform with feedback-controlled LED multiplexing. (E) Conversions of all monomers synthesized on the automated platform and calculated using ¹H-NMR spectroscopy. Homopolymers (HP) and copolymers (CP) were completed after three hours of polymerization.